The word Physics comes from the Greek word Phusis meaning
nature. Its Sanskrit equivalent is Bhautiki that is used to
refer to the study of the physical world. We can broadly
describe Physics as a study of the basic laws of nature and
their manifestation in different natural phenomena. We can
get some idea of the scope of Physics by looking at its
various sub-disciplines. Basically, there are two domains of
interest: macroscopic and microscopic.
The macroscopic domain includes phenomena at the
laboratory, terrestrial and astronomical scales. The
microscopic domain includes atomic, molecular and nuclear
phenomena. Classical Physics deals mainly with the
macroscopic phenomena and includes subjects like
Mechanics, Electrodynamics, Optics and Thermodynamics.
Mechanics is founded on Newton’s laws of motion and
the law of gravitation is concerned with the motion (or
equilibrium) of particles, rigid and deformable bodies, and
general systems of particles. The propulsion of a rocket by a
jet of ejecting gases, propagation of water waves or sound
waves in air, the equilibrium of a bent rod under a load, etc.,
are associated with the phenomenon of Mechanics.
Electrodynamics deals with electric and magnetic
phenomena associated with charged and magnetic bodies.
Its basic laws were given by Coulomb, Oersted, Ampere and
Faraday, and encapsulated by Maxwell in his famous set of
equations. The motion of a current-carrying conductor in a
magnetic field, the response of a circuit to an AC voltage
(signal), the working of an antenna, the propagation of radio
waves in the ionosphere, etc., are associated with the
phenomenon of Electrodynamics.
Optics deals with the phenomena involving light. The
working of telescopes and microscopes, colours exhibited
by thin films, etc., are topics in Optics.
Thermodynamics, in contrast to mechanics, does not
deal with the motion of bodies as a whole. Rather, it deals
with systems in macroscopic equilibrium and is concerned
with changes in internal energy, temperature, entropy, etc.,
of the system through external work and transfer of heat.
The efficiency of heat engines and refrigerators, the direction
of a physical or chemical process, etc., are phenomena of
interest in Thermodynamics.
The microscopic domain of Physics deals with the
constitution and structure of matter as the minute scales of
atoms and nuclei (and even lower scales of length) and their
interaction with different probes such as electrons, photons
and other elementary particles.
Some physicists from different countries of the
world and their major contributions
Name Major contribution/ Country of
Archimedes Principle of buoyancy; Greece
Principle of the lever
Galileo Galilei Law of inertia Italy
Christian Huygens Wave theory of light Holland
Isaac Newton Universal law of U.K.
gravitation; Laws of
Michael Faraday Laws of electro- U.K.
James Clerk Electromagnetic theory U.K.
Maxwell Light as an electromagnetic
Heinrich Rudolf Generation of electro- Germany
Hertz magnetic waves
J.C. Bose Ultra-short radiowaves India
W.C. Roentgen X-rays Germany
J.J. Thomson Electron U.K.
Marie Sklodowska Discovery of radium and Poland
Curie polonium; Studies on
Albert Einstein Explanation of photo- Germany
electric effect; Theory
Victor Francis Hess Cosmic radiation Austria
R.A. Millikan Measurement of U.S.A.
Ernest Rutherford Nuclear model New
of atom Zealand
Niels Bohr Quantum model Denmark
of hydrogen atom
C.V. Raman Inelastic scattering India
of light by molecules
Louis Victor Wave nature of matter France
M.N. Saha Thermal ionisation India
2 l Physics
S.N. Bose Quantum statistics India
Wolfgang Pauli Exclusion principle Austria
Enrico Fermi Controlled nuclear Italy
Werner Heisenberg Quantum mechanics; Germany
Paul Dirac Relativistic theory of U.K.
Edwin Hubble Expanding universe U.S.A.
Ernest Orlando Cyclotron U.S.A.
James Chadwick Neutron U.K.
Hideki Yukawa Theory of nuclear Japan
Homi Jehangir Cascade process of India
Bhabha cosmic radiation
Lev Davidovich Theory of condensed Russia
Landau matter; Liquid helium
S. Chandrasekhar Chandrasekhar limit, U.S.A.
structure and evolution
John Bardeen Transistors: Theory U.S.A.
C.H. Townes Maser, Laser U.S.A.
Abdus Salam Unification of weak Pakistan
Link between technology and physics
Technology Scientific Principle(s)
Steam engine Laws of thermodynamics
Nuclear reactor Controlled nuclear fission
Radio and Television Generation, propagation
Computer Digital logic
Laser Light amplification by
stimulated emission of
Production of ultra height Superconductivity
Rocket propulsion Newton’s laws of motion
Electric generator Faraday’s laws of
Hydroelectric power Conversion of
energy into electrical
Aeroplane Bernoulli’s principle in fluid
Particle accelerators Motion of charged
Sonar Reflection of ultrasonic
Optical fibres Total internal reflection of
Non-reflecting coatings Thin film optical
Electron microscope Wave nature of electrons
Photocell Photoelectric effect
Fusion test reactor (Tokamak) Magnetic confinement of
Giant Metrewave Radio Detection of cosmic
Telescope (GMRT) radio waves
Bose-Einstein condensate Trapping and cooling of
atoms by laser beams and
There are four fundamental forces in nature, which have
been described below.
The gravitational force is the force of mutual attraction
between any two objects by virtue of their masses. It is a
universal force. Every object experiences this force due to
every other object in the universe. All objects on the earth,
for example, experience the force of gravity due to the earth.
In particular, gravity governs the motion of the moon and
artificial satellites around the earth, the motion of the earth
and planets around the sun, and, of course, the motion of
bodies falling on the earth. It plays a key role in the largescale
phenomena of the universe, such as formation and
evolution of stars, galaxies and galactic clusters.
Electromagnetic force is the force between charged particles.
In the simpler case, when charges are at rest, the force is
governed by Coulomb’s law: attractive for unlike charges
and repulsive for like charges. Charges in motion produce
magnetic effects and a magnetic field gives rise to a force on
a moving charge. Like the gravitational force, electromagnetic
force acts over large distances and does not need any
Physics l 3
intervening medium. It is enormously strong compared to
gravity. The electric force between two protons is 1036 times
the gravitational force between them, for any fixed distance.
Since the electromagnetic force is so much stronger than the
gravitational force, it dominates all phenomena at atomic
and molecular scales. Thus it is mainly the electromagnetic
force that governs the structure of atoms and molecules,
the dynamics of chemical reactions and the mechanical,
thermal and other properties of materials.
Gravity is always attractive, while electromagnetic force
can be attractive or repulsive. The charge comes in two
varieties: positive and negative.
Strong Nuclear Force
The strong nuclear force binds protons and neutrons in a
nucleus. It is evident that without some attractive force, a
nucleus will be unstable due to the electric repulsion between
its protons. This attractive force cannot be gravitational since
the force of gravity is negligible compared to the electric
force. A new basic force must, therefore, be invoked. The
nuclear force is the strongest of all fundamental forces, about
100 times the electromagnetic force in strength. It is chargeindependent
and acts equally between a proton and a proton,
a neutron and a neutron, and a proton and a neutron. Its
range is, however, extremely small, of about nuclear
dimensions (10–15 m). It is responsible for the stability of
nuclei. The electron, it must be noted, does not experience
this force. Recent developments have, however, indicated
that protons and neutrons are built out of still more
elementary constituents called quarks.
Weak Nuclear Force
The weak nuclear force appears only in certain nuclear
processes such as the beta decay of a nucleus. In beta decay,
the nucleus emits an electron and an uncharged particle
called neutrino. The weak nuclear force is not as weak as the
gravitational force, but much weaker than the strong nuclear
and electromagnetic forces. The range of weak nuclear force
is exceedingly small, of the order of 10-16 m.
Fundamental forces of nature
Name Relative Range Operates among
Gravitational 10-39 Infinite All objects in
Weak nuclear 10-13 Very short, Some elementary
size (10-16 m) particularly
Electromagnetic 10-2 Infinite Charged particles
Strong nuclear 1 Short, Nucleons,
size (-10-15m) heavier than
Units and Measurements
The International System of Units
In earlier times scientists of different countries used different
systems of units for measurement. Three such systems, the
CGS, the FPS (or British) and the MKS system were in use
extensively till recently. The base units for length, mass and
time in these systems were as follows:
In CGS system they were centimetre, gram and second
In FPS system they were foot, pound and second
In MKS system they were metre, kilogram and second
The system of units which is at present
internationally accepted for measurement is Le Système
International d'Unités (French for International System
of Units), abbreviated as SI. The SI, with standard
scheme of symbols, units and abbreviations, was
Physical Quantity: The quantities which can be measured
are known as physical quantities. The measurement of any
physical quantity involves comparison with a certain basic,
arbitrarily chosen, internationally accepted reference standard
called unit. The units for the fundamental or base quantities
are called fundamental or base units. The units of all other
physical quantities can be expressed as combinations of the
base units. Such units obtained for the derived quantities are
called derived units. A complete set of these units, both the
base units and derived units, is known as the system of units.
Scalars and Vectors
A scalar physical quantity has magnitude only and no
direction. It is specified completely by a single number, along
with a proper unit, e.g. mass, length, time, temperature etc.
A vector quantity has both magnitude and a direction and
obeys the vector laws of addition (triangle law, parallelogram
law), e.g. displacement, velocity, acceleration etc.
4 l Physics
developed and recommended by General Conference
on Weights and Measures in 1971 for international
usage in scientific, technical, industrial and commercial
work. Because SI units used decimal system,
conversions within the system are quite simple and
SI Base Quantities and Units
Base SI Units
quantity Name Symbol Definition
Length metre m Metre is the length of the
path travelled by light in
vacuum during a time
interval of 1/299,792,458 of
a second. (1983)
Mass kilogram kg Kilogram is equal to the mass
of the international
prototype of the kilogram (a
cylinder) kept at
international Bureau of
Weights and Measures, at
Sevres, near Paris, France.
Time second s Second is the duration of
9,192,631,770 periods of the
radiation corresponding to
the transition between the
two hyperfine levels of the
ground state of the
Caesium-133 atom. (1967)
Electric ampere A Ampere is that constant
current current which, if maintained
in two straight parallel
conductors of infinite
length, of negligible circular
cross-section, and placed 1
metre apart in vacuum,
would produce between
these conductors a force
equal to 2×10-7 Newton per
metre of length. (1948)
Thermo- kelvin K Kelvin, is the fraction
dynamic 1/273.16 of the thermodynatemperature
mic temperature of the triple
point of water. (1967)
Amount of mole mol Mole is the amount of subsubstance
stance of a system, which
contains as many elementary
entities as there are
atoms in 0.012 kilogram of
Luminous candela cd Candela is the luminous
intensity intensity, in a given
direction, of a source that
radiation of frequency
540×1012 hertz and that has
a radiant intensity in that
direction of 1/683 watt per
Motion has been defined as a change in position of a
body with respect to time.
Motion may be classified into two types:
1. Linear Motion
2. Non-Linear motion
Linear Motion: When a body travels along a straight
line. For example, when a car travels on a straight road
without changing the direction.
Non-Linear Motion: When a body travels along a nonstraight
Further motion can be classified into two types:
(i) Uniform motion
(ii) Non-Uniform motion
Uniform motion: When the velocity of a body does not
change with respect to time.
Non-Uniform motion: When the velocity of a body
changes with respect to time.
Velocity: Velocity is displacement per unit time.
Unit : m/s
Acceleration: Acceleration is the rate of change of
velocity or change in velocity per unit time.
Negative acceleration is called retardation or
Types of acceleration: There are two types of
(a) Uniform acceleration
(b) Non-uniform acceleration or variable acceleration
Uniform acceleration: A body is said to possess uniform
acceleration if there are equal changes in its velocity in
Variable acceleration: A body is said to possess non-
Physics l 5
change its state of motion is called inertia of motion; e.g.
(i) If a horse suddenly stops galloping, the rider receives a
forward jerk. This is because the lower part of the rider
in contact with the horse comes to rest whereas the
upper portion of his body remains in a state of motion.
(ii) When a rotating fan is switched off it continues to rotate.
(iii) A ball thrown vertically upwards in a moving train comes
back into the hands of the thrower when the train is
having a uniform motion.
(iv) A person falls when he alights a moving train. This is
because inside the train his body is in a state of motion.
But when he jumps out of the train the lower portion of
his body is at rest while the upper motion remains in
motion due to inertia of motion.
Inertia of Direction
The inherent property of a body by virtue of which it cannot
change its direction of motion is called inertia of direction;
(i) Vehicles are provided with mudguards to save them from
being spoilt. This is due to the reason that the mud
sticking to the wheels flies off tangentially.
(ii) A stone tied to a string and whirled along a circular path
flies off tangentially when its string is suddenly broken.
(iii) While sharpening a knife, sparks fly off tangentially
from the grinding stone.
Inertia and Mass
The heavier or more massive objects offer larger inertia.
Quantitatively, the inertia of an object is measured by its
The momentum p of an object is defined as the product of its
mass m and velocity v.
i.e. p = mv
Momentum has both direction and magnitude.
The SI unit of momentum is kilogram-metre per second
By the definition of momentum, it is clear that a moving
object can have a large momentum if either its mass is large
or its velocity is large or both are large. A truck has more
momentum than a car moving with the same speed as that of
the truck. Huge objects having a small speed usually have a
Example: During the game of table tennis if the ball hits a
player it does not hurt him. On the other hand, when a fast
moving cricket ball hits a spectator, it may hurt him.
Newton’s Second Law of Motion
It states that the rate of change of momentum is directly
proportional to the applied force and the change takes place
in the direction of the applied force.
Newton's second law gives the quantitative definition
uniform or variable acceleration if there are unequal
changes in its velocity in equal intervals of time.
Speed: The speed of a body is the distance covered per
Laws of Motion
Galileo proposed the concept of acceleration. From
experiments on motion of bodies on inclined planes or falling
freely, he thus arrived at the law of inertia.
Newton built on Galileo’s ideas and laid the foundation
of mechanics in terms of three laws of motion that go by his
name. Galileo’s law of inertia was his starting point which he
formulated as the First Law of Motion.
Newton’s First Law of Motion or
Law of Inertia or Galileo's law
Every body in the universe stays in a state of rest or in
uniform motion along a straight line until and unless
compelled by an external force to change its state.
Newton’s first law gives the qualitative definition of
force, as it tells us about an agent without which,
acceleration is not possible. According to the law, force
is an agent which tends to change the state of rest or of
uniform motion of a body; e.g. a moving car stops only
when brakes are applied.
The concept of inertia
Inertia can be understood in the following two ways:
It is the inability of a body to change, by itself, its state
of rest or uniform motion.
It is the property of a body by virtue of which it opposes
any change in its state of rest or of uniform motion.
Inertia is of three types:
1. Inertia of rest
2. Inertia of motion
3. Inertia of direction
Inertia of rest
The inherent property of a body by virtue of which it cannot
change its state of rest is called inertia of rest; e.g.
(i) When we dust a carpet, the dust moves out of the carpet.
This is because the dust is set into motion whereas the
carpet remains in a state of rest due to inertia of rest.
(ii) If a horse suddenly starts galloping, the rider receives a
(iii) An apple falls down from the tree when its branch is
(iv) When a train starts suddenly, the passengers receive a
Inertia of Motion
The inherent property of a body by virtue of which it cannot
6 l Physics
of force. The mathematical form of Newton's second law of
motion may be written as
m × a = rate of change of momentum, therefore,
Force = rate of change of momentum
or, F = ma
Units of Force
(i) Newton (N) in SI system (ii) Dyne in CGS system (iii) Pound
(Lb) in British Engineering System.
If m = 1g, a = 1 cms-2 then F = 1 dyne
Newton's Third Law of Motion
According to this law, forces appear in pair. An isolated
force does not exist. Whenever one object exerts a force on
another, the second object exerts an equal and opposite
force on the first. This law states that to every action there is
an equal and opposite reaction. It can be expressed
Consider two bodies A and B. Let FAB be the force
experienced by body A due to body B, and FBA
be the force
experienced by B due to A. If the system is isolated, then by
Newton's third law we have FAB = –FBA. This is the
mathematical representation of the third law.
Examples of Newton's Third Law
1. When a person throws a package out of a boat, the boat
moves in the opposite direction. The person exerts a
force on the package. The package exerts an equal and
opposite force back on the person, and this force propels
the person (and the boat) backward slightly.
2. Rockets work on the same principle. A common
misconception is that rockets accelerate because the
gases rushing out of the back of the engine push against
the ground or the atmosphere. Actually, a rocket exerts
a strong force on the gases, expelling them. The gases
exert an equal and opposite force on the rockets and it
is this force that propels the rocket forward. Thus a
space vehicle moves in empty space by firing its rockets
in the direction opposite to that in which it wants to
3. A person begins walking by pushing with the foot
against the ground. The ground then exerts an equal
and opposite force back on the person and it is this
force on the person that moves him or her forward.
4. An automobile moves forward because of the force
exerted on it by the ground which is the reaction to the
force exerted on the ground by the tyres.
Law of Conservation of Momentum
The momentum of an object is the product of mass and
direction. Momentum is also a vector. Therefore the direction
of the object is important for the determination of the total
momentum of a system of objects. The Law of Conservation
of Momentum infers that the total momentum of a system of
objects remains the same. Therefore, if two objects collide
the total momentum before the collision is equal to the total
momentum after the collision. The velocities of the objects
involved in the collision can change in both magnitude and
direction. There are two types of collisions: elastic, where
the objects are only in contact with each other for a brief
period of time; and inelastic, where they remain fixed together
and move as one object with one velocity. The equations
for the conservation of momentum are given below:
m1v1i + m2v2i = m1v1f+ m2v2f
where i and f are initial and final velocities.
Applications of Law of Conservation of Momentum
When a bullet is fired from a gun, the gases produced in
the barrel exert a tremendous force on the bullet (action
force). As a result, the bullet moves forward with a great
velocity called the muzzle velocity. The bullet at the
same time exerts an equal force on the gun in the
opposite direction (reaction force). Due to this the gun
moves backwards. This backward motion of the gun is
called the recoil of the gun. The velocity with which the
gun moves backwards is called the recoil velocity.
The motion of a rocket is an application of Newton's
third law of motion and law of conservation of linear
momentum. A rocket is a projectile that carries the rocket
fuel and the oxidiser, which supplies the oxygen needed
for combustion. Liquid hydrogen, liquid paraffin, etc.,
are used as rocket fuels. Hydrogen peroxide, liquid
oxygen etc., are used as oxidisers. The fuel-oxidiser
combination in a rocket is called the propellant. The
simplest form of rocket consists of a combustion
chamber in which a solid or liquid propellant is burnt.
There is a nozzle at its tail through which the gaseous
products of combustion can escape. The rocket forces
a jet of hot gases downwards through the nozzle. This
is the action. The jet of gases exerts an equal force on
the rocket, pushing it forward. This is the reaction. This
force gives the rocket a forward acceleration.
Physics l 7
Work, Energy and Power
Work is said to be done only when the force applied on a
body makes the body move, i.e., there is a displacement of
The amount of work done (W.D.) by a force is equal to
the product of the force and the displacement of the point of
application of the force in the direction of the force.
The amount of W.D. by a force is zero
(i) When there is no displacement
(ii) When displacement is normal to the direction of
force ( = 90°),
cos 90° = 0.
The amount of W.D. will be maximum when the
displacement is in the direction of force applied
W.D. = f × s cos = f × s cos 0°
W.D. = f × s ( cos 0° = 1)
The amount of W.D. will be positive when the angle
between force and displacement will be less than 90°.
The amount of W.D. will be negative when the angle
between force and displacement will be 180° or greater than
W.D. = f × s cos 180°
= f × s(–1)
W.D. = –f × s ( cos 180° = –1)
SI unit: Joule
CGS unit: Erg
Work is a scalar quantity.
1 Joule of work is said to be done when a force of 1 Newton
displaces a body through 1 metre in its own direction.
1 erg of work is said to be done when a force of 1 dyne
displaces a body through 1 cm in its own direction.
1 Joule = 1 Nm 1 N = 105 dyne
1 m = 102 cm 1 Joule = 107 ergs
Energy of a body is its capacity to do work.
When a body does work, its energy decreases, while if work
is done on the body, its energy increases.
Unit of energy:
SI unit – Joule CGS unit – Erg
Commercial unit – kWh
There are two types of energy:
1. Potential energy
2. Kinetic energy
Potential energy is the energy possessed by a body by virtue
of its position or when the body is at rest or in deformed
state. It may also be defined as the amount of work done on
a body to change its position against the force of gravity.
P.E. = mgh,
where h is the height through which the body is lifted.
m= mass of the body.
g= acceleration due to gravity.
Kinetic energy is possessed by the body when it is in motion.
It may also be defined as the amount of work done (W.D.) in
increasing the velocity of the body from zero to some value.
K.E. = ½ mv2
where v is the velocity of the body.
Different forms of Energy
Thermal Energy: Thermal energy is the energy of an object
because of the kinetic energy and potential energy of the
molecules. The higher the temperature, the faster the
molecules move, the greater the thermal energy of a
substance. It is commonly called heat energy.
Chemical Energy: The energy released or absorbed
during a chemical reaction, depending on whether the total
energy of the reactant is more or less than the product, e.g.
hydrolysis, burning of coal.
Electrical Energy: Work needs to be done with respect
to attraction or repulsion of electrical changes. The energy
by virtue of this work is electrical energy.
Nuclear Energy: The energy produced due to fission of
a heavy nucleus molecule to two lighter fragments or fusion
of two light nuclei to give a heavier nucleus, is called nuclear
Conservation of Energy
According to the law of conservation of energy, energy can
neither be created, nor be destroyed; it can only be converted
from one form to another, i.e. total energy in a closed system
The general law of conservation of energy is true for all
forces and for any kind of transformation between different
forms of energy.
Energy related to Mass: According to Einstein's
hypothesis, mass can be converted into energy according to
the relation E = mc2, where 'm' is mass and 'c' is the speed of
light. The value of 'c' = 3 × 10 m/s. The law cannot be proved
8 l Physics
mathematically, but is an empirical one. It forms one of the
fundamental principles of physics.
Equipment/Instrument Transformation of energy
Loudspeaker Electrical energy into Sound
Musical Instruments Mechanical energy into Sound
Bulb/Tube Electrical energy into Light energy
Heater Electrical energy into Thermal
Coal Chemical energy into Thermal
Electric Cell Chemical energy into Electrical
Heat Engine Thermal energy (Heat energy) into
Solar cell Light energy into Electrical energy
Petrol engine Chemical energy into Mechanical
Microphone Sound energy into Mechanical
The rate of doing work is called power. It depends on two
factors: (i) Amount of work done (ii) Time taken to do work.
Power is also calculated from the product of force (f)
and average speed (v).
SI unit: Watt or Joule/s or 1 kg m2s–3. The other unit is horse
1 hp = 746 watt
1 KW = 1000 watt
1 MW = 106 watt
CGS unit: erg/s
1 W = 1 J/s or 107 erg/s
The effect of a force applied for a very short period of time is
called impulse. It is equal to the change in the momentum of
Impulse = F × t = mv – mu
Units: kg m/s or Ns
Application of Impulse
Thick mattresses with soft surfaces are used in events
such as high jump so that the time interval of impact on
landing is extended, thus reducing the impulsive force.
This can prevent injuries to the participants.
Goalkeepers will wear gloves to increase the collision
time. This will reduce the impulsive force.
A high jumper will bend his legs upon landing. This is to
increase the time of impact in order to reduce the impulsive
force acting on his legs. This will reduce the change of
getting a serious injury.
A baseball player must catch the ball in the direction of
the motion of the ball. Moving his hand backwards when
catching the ball prolongs the time for the momentum to
change so as to reduce the impulsive force.
Force and Pressure
Non-Contact Force: The force which does not involve
physical contact between the two objects but acts through
empty space is called a non-contact force.
Force/Weight is a vector quantity.
1 Newton is the force which when acting on a body of
mass 1 kg produces an acceleration of 1 ms-2 in it.
1 dyne is the force which when acting on a body of mass
1 g produces an acceleration of 1 cms-2 in it.
Force is that physical cause which changes or tends to
change the state of rest or the state of motion of the body. It
can also bring about a change in the dimension of the body.
Contact Force: The force between two bodies
when they are physically in contact is called a contact
Physics l 9
1 kgf is the force with which the earth attracts a mass of
1 gf is the force with which the earth attracts a mass of
Relation between Newton and dyne:
(i) 1 N = 105 dyne (ii) 1 kgf = 9.8 N
(iii) 1 gf = 980 dyne (iv) 1 kgf = 1000 gf
Centripetal Force: For a body to move in a circle, there
must be a force on it directed towards the centre. This is
called the centripetal force and is necessary to produce
continuous change of direction in a circular motion. In case
of the moon, the gravitational force between the earth and
the moon acts as the centripetal force. When a stone tied at
one end of a string is whirled in a circle, the pull in the string
provides the centripetal force.
The magnitude of the centripetal force, Fc, required to
cause an object of mass m and speed v to travel in a circular
path of radius r is given by the relation
It is a real force.
Centrifugal Force: This force is supposed to be acting
on a body revolving in a circle. Centrifugal force is equal and
opposite to the centripetal force, i.e. it acts outwards.
It is not a real force.
Application of Centrifugal Force
Centrifuge: A device by means of which light particles and
heavy particles are separated from each other.
Cream Separator: In a cream separator, a vessel
containing milk is rotated fast. Being lighter, the cream collects
in a cylindrical layer around the axis, whence it is drawn off
and the skimmed milk is drained through an outlet fitted on
the wall of the vessel. The particles whose density is less
than that of the liquid are driven towards the axis of rotation
and those whose density is greater than that of the liquid are
driven away from the axis. Cream is lighter than milk, so it is
separated from milk and collected at the axis.
Centrifugal Drier: In laundries, wet clothes are dried
by packing them in a cylindrical vessel with perforated walls
which is rotated at a very high speed. Water particles stick
to the clothes with a certain force which is called adhesive
force. The water particles are not sufficient to keep them
moving uniformly in a circle.
The force acting on a unit area of a surface is called pressure.
Pressure = Area
SI unit: Nm-2 = Pascal = Pa
Thus, the same force acting on a smaller area exerts a
larger pressure, and a smaller pressure on a larger area.
Examples which show that decrease in area increases the
A sharp knife has a very small surface area on its cutting
edge so that high pressure can be exerted to cut the
The studs on a football boot have only a small area of
contact with the ground. The pressure under the studs
in high enough for them to sink into the ground, which
gives extra grip.
Nails, needles and pins have very sharp ends with very
small surface areas.
The sole of an ice skater is a fixed narrow metal bar. The
high pressure on the surface of the ice makes the ice
melt and allows the ice skater to glide smoothly.
Racing bicycles need very high air pressure inside the
tyres, because the narrow tyres have a very small
contact area with the road. The hard road surface can
support the high pressure under the wheels.
Examples which illustrate that increase in area decreases
Skis have a large area to reduce the pressure on the
snow so that they do not sink in too far.
A wide shoulder pad of a heavy bag will reduce the
pressure exerted on the shoulder of the person carrying
Pressure in Liquids
A liquid in a container exerts pressure because of its weight.
The pressure in a liquid is directly proportional to the
depth. The pressure in a liquid increases with depth.
The pressure in a liquid is directly proportional to the density
of the liquid.
Pressure in liquid (P) = gh
Where = density, h = depth, g = gravitational
Characteristics of pressure in a liquid
The pressure at any point in a liquid, at a particular
depth, acts equally in all directions.
The pressure in a liquid does not depend on the area of
The pressure in a liquid acts equally in all directions
and does not depend on the shape of the container.
Applications of pressure in a liquid
The wall of a dam is much thicker at the bottom than at
the top because it must withstand the increased lateral
pressure in the depths of water.
Normally a water tank is placed at a higher level so as to
supply water at a greater pressure.
10 l Physics
The submarine is built with a thick wall so as to
withstand enormous pressure at a greater depth.
The liquid solution is at a higher pressure so that it has
sufficient pressure to flow into the veins of the patient.
The pressure exerted on an enclosed liquid at one place is
transmitted equally throughout the liquid. This is called
Pascal’s law. Hydraulic presses, hydraulic brakes, hydraulic
door closers, etc. are applications of this principle.
Atmospheric Pressure and Gauge Pressure: The
pressure of the atmosphere at any point is equal to the weight
of a column of air of a unit cross-sectional area extending
from that point to the top of the atmosphere. At sea level it is
1.013 × 105 Pa (1 atm). Italian scientist Evangelista Torricelli
(1608-1647) devised for the first time a method for measuring
atmospheric pressure. This device is known as a mercury
barometer. By experiment it is found that the mercury column
in the barometer has a height of about 76 cm at sea level
equivalent to one atmosphere (1 atm).
Atmospheric pressure varies with the height of the
object above sea level. It decreases with the altitude or
the height above sea level. At higher altitudes, the
density and the temperature of the air are lower. As a
result, the frequency of collisions of the molecules is
lower. Hence, atmospheric pressure is lower.
Common Manifestations of Atmospheric
When we suck through a straw, the air pressure in the
straw is lowered. Then the pressure of the atmosphere
acting on the surface of the drink in the glass pushes
the water up the straw and into our mouth.
When the sucker is pressed into place, most of the air
behind it is squeezed out. The sucker is held in position
by the pressure of the atmosphere on the outside surface
of the rubber. If the seal between the sucker and the
surface is airtight, the sucker will stick permanently.
Pulling up the piston reduces the atmospheric pressure
inside the cylinder. The atmospheric pressure on the
liquid surface then pushes the liquid up into the syringe.
If we then hold the plunger in place and lift the syringe
out of liquid, none will fall out. This is again due to
Fountain pen: Ink can be filled in a fountain pen with
the help of atmospheric pressure. When the tube of the
pen is squeezed, the air in it rushes out of that and the
pressure in the tube decreases. The air pressure outside
the tube now pushes the ink into the pen. When we go
to a higher altitude, the atmospheric pressure decreases,
and the pressure inside the pen in comparison to the
atmospheric pressure increases. That is why fountain
pens leak at higher altitudes.
A vacuum cleaner produces only a partial vacuum. The
fan inside the cylinder blows air out of the vents. With
less air inside, the air pressure drops. The atmospheric
pressure outside then pushes the air up the cleaner
hose, carrying dust and dirt with it.
In an aircraft flying at high altitude, normal atmospheric
pressure is maintained by the use of air pumps.
The atmospheric pressure is measured with an
instrument called the barometer.
Since atmospheric pressure varies with altitude, a
barometer can be used for determining altitudes. An
aneroid barometer calibrated for determining altitudes
is called an altimeter. Barometers are also used for
weather forecasting. If the barometric height falls
suddenly, it indicates the coming of a storm. A gradual
fall in the barometric height indicates the possibility of
rain. A gradual increase in the barometric height indicates
An open-tube manometer is a useful instrument for
measuring pressure differences.
1 bar = 105 Pa
body usually floats in water, and a helium-filled balloon
floats in air.
Upthrust depends upon two factors:
(i) Volume of the body
(ii) Density of the fluid
It is found that the greater the volume of a body, the
greater the upthrust it experiences when placed inside a
Upthrust and Buoyancy
The upward force experienced by a body immersed
partially or fully in a fluid (liquid or gas) is called upthrust
or buoyant force (FB).
Buoyancy is a familiar phenomenon: a body immersed
in water seems to weigh less than when it is in air. When
the body is less dense than the fluid, it floats. The human
Physics l 11
It is also found that the greater the density of the fluid
the greater the upthrust it applies on the body. For
example when a piece of wood is pushed into different
fluids, different forces have to be applied to do so. The
denser the fluid, more is the force required to push the
body into it. This is the reason that we apply more force
to push a body into salt water than into ordinary water.
(i) If we place an iron nail on the surface of water, it sinks.
This is because the density of iron is greater than that
of water, so the weight of the nail is more than the
upthrust of water on it. On the other hand a ship made
of iron does not sink. This is because the ship is hollow
and the empty space contains air which makes the
average density of the ship less than that of water.
Therefore, even with a small part of its submerged into
water, the weight of the water displaced becomes equal
to the total weight of the ship and hence the ship floats.
(ii) Due to the presence of minerals, the density of sea water
is more than the density of river water, therefore upthrust
is large. Therefore, it is easier to swim in sea water than
in river water.
(iii) Dead bodies always float on the surface of water, but
the head stays within water. The reason is that when
the dead body decays its volume increases. Thus, it
becomes lighter than water and, hence, floats. However,
the head being heavy cannot displace water more than
its own weight, hence it remains under water.
(iv) The mass of a balloon filled with helium is less than the
mass of the air displaced by it. Hence upthrust acting
on the balloon is more than its weight. As a result the
balloon experiences a net upthrust which makes it rise.
As the balloon rises it experiences lesser and lesser
upthrust due to the fact that with height the density of
air decreases. At a certain point the weight of the balloon
may be completely balanced by the upthrust acting on
it. Therefore the balloon stops rising.
The principle states that when a body is wholly or partially
immersed in a fluid, it experiences an upthrust equal to the
weight of the fluid displaced. When an object is immersed in
a fluid, two forces act on it: (i) the weight (W) of the object
acting downward through the centre of the body, and (ii)
upthrust (U) acting upward through the centre of gravity of
the body. It is due to upthrust that objects apparently weigh
less when immersed in fluids.
Effect of buoyancy on bodies of different weights:
1. When W < U, the body floats.
2. When W > U, the body sinks.
3. When W = U, the resultant force acting on the body
when fully immersed in the fluid in zero. The body is at
rest anywhere within the fluid. The apparent weight of
the body is zero for all such positions.
The density of a substance is defined as its mass per unit
Density () = volume(V)
The density of most of the substances decreases with
an increase in temperature and increases with a decrease in
temperature. Water is an exception. Water contracts when
cooled upto 4°C and expands when cooled further, below
4°C. Thus the density of water is a maximum at 4°C. At 4°C,
the density of water is 1 gcm-3 or 1000 kg m–3.
Law of Floatation
A floating body displaces its own weight of the fluid in
which it floats. The Archimedes' principle and the law of
floatation can explain several phenomena.
An iron nail sinks in water whereas a ship made of iron
and steel floats. This is due to the fact that a ship is hollow
and contains air and, therefore, its density is less than that
The density of sea water, due to the presence of
impurities like salt, etc. is greater than that of river water.
Therefore, lesser volume of ship will be immersed in sea
water to balance its weight. So the ship sinks to a great
depth in river water than in sea water.
It is because of the higher density of sea water that it is
easier to swim in the sea.
A submarine has large ballast tanks. When these tanks
are filled with water, the average density of the submarine
becomes more than that of water and it can dive easily. When
the submarine is ready to surface, compressed air is forced
into the ballast tanks, forcing the water out, thus reducing
the density of the submarine, which can then rise.
A solid chunk of iron will sink in water but float in mercury
because the density of iron is more than that of water but
less than that of mercury.
Ice, being less dense than water, floats in it with one
tenth of its volume above the surface. When ice melts it
contracts by as much of its volume as was above the surface
and, therefore, the level of water remains unchanged.
A hydrometer is an instrument used for measuring the density
or relative density of liquids. It is based on the principle of
floatation. A special type of hydrometer is used to measure
the density of acid in a car battery. Another special type of
hydrometer called lactometer is used for testing the purity
of milk by measuring its density.
12 l Physics
The flow of a fluid is said to be steady if at any given point,
the velocity of each passing fluid particle remains constant
in time. The path taken by a fluid particle under a steady flow
is a streamline. It is defined as a curve whose tangent at any
point is in the direction of the fluid velocity at that point.
This states that for all points along a streamline in an
incompressible and non-viscous fluid flowing steadily, the
sum of pressure energy, potential energy and kinetic energy
per unit volume is constant.
Thus, if p be the pressure energy per unit volume, be
the density of the fluid, h be the height from the ground level,
then by Bernoulli's theorem,
p 1 2 = constant
In fact, Bernoulli’s theorem is nothing but the law of
conservation of energy for an ideal fluid.
Note: Bernoulli’s equation ideally applies to fluids with zero
viscosity or non-viscous fluids.
Applications of Bernoulli's Theorem
Dynamic Lift: Dynamic lift is the force that acts on a
body, such as airplane wing, a hydrofoil or a spinning
ball, by virtue of its motion through a fluid. In many
games such as cricket, tennis, baseball and golf, a
spinning ball deviates from its parabolic trajectory as it
moves through air. This deviation can be partly explained
on the basis of Bernoulli’s principle. When a bowler spins
a ball, it changes its direction (swings) in the air due to
unequal pressure acting on it.
Lift on an Aeroplane Wing: The shape of the aeroplane
wings is such that the upper surface is more curved than
its lower surface and its head is thicker than its tail. As
the aeroplane moves forward, the speed of air above the
wings is larger than the speed of the air below the wings.
According to Bernoulli's theorem, the pressure above
the wings is less than the pressure below the wings. Due
to the pressure difference, the aeroplane gets the vertical
lift, which is sufficient to overcome the force of gravity
and the aeroplane is lifted up.
Blowing Off the Roofs During Storm : During storms or
cyclones, the roofs of the huts or tinned roofs are blown
off because of the high-speed wind. According to
Bernoulli's theorem, the pressure over the roof becomes
less, while the pressure of air under the roof is very
large, i.e. P1 > P2. This pressure difference gives an
erratical lift to the roof and it is blown off.
Bernoulli’s principle helps in explaining blood flow in
The word capilla means 'hair' in Latin; if the tube were hairthin,
the rise would be very large. The phenomenon of rise or
fall of a liquid in a capillary tube is known as capillarity.
The melted wax of a candle is drawn up into the wick by
capillary action. Oil rises up a lamp wick for the same reason.
If one end of a sugar cube is dipped into tea, the entire cube
is quickly wet on account of capillary action.
The fine pores of a blotting paper act as tiny capillary
tubes. The ink rises into the blotting paper through these
pores. The capillary action in soil is important in bringing
water to the roots of plants.
Bricks are porous and, therefore, subsoil water can seep
up them by capillary action. To avoid dampness in a building,
a layer of nonporous material, such as slate, is necessary in
A towel soaks water due to capillary action.
Most of the fluids are not ideal ones and offer some resistance
to motion. This resistance to fluid motion is like an internal
friction analogous to friction when a solid moves on a surface.
It is called viscosity.
The viscous force exists when there is relative motion
between layers of the liquid. Suppose a fluid like oil is enclosed
between two glass plates. The bottom plate is fixed while the
top plate is moved with a constant velocity v relative to the
fixed plate. If oil is replaced by honey, a greater force is required
to move the plate with the same velocity. Hence we say that
honey is more viscous than oil.
It is measured in terms of the coefficient of viscosity, .
Its SI unit is pa–s (Pascal second).
Generally thin liquids like water, alcohol etc. are less
viscous than thick liquids like coal tar, blood, glycerin etc.
The viscosity of liquids decreases with temperature while
it increases in the case of gases. In a gas, the temperature rise
increases the random motion of atoms and viscosity increases.
The viscous force F acting on an object falling through
a fluid of coefficient of viscosity depends on its size r (in
case of a ball, r is its radius) and its velocity .
F = 6r
This is Stokes' law.
Flow in Fluids
Physics l 13
Intermolecular force: The force of attraction or repulsion
acting between the molecules is known as intermolecular
force. The nature of intermolecular force is electromagnetic.
Cohesive force: The force of attraction between the
molecules of the same substance is called cohesive force.
Adhesive force: The force of attraction between the molecules
of different substances is called adhesive force.
Examples of Cohesive Force
1. Two drops of liquid coalesce, i.e. join into one, when
brought in mutual contact.
2. It is difficult to separate two sticky plates of glass welded
3. It is difficult to break a drop of mercury into a small
droplet because of large cohesive force between the
Examples of Adhesive Force
1. Adhesive force enables us to write on the blackboard
with a chalk.
2. A piece of paper sticks to another due to large force of
adhesion between the paper and gum molecules.
3. Water wets the glass surface because the force of
adhesion between glass molecules and water molecules
is greater. But mercury does not wet glass because force
of cohesion is greater than force of adhesion.
Surface Tension: The property of a liquid to have minimum
surface area and behave as if it were under tension,
somewhat like a stretched elastic membrane, is called surface
Phenomena due to surface tension:
A small liquid drop/raindrop has spherical shape due to
Lead balls are spherical in shape.
The bristles of shaving brush/painting brush, when
dipped in water, spread out; but as soon as the brush is
taken out of water, its bristles stick together.
Similarly, insects can walk on the free surface of water
The liquid surface tries to have minimum surface area,
and for a given volume, the sphere has minimum surface area.
Surface tension of a liquid is measured by the force acting
per unit length on either side of an imaginary line drawn on
the free surface of liquid, the direction of this force being
perpendicular to the line and tangential to the free surface of
liquid. So if F is the force acting on one side of imaginary line
of length (L), then
Note:1. It depends only on the nature of the liquid and is
independent of the area of surface or length of line
2.It is a molecular phenomenon and its root cause is
the electromagnetic forces.
SI unit of surface tension is N/m.
The value of surface tension depends on temperature.
Like viscosity, the surface tension of a liquid usually
falls with temperature.
Applications of surface tension
(a) Manufacture of lead shots
(b) Oils and paints: they spread easily and uniformly.
(c) Destruction of mosquito breeding
Effect of impurity
It is found that when inorganic substances such as sodium
chloride (common salt) are dissolved into water, the surface
tension of water increases. On the other hand, when organic
substances such as soap solution is added to water, the
surface tension of water decreases. Generally, if a highly
soluble substance is added to a liquid, its surface tension
Simple Harmonic Motion
Its SI unit is second.
The reciprocal of T gives the number of repetitions that
occur per unit time. This quantity is called the frequency
of the periodic motion. It is represented by the symbol
The relation between and T is = 1/T
A motion that repeats itself at regular intervals of time is
called periodic motion.
The smallest interval of time after which the motion is
repeated is called its period.
The period is denoted by the symbol T.
14 l Physics
The unit of is thus s–1. It is also known as Hertz
(abbreviated as Hz).
If a body in periodic motion moves back and forth over
the same path, then the motion is said to be vibratory or
Every oscillatory motion is periodic, but every periodic
motion need not be oscillatory.
Circular motion is a periodic motion, but it is not
oscillatory. The motion of the earth around the sun is
periodic but not oscillatory.
There is no significant difference between oscillations
and vibrations. It seems that when the frequency is small,
we call it oscillation (like the oscillation of a branch of a
tree), while when the frequency is high, we call it
vibration (like the vibration of a string of a musical
Simple harmonic motion (SHM) is the simplest form of
oscillatory motion. This motion arises when the force
on the oscillating body is directly proportional to its
displacement from the mean position, which is also the
In SHM, forces acting on the particle is always directed
towards a fixed point known as equilibrium position and
the magnitude of force is directly proportional to the
displacement of the particle from the equilibrium position
and is given by
F = -kx
where k is the force constant, x = displacement of the
particle from the fixed point and the negative sign shows
that force opposes increase in x.
The SI unit of force constant k is N/m and the magnitude
of k depends on elastic properties of the system under
Characteristics of SHM
It is also known as restoring force, which takes the
particle back towards the equilibrium position, and
opposes increase in displacement.
The period of SHM does not depend on amplitude or
energy or the phase constant.
An ideal simple pendulum consists of a heavy point mass
(called bob) tied to one end of a perfectly inextensible, flexible
and weightless string.
It is the maximum displacement of the pendulum from its
Factors on which T of a pendulum depends
The time period of a simple pendulum is given by the
T 2 L . It follows, from this formula, that
The time period of a pendulum depends directly upon
the square root of its length (L), i.e. T L . The
larger the length of a pendulum the more is its time period.
The time period of a pendulum depends inversely upon
the square root of acceleration due to gravity (g), i.e.
. Since g is different at different places,
therefore even for the same length, the time period will
be different at different places.
The time period of a pendulum does not depend upon
the mass of the bob.
The time period of a pendulum also does not depend
upon its amplitude of vibration so long as it remains
If a pendulum clock is brought inside an artificial satellite,
then due to the state of weightlessness inside the satellite
(g = 0), the time period of the clock becomes infinite
( ). That's why such clocks don't work inside artificial
In summer season, the effective length of the pendulum
clock is lengthened (increased length), so its time period
is also increased and consequently the clock becomes
slow. But in winter season the effective length is reduced,
thus the time period is decreased and the clock becomes
On the moon the value of acceleration due to gravity is
g/6, where g is acceleration due to gravity on the earth's
surface. Thus, the period of oscillation of the pendulum
clock is increased on the moon and so it (pendulum
clock) is slowed down.
Physics l 15
Temperature is a relative measure or indication of hotness or
coldness of a body.
Concept of Heat
Energy transfer that takes place solely because of a
temperature difference is called heat flow or heat transfer,
and the energy transferred in this way is called heat.
On the basis of kinetic model of matter, heat energy is
the sum of total kinetic and potential energies of all the
molecules of a given substance.
In other words, heat is the form of energy transferred
between two (or more) systems or a system and its
surroundings by virtue of temperature difference. The SI
unit of heat energy transferred is expressed in Joule (J)
while the SI unit of temperature is Kelvin (K), and °C is a
commonly used unit of temperature.
Calorie: The amount of heat required to raise the
temperature of 1 gm water by 1° C is called a calorie.
By Joule's experiment, it was observed that heat is a
form of energy by which various works can be performed.
Joule also asserted that heat and mechanical work are intertransferable
and the ratio of mechanical work to heat energy
by which work is done is a fixed ratio called mechanical
equivalent of heat and basically it is a conversion unit. If
mechanical work W is produced by amount of heat H, then J
= W/H or W = JH,
where J = mechanical equivalent of heat
= 4186 Joule/kilo cal. = 4.186 Joule/cal
= 4.186 × 107 erg/cal.
A thermometer is a device for measuring the
temperature of a body, often a sealed glass tube that contains
a liquid, such as mercury, that expands and contracts with
rise and fall in temperature.
Types of Thermometer
Thermometers are classified in accordance with the type of
thermometric substance used and on the type of property
which varies with temperature. They are of the following
1. Bimetallic thermometer: A bimetallic thermometer uses
a bimetallic strip made by bonding strips of two different
metals together. When the system gets hotter, one metal
expands more than the other, so the composite strip
bends when the temperature changes. This strip is
usually formed into a spiral, with the outer end anchored
to the thermometer case and the inner end attached to a
pointer. The pointer rotates in responses to temperature
2. Platinum resistance thermometer: In a resistance
thermometer, the changing electrical resistance of a coil
of fine wire, a carbon cylinder, or a germanium crystal is
measured. Because resistance can be measured very
precisely, resistance thermometers are usually more
precise than most other types.
3. Optical pyrometer: To measure very high temperatures,
an optical pyrometer can be used. It measures the
intensity of radiation emitted by a red-hot or white-hot
substance. The instrument does not touch the hot
substance, so the optical pyrometer can be used at
temperatures that would destroy most other
4. Thermoelectric thermometer: The thermoelectric
thermometer works on the principle that the difference
in temperature between two junctions produces a
current. This current can be measured and the
temperature can be estimated.
5. Mercury-in-glass thermometer: It uses mercury and is
based on the volume expansion of mercury. Mercury is
mainly used in thermometers because it does not stick
to the glass surface.
6. Alcohol-in-glass thermometers: It is based on the
expansion of alcohol.
7. Constant volume gas thermometers: It is based on the
concept of increase in pressure of a gas with rise in
The two most commonly used temperature scales are:
1. Celsius scale and 2. Fahrenheit scale
1. Celsius Scale (°C): The Celsius (earlier called
centigrade) was put forward by Anders Celsius in 1742.
Its minimum point (freezing point of pure water at
atmospheric pressure or melting point of pure ice) is at
0°C and its upper end is at 100°C (boiling point of pure
water at atmospheric pressure). It is divided into 100
equal divisions. Each division is called one degree
Celsius. A degree on the Celsius scale is 1/100th of the
Heat and Thermodynamics
16 l Physics
2. Fahrenheit Scale (°F): It was devised by Daniel
Fahrenheit in 1717. Its lower point (freezing point of
pure water at atmospheric pressure) is at 32 °F and the
upper point (boiling point of pure water at atmospheric
pressure) is at 212°F. It is divided into 180 equal
divisions. Each division is called one degree Fahrenheit.
A degree on the Fahrenheit scale is 1/180th of the
Kelvin or Absolute Scale (K): It was devised by Lord
Kelvin. Its lower point (freezing point of water at atmospheric
pressure) is at 273.15 or 273 K and the upper point (boiling
point of water at atmospheric pressure) is at 373 K. It has
Relation between the Celsius and the Fahrenheit scales of
TC 0 F K
TC F K
Thermal equilibrium is a situation in which two objects in
thermal contact with each other cease (stop) to have any net
energy exchange due to a difference in their temperatures.
In other words, temperature can also be defined as a
quantity which determines the direction of flow of heat when
two bodies are placed in contact.
Heat always flows from a higher to a lower temperature.
In SI, temperature is measured in kelvin (K) and in CGS system
in Celsius (°C). The two scales are related as
T K = 273 + t°C
Absolute zero: Theoretically there is no limit to maximum
temperature but there is a limit or restriction on the minimum
temperature. The lowermost temperature is –273.15°C and it
is called absolute temperature.
The temperature on various scales
Temperature Celsius Fahrenheit Kelvin (K)
Freezing of water 0°C 32°F 273 K
Normal temperature 27°C 80.6° F 300 K
of the room
Normal temperature 37°C 98.6°F 310 K
of the human body
Boiling point of 100°C 212°F 373K
Matter (solids, liquids and gases), in general, expands on
heating and contracts on cooling (with the exception of water
that contracts from 0°C to 4°C).
The expansion of a solid with change in temperature is
called thermal expansion. If there is change in the length of
a solid due to expansion it is termed as linear expansion. If
there is a change in the surface area of the solid then the
expansion is called superficial expansion. When volume
undergoes a change in the expansion then it is called cubical
expansion. With a small change in temperature, there is a
large change in the volume of liquids and gases, therefore
liquids and gases have cubical expansion.
Simple examples of the uses of
expansion of solids
1. Iron tyres of horse-carts: Many bullock-carts and horsecarts
(tongas) have large wooden wheels with iron rims.
To fit an iron rim tightly on the wheel, it is made with its
diameter slightly smaller than the diameter of the wheel.
It is then heated until it expands and has its diameter
slightly greater than the diameter of the wheel. The hot
iron rim is then made to slide over the wheel and left to
cool. As iron rim cools, it contracts and fits very tightly.
Steel rims are fitted on railway carriage wheels in a similar
manner. The rim is heated until it slides easily on to the
wheel; when it cools it fits very tightly.
2. The jam bottle and its metal cap: A metal screw cap on a
glass bottle (or on a jam bottle) is sometimes very
difficult to remove. If the metal cap is slightly heated
(carefully turning it around to avoid direct heating of
the glass), the metal will expand more than the glass and
will unscrew very easily.
3. Riveting: A rivet is a metal pin or bolt used for fastening
metal plates. Riveting is the act of using rivets to secure
two overlapping plates very tightly so that the joint
becomes leak-proof. Steel plates, such as those used in
ship-building or in large boilers, are usually riveted
together using red-hot rivets. This provides a good seal
against sea water for ship plates and against water and
steam in large boilers.
4. Fixing a metal wire into glass: Platinum and soda glass
have equal expansion coefficients. This is made use of
in fixing a platinum wire into soda glass. Changes in
temperature will not affect the seal as both materials
expand at approximately the same rate.
5. Railway tracks: The rails are made of steel, which expands
in summer due to rise in atmospheric temperature.
Therefore, a little space is left between the two rails to
accommodate their expansion on the railway track. If no
gap is left the expansion will cause the rails to bend
6. Similarly, in an iron bridge, one end of the girder (beam)
is fixed while the other end is left free and supported by
Physics l 17
rollers. In summer, when the iron girder expands, the
rollers roll forward, and in winter when the girder
contracts, the rollers roll backwards. Thus, there is less
pressure on the girders.
7. Error in metal measuring scales: Measuring tapes and
metal scales are calibrated accurately by the maker at a
particular temperature. A rise or fall in temperature causes
the tape/scale to expand or contract and introduces error
in the measurements made.
8. Telegraph/Telephone/Electric lines: The wires used in
these lines expand or contract as the weather changes
and temperature rises or falls. Therefore, these are kept
slightly sagging in summer so that when they contract
in winter, these do not break.
9. Pendulum of a clock: In summer, the pendulum of a
clock, which is made of metal, expands. Therefore, time
taken for one oscillation increases and the clock loses
time. In winter, the length of the pendulum decreases
and the clock gains time, i.e. it becomes fast.
Anomalous Behaviour of water
While most substances expand when heated, a few do
not. For instance, if water at 0°C is heated, its volume
decreases until the temperature reaches 4°C. Above 4°C water
behaves normally, and its volume increases as the
temperature increases. Because a given mass of water has a
minimum volume at 4°C, the density of water is greatest at
4°C. This behaviour of water is called its anomalous
behaviour or its unusual behaviour.
Consequences of anomalous expansion of water
1. Bursting of water pipe in winter: When water freezes it
expands and exerts a great pressure inside the pipe as a
result of which the pipe bursts. So, enough space is left
in the pipe for the expansion to take place and prevent
2. Other effects: Expansion on freezing causes weathering
of rocks (as water in its crevices freezes) and weathering
of soil. Vegetables and plants get damaged in winter as
the water inside them, expands on freezing and bursts
the cell walls.
2. Aquatic animals are able to survive in frozen ponds:
The temperature of water falls in lakes, ponds etc. till
4°C, at which its density becomes the maximum. Due to
the anomalous expansion of water, its density decreases
when the temperature drops to 0°C. At this temperature
water changes to ice, which floats on the surface. Water
inside the pond does not freeze completely since ice
formed on the top is a bad conductor of heat. Thus
fishes and other aquatic animals can easily survive in
the lower layers of the frozen lakes as the water below
stays at 4°C.
Change of State
Matter is found usually in three states: solid, liquid and gas.
Water exists in the solid state as ice, in the liquid state as
water, and in the gaseous state as steam.
A transition from one state (or phase) to another is called
a change of state (or a phase change or phase transition). For
any given pressure, a phase change takes place at a definite
temperature, usually accompanied by absorption or emission
of heat and a change in volume and density.
A familiar example of a change of state is the melting of ice.
When we slowly heat ice, the temperature of ice does not
increase until all the ice is melted. The effect of adding heat to
ice is not to raise its temperature but to change its phase from
solid to liquid. Thus, the two states of a substance coexist in
thermal equilibrium during a phase transition.
A change of state from solid to liquid is called melting and
from liquid to solid is called freezing.
Melting Point: “The temperature at which a solid changes to
liquid state without any change in its temperature, is called
its melting point”.
Heat is absorbed from the surroundings in melting.
Melting point depends on pressure. The melting point
at standard atmospheric pressure of 76 cm of mercury is
called normal melting point.
Normal melting point of ice is 0°C.
The melting point of substances which contract on
melting like ice, cast iron, antimony, bismuth, brass etc.
decreases with increase in pressure.
Due to absorption of heat, melting of ice increases the
internal energy without change in temperature. Due to
removal of heat, the internal energy decreases in freezing.
The melting point of other substances which expand on
melting like wax, glass, gold, silver, copper etc. increases
with increase in pressure.
Melting point decreases on adding impurity.
At the melting (or freezing) temperature, the liquid and
solid phases can coexist in a condition called phase
equilibrium, hence water at 0°C cannot be converted
into ice by adding any amount of ice at 0°C.
Cooling a liquid below freezing point without turning it
into solid is called supercooling. Water can be
supercooled up to -12°C.
Vaporisation and Condensation
The phase transition from liquid or solid to gas (or vapour) is
vaporisation and from gas to liquid is called condensation.
Vaporisation has three types: boiling, evaporation and
Boiling Point: It is the transition from liquid phase to gas
phase which takes place at or above the boiling temperature
and it occurs below the surface.
We have similar concepts (as of melting and melting point)
18 l Physics
for vaporisation and boiling as well.
1. At a given pressure, the temperature of boiling and
condensation are always the same. At this temperature,
the liquid and gaseous phases can coexist in phase
2. The boiling point always increases with increase in
pressure. Hence, the cooking in a pressure cooker is
faster but cooling becomes difficult on hills.
3. Normal boiling point of water is 100°C. It decreases by
1°C for a decrease in pressure by nearly 26.8 mm of
mercury, i.e., with an increase in elevation above the
earth’s surface by 300 metres.
4. Water can be made to boil at 0°C if the surrounding
pressure is 4.6 mm of Hg.
5. Impurity increases the boiling point.
6. The internal energy of a body in vapour phase is greater
than that in liquid phase.
Sublimation: There are some substances which normally pass
from the solid state to the vapour state directly and vice
versa. The process of changing from solid state to vapour
state without passing through the liquid state is called
“sublimation” and the solid is said to sublime.
The phenomenon of change of liquid into vapour at any
temperature below its boiling point is called evaporation.
Factors Affecting Evaporation
You must have observed that the rate of evaporation
An increase of surface area: If the surface area is
increased, the rate of evaporation increases. For example,
while putting clothes for drying up we spread them out.
An increase of temperature: With the increase of
temperature, more number of particles get enough kinetic
energy to go into the vapour state.
A decrease in humidity: Humidity is the amount of water
vapour present in the air. If the amount of water in air is
low, the rate of evaporation increases.
An increase in wind speed: It is a common observation
that clothes dry faster on a windy day. With the increase
in wind speed, the particles of water vapour move away
with the wind, decreasing the amount of water vapour in
Latent heat of a substance is defined as the amount of heat
absorbed or given out by a unit mass of it during the change
of state while its temperature remains constant. It is of two
(i) Latent heat of fusion
(ii) Latent heat of vaporisation
Latent heat of fusion: The latent heat of fusion of a substance
is the quantity of heat absorbed by (or removed from) the
substance to change a unit mass of it from solid state to
liquid state (or from liquid state to solid state), while
temperature remains constant. It is also called heat effusion.
Latent heat of vaporisation: The latent heat of vaporisation
of a substance is the quantity of heat absorbed by (or removed
from) the substance to change a unit mass of it from liquid
state to gaseous state (or from gaseous state to liquid state),
while temperature remains constant. It is also called heat of
Specific Latent Heat: The amount of heat required to change
the phase of 1 kg of the substance at a constant temperature.
Q = latent heat absorbed or released by the substance
m = mass of the substance.
SI unit: J kg-1
Applications of Specific Latent Heat
Drinks can be cooled by adding in several cubes of ice.
When ice melts a large amount of heat is absorbed and
this lowers the temperature of the drink.
The freshness of fish and meat can be maintained by
placing them in contact with ice. With its larger latent
heat, ice is able to absorb a large quantity of heat from
the fish as its melts. Thus, food can be kept at a low
temperature for an extended period of time.
Water has a large specific latent heat of vaporisation.
This property enables steam to be used for cooking by
the method of steaming. When steam condenses on the
food, the latent heat is released directly onto the food
and enables the food to be cooked at a faster rate.
Our bodies feel cool after sweating. This is because
latent heat of vaporization is absorbed from the body
when sweat evaporates. As a result, the body is cooled
by the removal of heat.
Specific Heat Capacity (c): The amount of heat that
must be supplied to increase the temperature by 1°C for a
mass of 1 kg of a substance is called its Specific Heat Capacity.
Specific heat capacity (c) = m
Q = heat absorbed/released, unit J
m = mass of the substance, unit kg
= temperature difference, unit °C
SI unit: J kg–1 °C-1
Note: A substance with a small value of specific heat capacity
1. heats up and cools at a faster rate. For example, metals
like iron, steel, copper and aluminium are used for pots
and pans because they can be quickly heated up when
there is only small heat absorption.
Physics l 19
2. is sensitive to temperature changes. A thermometer has
low specific heat capacities, so it enables heat to be
easily absorbed and released even when small quantities
of heat are involved.
Heat is energy transfer from one system to another or from
one part of a system to another, arising due to temperature
There are three distinct modes of heat transfer:
2. Convection and
The process by which heat is transferred from the hotter end
to the colder end of an object is known as conduction. In
other words, conduction is the mechanism of transfer of
heat between two adjacent parts of a body because of their
temperature difference. In conduction, heat is transferred
between neighbouring parts of a body through molecular
collisions, without any flow of matter. If one end of a metallic
rod is put in a flame, the other end of the rod will soon be so
hot that we cannot hold it with our bare hands.
Gases are poor thermal conductors while liquids have
conductivities intermediate between solids and gases.
The materials which allow heat to pass through them easily
are good conductors of heat. Metals like aluminum, iron and
copper, human body, acidic water are the examples of good
The materials which do not allow heat to pass through them
easily are bad conductors of heat. Poor conductors are known
as insulators. Examples: water, air, wood, fibre, glass, rubber
The substances through which heat is not transmitted by
any means (method) are called thermal insulators. In fact
bad conductors are sometimes synonymously used as
thermal insulators. Examples: Ebonite, asbestos etc.
Houses made of concrete roofs get very hot during
summer days, because thermal conductivity of concrete
(though much smaller than that of a metal) is still not small
enough. Therefore, people usually prefer to give a layer of
earth or foam insulation on the ceiling so that heat transfer is
obstructed and it keeps the room cooler.
Thermal conductivities of some materials
Materials Thermal conductivity
(J s–1 m-1 K-1)
Insulating brick 0.15
Body fat 0.20
Gases Air 0.024
Application of conduction
Drinking tea in a metallic cup is somewhat painful but
drinking tea in a ceramic cup is pleasant. The simple
reason behind it is that heat from the tea goes into the
metallic cup, which becomes hot and one’s lips have a
painful and bitter experience due to good conduction of
heat. But due to bad conduction of heat in ceramic or
fibre cup, heat doesn't travel from tea to the cups and
does not make the cup hot.
Refrigerators and ice-boxes have similar double walls to
minimise heat gain by conduction.
Eskimos live in snow huts called igloos. Snow, being a
poor conductor, shields them from cold. It prevents the
heat they generate from escaping and keeps them warm.
Convection is a mode of heat transfer by actual motion of
matter. It is possible only in fluids (liquids and gas).
Convection can be natural or forced.
In forced convection, material is forced to move by a
pump or by some other physical means.
Examples of forced convection systems – air heating
systems in home, the human circulatory system, and the
cooling system of an automobile engine. In the human body,
the heart acts as the pump that circulates blood through
different parts of the body, transferring heat by forced
convection and maintaining it at a uniform temperature.
Natural convection is responsible for many familiar
20 l Physics
phenomena. During the day, the ground heats up more quickly
than large bodies of water do. This occurs both because the
water has a greater specific heat and because mixing currents
disperse the absorbed heat throughout the great volume of
water. The air in contact with the warm ground is heated by
conduction. It expands, becoming less dense than the
surrounding cooler air. As a result, the warm air rises (air
currents) and other air moves (winds) to fill the space,
creating a sea breeze near a large body of water. Cooler air
descends, and a thermal convection cycle is set up, which
transfers heat away from the land. At night, the ground loses
its heat more quickly, and the water surface is warmer than
the land. As a result, the cycle is reversed.
Another example of natural convection is the steady
surface wind on the earth blowing in from north-east towards
the equator, the so called trade wind.
Heating elements in geysers and water heaters are fitted
near the bottom so that water can be heated by convection
currents. Heating elements in electric ovens are fitted near
the bottom to heat the entire enclosed air by convection.
The cooling unit (freezer) in a refrigerator is fitted near
the top to cool the whole of the interior. The air near the top
cools and descends due to increased density. Its place near
the top is taken by warm air and in this way convection
currents are set up, which cool the entire interior.
To avoid the burning of the filaments of the electric
bulbs, which are made of tungsten (high atomic weight and
high melting point), the bulb is evacuated (creation of
vacuum). Also, to avoid the melting of filaments in the bulb,
some inert gases like argon or krypton are filled up and thus
thermal radiations generated by filaments form a convectional
current inside the bulb and a tremendous quantity of thermal
energy produced doesn't melt the filament.
The third mechanism for heat transfer needs no medium; it is
called radiation and the energy so radiated by
electromagnetic waves is called radiant energy.
Electromagnetic waves can travel in vacuum with the same
speed as the speed of light, i.e. 3 × 108 ms–1.
Heat transfer by radiation does not need any medium
and it is very fast.
Examples of radiation
The heat is transferred to the earth from the sun through
the empty space.
When we sit in front of a room heater, we get heat by
A hot utensil kept away from the flame cools down as it
transfers heat to the surroundings by radiation.
Our body, too, gives heat to the surroundings and
receives heat from it by radiation.
All bodies emit radiant energy, whether they are solids,
liquids or gases. The electromagnetic radiation emitted by a
body by virtue of its temperature, like the radiation by a red
hot iron or light from a filament lamp, is called thermal
When this thermal radiation falls on other bodies, it is
partly reflected and partly absorbed. The amount of heat
that a body can absorb by radiation depends on the colour
of the body. Black bodies absorb and emit radiant energy
better than bodies of lighter colours. Note the following
We wear white or light-coloured clothes in summer so
that they absorb the least heat from the sun. However,
during winter, we use dark-coloured clothes, which
absorb heat from the sun and keep our body warm.
The bottoms of the utensils for cooking food are
blackened so that they absorb maximum heat from the
fire and give it to the vegetables to be cooked.
The base of an electric iron is highly polished so that it
does not lose heat by radiation.
A cloudy night in winter is warmer than a night with
In a desert, days are too hot and nights too cold.
Note: Convection and conduction are not possible in vacuum
while radiation is possible in vaccum.
Newton's Law of Cooling: This law states that the rate
at which a hot body loses heat is directly proportional to the
difference between its temperature and the surrounding
temperature. For example, hot water takes much less time in
cooling from 90°C to 80°C than in cooling from 40°C to 30°C.
If hot water and fresh tap water are kept in a refrigerator,
the rate of cooling of hot water will be faster than that of tap
water. Suppose a person is served hot coffee with added
cream (at room temperature), but he wants to drink it after a
while. It is then advisable to add cream right in the beginning
rather than at the time of taking the coffee because this way
the coffee will remain hotter.
It is the branch of natural science concerned with heat and
its relation to other forms of energy and work. It defines
macroscopic variables (such as temperature, internal energy,
entropy, and pressure) that describe average properties of
material bodies and radiation, and explains how they are
related and by what laws they change with time.
Thermodynamics does not describe the microscopic
constituents of matter, and its laws can be derived from
Thermodynamics can be applied to a wide variety of topics
in science and engineering, such as engines, phase
transitions, chemical reactions, transport phenomena, and
even black holes.
Physics l 21
Much of the empirical content of thermodynamics is
contained in its four laws:
The Zeroth Law of Thermodynamics is a generalization
principle of thermal equilibrium among bodies, or
thermodynamic systems, in contact. It states: If two
systems are each in thermal equilibrium with the third,
they are also in thermal equilibrium with each other.
The First Law of Thermodynamics is simply the law of
conservation of energy applied to the thermodynamic
system. According to this law, if a substantial amount of
thermal energy is supplied to a thermodynamic system,
then it is partially utilised in changing its internal energy.
U = Q – W
Q = Thermal energy supplied
U = Change in internal energy
W = External work done
The Second Law of Thermodynamics concerns a
quantity called entropy that expresses limitations, arising
from what is known as irreversibility, on the amount of
thermodynamic work that can be delivered to an external
system by a thermodynamic process. It states: Heat
cannot spontaneously flow from a colder location to a
The Third Law of Thermodynamics is a statistical law of
nature regarding entropy and the impossibility of
reaching absolute zero of temperature. This law provides
an absolute reference point for the determination of
Note: 1. Absolute zero is –273.15 °C (Degrees Celsius), or
–459.67 °F (Degrees Fahrenheit) or 0 K (kelvin).
2. Entropy is a thermodynamic property that is the
measure of a system's thermal energy per unit
temperature that is unavailable for doing useful
Several commonly studied thermodynamic processes are:
Isobaric process: It occurs at constant pressure.
Isochoric process: It occurs at constant volume (also
Isothermal process: It occurs at a constant temperature.
Adiabatic process: It occurs without loss or gain of
energy as heat.
When a motor tyre bursts, the sudden expansion of air
into the atmosphere is an adiabatic process in which the
tyre is cooled down.
To prepare dry ice (solid CO2), carbon dioxide is
suddenly expanded and consequently it converts into ice
called dry ice.
On shaking, a thermos containing tea becomes warm,
because on shaking the existing viscous forces between
various layers of the tea do external work and this work is
transformed into thermal energy. So its internal energy
increases and consequently the temperature rises.
Isentropic process: A reversible adiabatic process that
occurs at a constant entropy, but is a fictional
Isolated process: It occurs at constant internal energy
and elementary chemical composition.
The elastic behaviour of materials plays an important
role in engineering design. For example, while designing a
building, knowledge of elastic properties of materials like
steel, concrete etc. is essential. The same is true in the design
of bridges, automobiles, ropeways etc.
Note: Steel is more elastic than rubber, copper, brass
and aluminium. It is for this reason that steel is preferred in
heavy-duty machine and in structural designs.
By the process of hammering or rolling, the elasticity of
a body increases.
By the process of annealing, the elasticity of a body is
Stress and Strain
When a force is applied on a body, it is deformed to a small
or large extent, depending upon the nature of the material of
the body and the magnitude of the deforming force.
A rigid body generally means a hard solid object having a
definite shape and size. But in reality, bodies can be stretched,
compressed and bent. Even the appreciably rigid steel bar
can be deformed when a sufficiently large external force is
applied on it. This means that solid bodies are not perfectly
rigid. A solid has definite shape and size. In order to change
(or deform) the shape or size of a body, a force is required.
The property of a body, by virtue of which it tends to regain
its original size and shape when the applied force is removed,
is known as elasticity, and the deformation caused is known
as elastic deformation. Examples: steel, rubber and quartz.
However, if we apply force to a lump of putty or mud, it
has no gross tendency to regain its previous shape, and it
gets permanently deformed. Such substances are called
plastic and this property is called plasticity. Examples: putty,
clay, mud, wax, lead and chewing gum. Putty and mud are
close to ideal plastics.
22 l Physics
When a body is subjected to a deforming force, a
restoring force is developed in the body. This restoring force
is equal in magnitude but opposite in direction to the applied
force. The restoring force per unit area is known as stress.
If F is the force applied and A is the area of cross-section of
the body, magnitude of the stress = F/A
The SI unit of stress is Nm–2 or Pascal (Pa) and its
dimensional formula is [ML–1 T–2 ].
When a body suffers a change in its size or shape under the
action of external forces, it is said to be deformed and the
corresponding fractional change is called strain. The strain
is a ratio and it has no unit, no dimension.
There are three types of strain — longitudinal (linear)
strain, volume strain and shearing (shape) strain.
For small deformations, stress and strain are proportional to
each other. This is known as Hooke’s law.
Thus, stress strain
or, stress = × strain
where is the proportionality constant and is known as
modulus of elasticity.
Hooke’s law is an empirical law and is found to be valid for
The ratio of stress to strain, called modulus of elasticity, is
found to be a characteristic of the material. There are three
types of elasticity:
1. Young’s Modulus 2. Shear Modulus 3. Bulk Modulus
The ratio of longitudinal stress () to longitudinal strain (L
is defined as Young’s modulus and is denoted by the symbol
Y = (F/A)/(L/L)
= (F × L) /(A × L)
Since strain is a dimensionless quantity, the unit of
Young’s modulus is the same as that of stress, i.e. Nm–2 or
The ratio of shearing stress to the corresponding shearing
strain is called the shear modulus of the material and is
represented by G. It is also called the modulus of rigidity.
The SI unit of shear modulus is N m–2 or Pa. It can be
seen that shear modulus (or modulus of rigidity) is generally
less than Young’s modulus. For most materials G = Y/3.
When a uniform pressure is applied all over the surface of a
body, the volume of the body changes. The change in volume
per unit volume of the body is called volume strain and the
applied pressure is called normal stress. Thus the ratio of
normal stress to volume strain is called bulk modulus of the
material of the body.
B = – p/(V/V)
Where p = normal stress v
= volume strain
The negative sign indicates the fact that with an increase
in pressure, a decrease in volume occurs. That is, if p is
positive, V is negative.
Thus for a system in equilibrium, the value of bulk
modulus B is always positive. The SI unit of bulk modulus is
the same as that of pressure, i.e. Nm–2 or Pa.
The reciprocal of bulk modulus is called compressibility
and is denoted by k. It is defined as the fractional change in
volume per unit increase in pressure.
k = (1/B) = – (1/p) × (V/V)
Note: 1. The bulk moduli for solids are much larger than for
liquids, which are again much larger than the bulk
modulus for gases (air). Thus solids are least
compressible whereas gases are most compressible.
Gases are about a million times more compressible
than solids. Gases have large compressibility, which
vary with pressure and temperature.
2. The Young’s modulus and Shear modulus are
relevant only for solids since only solids have
lengths and shapes.
due to gravity. It is denoted by g. It is independent of the
mass of the body. It varies with altitude and depth.
The value of acceleration due to gravity is taken as 9.8
ms-2 at 45° latitude at the sea level.
Weight: It is defined as the force with which a body is
attracted towards the centre of the earth.
Gravitation: It is the force of attraction between any two
bodies in the universe.
Gravity: It is the force of attraction between a planet
and a body.
Acceleration due to gravity: The acceleration produced
in a body due to the force of gravity is called acceleration
Physics l 23
Newton's Law of Gravitation
The law of gravitation states that the force of attraction
between any two objects is proportional to the product of
their masses and inversely proportional to the square of the
distance between them. The law applies to objects anywhere
in the universe. Such a law is said to be universal.
Consider two particles of masses m1 and m2
by a distance r, then by Newton's law of gravitation we have
F m1m2 .... (1)
Combining equations 1 and 2, we have
or, F = 2
Here G is a constant called the universal gravitational
constant. In SI it has a value 6.67 × 10–11 Nm2 kg–2, which is a
small value. The above expression is valued only for bodies
having spherical shape. The gravitational unit of force in SI
is kilogram weight (kgwt). 1 kgwt = 9.8 N
Acceleration due to Gravity at
the Surface of the Earth
Suppose a body of mass m is placed on the surface of the
earth. Let the mass of the earth be M and its radius be R.
Since the radius of the body is very small, the centre-tocentre
distance between the bodies is R. Now, by Newton's
gravitational law, the force of attraction between these two
F = 2 R
Now, the weight of a body is defined as the force with
which a body is attracted to the centre of the earth. Therefore,
the Weight (W) of the body is given by
W = mg ... (2)
Therefore, from equations 1 and 2, we have
mg = 2 R
or g = 2 R
If the body lies at a distance r from the centre of the
earth then the acceleration due to gravity is given by the
expression g = 2 r
The acceleration due to gravity on the surface of the
earth is given by 2 R
g GM .
It should be noted that the acceleration due to gravity
1. decreases with height from the surface of the earth.
2. decreases with depth from the surface of the earth.
3. is a maximum at the surface of the earth.
4. is more at the poles than at the equator.
Difference between mass and weight
It is the amount of It is a force equal to the
matter contained in gravitational pull exerted
a body. by a planet.
It is a constant quantity It is a variable quantity and
and does not change changes with the change in
with respect to position acceleration due to gravity at
or place. a place.
Mass of a body can Weight of a body can be zero
never be zero. during free fall.
It is measured by using It is measured by using a
a physical balance. spring balance.
It is measured in It is measured in Newton.
Applications of Newton’s
Law of Gravitation
Newton's law of gravitation has a large number of
applications. Some of these are:
1. It can be used to estimate the mass of the earth by using
the expression 2 R
g GM .
2. It can also be used to estimate the mass of the moon,
the sun and other planets.
3. It is used to study the binary stars — a system of two
stars orbiting around their common centre, the
centripetal force being provided by the gravitational
force of attraction between them. Any distortion in the
orbit of a star indicates the presence of a companion
such that the system moves as a double star. Newton's
law of gravitation can be used to estimate the masses of
4. The wobbling (irregularity in the motion) of a star can
be detected by modern techniques. By using the law of
gravitation we can estimate the mass of these stars.
Weight of an Object on the Moon: The weight of an object
on the surface of the moon would be the magnitude of mass
in kilograms multiplied by 1.67 N.
24 l Physics
and the orbital velocity of the body around the earth
Thus, if the velocity of an orbiting satellite close to the
earth is increased 2 times or 41%, then the satellite will
leave the orbit and will escape. In other words, if the kinetic
energy of an orbiting satellite is doubled immediately, then
the satellite will escape. The escape velocity is maximum on
the sun and it is 614 km/sec. That's why even lighter gases
like hydrogen and helium do not escape. The moon has the
least volume of escape velocity, i.e. 2.4 km/sec. So, no gases
exist on the moon. That's why the moon has no atmosphere
and, from the surface of the moon, the sky appears black.
The escape velocity of the gaseous molecules must be more
than the root mean square (rms) velocity for the existence of
Note: An astronaut experiences weightlessness in a space
satellite. This is not because the gravitational force is
small at that location in space. It is because both the
astronauts and the satellites are in “free fall” towards
The escape velocity is the minimum required velocity of a
body through which it is projected. It goes beyond the
gravitational pull and never comes back.
If m be the mass of a body projected from the earth's
surface of radius Re and mass Me, the kinetic energy of the
body must be equal to the gravitational potential energy.
or, ve =
where ve = escape velocity.
ve = escape velocity = 2gRe 11.2 km / sec
This implies that if a body is thrown from the earth's
surface with the velocity of 11.2 km/sec, then the body will
never come back to the earth.
Here we observe that escape velocity of a body on the
earth's surface is
or (transmits) along the perpendicular direction of the particle's
vibration, it is called transverse mechanical wave.
Transverse mechanical waves can be generated in solids
and upper surfaces of the liquids. But it cannot be generated
through gases and inside liquids due to lack of rigidity. The
transverse wave propagates in the form of crest (max. upward
displacement) and trough (max. downward displacement).
The distance between two adjacent crests or trough is called
wavelength and it is represented by .
Longitudinal mechanical wave: If, in an elastic medium,
a wave propagates (transmits) along the direction of particle's
vibration, it is called longitudinal mechanical wave.
Longitudinal waves can be generated (produced) in all
mediums — solids, liquids and gases — and such waves
transmit through compression and rarefaction. In
compression, the pressure and the density of the medium is
maximum, while in rarefaction, the pressure and the density
of the medium is minimum. Examples — Sound waves in air,
earthquake waves, water waves etc. are longitudinal
Waves can occur whenever a system is disturbed from its
equilibrium position and when the disturbance can travel or
propagate from one region of the system to the other. Sound,
light, ocean waves, radio and television transmission, and
earthquakes are all wave phenomena.
The waves are mainly of three types: (a) mechanical
waves, (b) electromagnetic waves and (c) matter waves.
Mechanical wave: Mechanical waves require a material
medium for their propagation, i.e. they cannot travel in
vacuum. Example: Water waves, sound waves, seismic waves,
Types of mechanical wave:
(a) Transverse mechanical wave
(b) Longitudinal mechanical wave
Transverse mechanical wave: If in an elastic medium
Physics l 25
The orderly distribution of electromagnetic radiations
according to their frequency or wavelength is termed as
For an electromagnetic wave, the frequency, wavelength
and the velocity of the wave are related by the
expression; c = v.
An electromagnetic wave is a wave generated by the
oscillations of electric and magnetic fields oscillating
mutually perpendicular to each other. Electromagnetic
waves do not require a medium to propagate.
Types of Electromagnetic Waves
The various types of electromagnetic waves (in the order of increasing wavelength) are:
Name Wavelength Discoverer Origin
Gamma rays 1 × 10-13 1 × 10-10 Becquerel and Curie Nuclei of atoms
X-rays 1 × 10-10 3 × 10-8 Roentgen Bombardment of high Z targets
Ultraviolet rays 3 × 10-8 4 × 10-7 Ritter Excitation of atoms
Visible light 4 × 10-7 8 × 10-7 Newton Excitation of atoms
Infrared rays 8 × 10-7 3 × 10-5 Herschel Heating
Microwaves 1 × 10-3 3 × 10-1 Hertz Oscillating circuits
Ultra High Frequency (UHF) 1 × 10-1 1 Marconi Oscillating circuits
Very High Frequency (VHF) 1 10 Marconi Oscillating circuits
Radio Frequency (RF) 10 104 Marconi Oscillating circuits
Power Frequency (PF) 5 × 106 6 × 106 Marconi Weak radiations from ac circuits
Uses of Electromagnetic Waves
The study of gamma rays provides valuable information
about the structure of the atomic nucleus.
X-rays are used as a diagnostic tool in medicine and
used in the study of crystal structure.
UV radiations are used for sterilisation, to check the
purity of gems, eggs, etc., and to check counterfeit
UV radiations are the cause of sun tanning and skin
Infrared radiations are used in physical therapy, infrared
photography, and vibrational spectroscopy, in warfare,
for looking through haze, in greenhouses and in the
electronic industry as infrared remote control.
Microwaves are used in communication, navigation,
studying atomic and molecular structure, and also in
Radio waves are used in radio and TV communication.
Properties of Electromagnetic waves
The range of wavelength of the visible part of the spectrum
is as below:
Violet/Indigo 400 to 440 nm
Blue 440 to 480 nm
Green 480 to 560 nm
Yellow 560 to 590 nm
Orange 590 to 630 nm
Red 630 to 790 nm
The following properties are common to all
(i) They are produced by accelerated charges.
(ii) They do not require any material medium to travel.
(iii) An electromagnetic wave consists of an electric and
a magnetic field which are mutually perpendicular
and also perpendicular to the direction of motion.
(iv)Both the electric and magnetic field vectors vary
both with respect to time and space.
(v) They are transverse in nature.
(vi)They exhibit the phenomenon of reflection,
refraction, polarisation and diffraction.
(vii) They are unaffected by electric and magnetic fields.
(viii) All the components of the electromagnetic spectrum
have a velocity of 3 × 108 ms-1 in vacuum. However
their velocity decreases in other medium.
Thus all electromagnetic waves travel through vacuum
at the same speed c, given by c = 3 × 108 ms–1 (speed of light).
Unlike the mechanical waves,the electromagnetic waves do
not require any medium for their propagation.
Note: Cathode rays, sound waves, alpha, beta and ultrasonic
waves are not electromagnetic waves.
Matter wave: Matter waves are associated with moving
electrons, protons, neutrons and other fundamental particles,
and even atoms and molecules. Because we commonly think
of these as constituting matter, such waves are called matter
waves. Matter waves are conceptually more abstract than
mechanical or electromagnetic waves.
26 l Physics
Sound and its production
Sound is a form of energy which excites in our ears the
sensation of hearing. It is a longitudinal wave that is created
by a vibrating object, such a guitar string, the human vocal
cords, or the diaphragm of a loudspeaker.
Sound needs a medium to travel
Sound is a mechanical wave and needs a material medium
like air, water, steel etc. for its propagation. It cannot travel
On the moon, there is vacuum, i.e. there is no air,
therefore sound cannot propagate on the moon. Thus the
astronauts cannot hear each other.
Terms related to sound wave
Air is the most common medium through which sound travels.
When a vibrating object moves forward, it pushes and
compresses the air in front of it, creating a region of high
pressure. This region is called a compression (C). This
compression starts to move away from the vibrating object.
When the vibrating object moves backwards, it creates
a region of low pressure called rarefaction (R). As the object
moves back and forth rapidly, a series of compressions and
rarefactions is created in the air. These create sound wave
that propagates through the medium.
Compression is the region of high pressure and
rarefaction is the region of low pressure. Pressure is related
to the number of particles of a medium in a given volume.
More density of the particles in the medium gives more
pressure and vice versa. Thus, propagation of sound can be
visualised as propagation of density variations or pressure
variations in the medium.
Relation between velocity,
wavelength and frequency
Frequency (): It is defined as the number of waves per
Wavelength (): It is defined as the distance travelled
by a wave when the particles of a medium complete one
vibration. It is also defined as the distance between two
consecutive crests or troughs or two consecutive
compressions or rarefactions. The particle takes time T equal
to the time period to complete one vibration. In this time, the
wave travels a distance . Let v be the velocity of the wave,
time taken T
V Dis tan ce travelled by the wave
Thus velocity = frequency × wavelength
The frequency range of a sound wave
Sound is produced by a vibrating body. But not all vibrating
bodies produce sound which we can hear.
A sound with a single frequency is called a pure tone.
A healthy young person hears all sound frequencies in the
range from approximately 20 to 20,000 Hz (20 kHz). This range
of frequency is called the audible range. The ability to hear
high frequencies decreases with age, however, and a normal
middle-aged adult hears frequencies only up to 12-14 kHz.
Sound waves whose frequency lie above 20 kHz are
called ultrasonic waves while those with a frequency below
20 Hz are called infrasonic waves. Infrasonic waves are
produced by a vibrating pendulum, whales, elephants and
It is observed that some animals get disturbed before
earthquakes. Earthquakes produce low-frequency infrasound
before the main shock waves begin. It is possibly this that
alerts the animals. These waves are extensively used for
Ultrasound is produced by dolphins, bats and porpoises.
Rats also play games by producing ultrasound.
Applications of Ultrasound
Ultrasounds are high-frequency waves. They are able
to travel along well-defined paths even in the presence
of obstacles. They are used extensively in industries
and for medical purposes.
Ultrasound is generally used to clean parts located in
hard-to-reach places; for example, spiral tube, oddshaped
parts, electronic components etc.
Objects to be cleaned are placed in a cleaning solution
and ultrasonic waves are sent into the solution. Due to
the high frequency, the particles of dust, grease and dirt
get detached and drop out. The objects thus get
Ultrasounds can be used to detect cracks and flaws in
Physics l 27
Metallic components are generally used in construction
of big structures like buildings, bridges, machines and
also scientific equipment. The cracks or holes inside
the metal blocks, which are invisible from outside,
reduces the strength of the structure.
Ultrasonic waves are allowed to pass through metal
blocks and detectors are used to detect the transmitted
waves. If there is even a small defect, the ultrasound
gets reflected back, indicating the presence of the flaw
Ultrasonic waves are made to reflect from various parts
of the heart and form the image of the heart. This
technique is called echocardiography.
An ultrasound scanner is an instrument which uses
ultrasonic waves for getting images of internal organs
of the human body. It helps the doctor detect
abnormalities such as stones in the gall bladder and
kidney, or tumours in different organs. These waves are
converted into electrical signals that are used to generate
images of the organ. These images are then displayed
on a monitor or printed on a film. This technique is called
Ultrasound may be employed to break small ‘stones’
formed in the kidneys into fine grains. These grains
later get flushed out with urine.
The acronym SONAR stands for Sound Navigation and
Ranging. Sonar is a device that uses ultrasonic waves to
measure the distance, direction and speed of underwater
Sonic boom: When the speed of any object exceeds the
speed of sound, it is said to be travelling at supersonic speed.
Bullets, jet aircraft, etc. often travel at supersonic speeds.
When a sound-producing source moves with a speed higher
than that of sound, it produces shock waves in air. These
shock waves carry a large amount of energy. The air pressure
variation associated with this type of shock waves produces
a very sharp and loud sound called sonic boom. The shock
waves produced by a supersonic aircraft have enough energy
to shatter glass and even damage buildings.
Frequency is an objective property of a sound wave,
because frequency can be measured with an electronic
Speed of Sound
The speed of sound in a medium depends upon
1. the elasticity of the medium (E) and
2. the density of the medium ()
According to Newton, the velocity of sound in an elastic
medium is given by
Newton assumed that the propagation of sound waves
through air (a gas) takes place at a constant temperature, i.e.
it is an isothermal process. For such a process, the elasticity
is equal to the pressure (P) of air/gas. Therefore, the speed
of sound, according to Newton, is
This formula gives the value of the speed of sound at
0°C as 280 ms–1, which is much lower than the experimental
value.This discrepancy was overcome by Laplace. According
to Laplace, the propagation of sound through air is an
adiabatic process (no exchange of heat) and not an isothermal
process. In this process we have
E = P
where is the ratio of the specific heat at constant
pressure to the specific heat at constant volume of the
Hence, according to Laplace, the speed of sound in air
is given by
For air, 1.4
Sound travels through gases, liquids and solids at
considerably different speeds as revealed. At room
temperature, the speed of sound in air is about 343 ms–1 and
is markedly greater in liquids and solids. For example, sound
travels more than four times faster in water and more than
seven times faster in steel than it does in air. In general,
sound travels slowest in gases, faster in liquids and fastest
The speed of sound depends on the nature of the
medium. In air, the speed of sound depends upon its
temperature, its density and its humidity.
Factors on which the velocity of sound depends
The velocity of sound depends upon the following factors:
(i) Pressure (ii) Density (iii) Temperature (iv) Humidity (v)
Velocity of wind.
Effect of pressure: Pressure does not affect the velocity
of sound because an increase in pressure increases the
density and a decrease in pressure decreases the density of
Effect of density: Velocity of sound varies inversely as
square root of density.
28 l Physics
Ex: Velocity of sound is more in H2 than in O2 as the
density of oxygen is greater than the density of hydrogen.
Effect of temperature: The velocity of sound in air
depends upon temperature as v T , where T is the
absolute temperature. Therefore, if other factors remain the
same, the velocity of sound in air increases with an increase
in the temperature of the air.
Effect of humidity: The velocity of sound increases with
humidity. This shows that the velocity of sound in moist air
is greater than the velocity of sound in dry air. As a result,
the velocity of sound is more during the night than during
the day. Also, it is more during the rainy season than during
the dry season.
Speeds of sound in various media
When sound travels in a denser medium its velocity
increases. Therefore, velocity of sound is more in solids
than in liquids or gases.
Medium Speed of sound in
m/s at 0°C
Vapour (100°) 405
Sea water 1533
An echo is the phenomenon of repetition of sound of a
source by reflection from an obstacle reaching ear drum after
0.1s. Let d be the minimum distance of a reflector from the
source, v the velocity of sound in air at room temperature,
and t the total time taken by sound to reach the listener after
Now, total distance travelled (2d)
= velocity (v) × total time (t)
Therefore, we have
Speed of sound
t Total distance travelled
For simple sounds, the condition of echo is that the
reflecting surface should be at a minium distance of 17m
from the source of sound. The sensation of sound persists
in our ear for th
of a second, so to hear an echo, the
sound after reflection should fall on the eardrum after th
of a sound.
For articulate sounds, the condition of echo is that the
reflecting surface should be at a minimum distance of 34m
from the source of sound.
The phenomenon of echo is utilised by bats and
dolphins. The phenomenon of echo is used in fishing boats
and in SONAR and RADAR and in ultrasonography.
Ultrasonography is the technique in which the ultrasonic
waves are used for taking the photograph of uterus, gall
bladder, liver etc. Using ultrasonic waves, a detailed
photograph of human heart can also be taken. This is called
Pitch (Shrillness) and Frequency
Pitch is that characteristic of sound by which an acute or
shrill note can be distinguished from a flat or grave note.
The frequency of note produced by a string in stringed
instruments can be changed by changing the place of
plucking or by increasing the tension on the string or by
using the string of less or more thickness. The pitch of a
woman's voice is higher than that of a man.
The pitch of sound depends on (i) frequency and (ii)
relative motion between source and listener. Pitch is a
Reflection of Sound
Sound bounces off a solid or a liquid like a rubber ball bounces
off a wall. Like light, sound gets reflected at the surface of a
solid or liquid and follows the same laws of reflection. The
directions in which the sound is incident and is reflected
make equal angles with the normal to the reflecting surface,
and the three are in the same plane. An obstacle of large size
which may be polished or rough is needed for the reflection
of sound waves. The rolling of thunder is due to the
successive reflections of sound from a number of reflecting
surfaces, such as the clouds and the land.
Uses of Multiple Reflection of Sound
1. Megaphones or loudhailers, horns, musical instruments
such as trumpets and shehnais, are all designed to send
sound in a particular direction without spreading it in all
2. Stethoscope is a medical instrument used for listening
to sounds produced within the body, chiefly in the heart
or lungs. In stethoscopes the sound of the patient’s
heartbeat reaches the doctor’s ears by multiple reflection
3. Generally, the ceilings of concert halls, conference halls
and cinema halls are curved so that sound reaches all
corners of the hall after reflection. Sometimes a curved
sound board may be placed behind the stage so that the
sound, after reflecting from the sound board, spreads
evenly across the width of the hall.
Physics l 29
People with hearing loss may need a hearing aid. A hearing
aid is an electronic, battery-operated device. The hearing
aid receives sound through a microphone. The microphone
converts the sound waves to electrical signals. These
electrical signals are amplified by an amplifier. The amplified
electrical signals are given to a speaker of the hearing aid.
The speaker converts the amplified electrical signal to sound
and sends to the ear for clear hearing.
A sound created in a big hall will persist by repeated
reflection from the walls until it is reduced to a value where it
is no longer audible. The repeated reflection that results in
this persistence of sound is called reverberation. In an
auditorium or big hall, excessive reverberation is highly
undesirable. To reduce reverberation, the roof and walls of
the auditorium are generally covered with sound-absorbent
materials like compressed fibreboard, rough plaster or
draperies. The seat materials are also selected on the basis
of their sound-absorbing properties.
When the frequency of an externally applied periodic force
is either equal to or an integral multiple of the natural
frequency of the body, the amplitude of oscillation is
increased to a high value. This phenomenon is known as
The phenomenon of resonance occurs in:
(i) Machine parts
(ii) Radio and TV reception
(iii) Air columns and
(iv) Sound boxes of musical instruments
Applications of Resonance
(i) When the soldiers walk in step, they produce some fixed
frequency. If this frequency corresponds to the natural
frequency of the bridge, resonance can take place. This
will make the bridge vibrate violently, and hence it may
collapse. To avoid such a situation, the soldiers are
asked not to march in step.
(ii) When the glass rattles, its natural frequency
corresponds with the frequency with which the piano is
being played. Thus, resonance takes place, which makes
the glass to vibrate violently.
(iii) When the frequency of the engine of a motorbike
corresponds with the natural frequency of the rear view
mirror, resonance takes place. Thus, the rear view mirror
vibrates with larger amplitude.
(iv) A tuning fork held close to the ear, disturbs a small
amount of air and thence sound heard is faint. When
the handle of the vibrating tuning fork is held against a
table, it sets up forced vibrations in the table top. As the
table top has a larger surface area, large volume of air is
set into vibrations, thereby producing a loud sound.
(v) A large sound box contains a large amount of trapped
air. When the vibrations of the vibrating string are
impressed upon the air, it sets up forced vibrations in
the air column. As a large volume of enclosed air
vibrates, a loud sound is produced.
Loudness and Intensity
Loundess is the property by virtue of which a loud sound
can be distinguished from a faint one, both having the same
pitch and quality.
Loudness depends on the following:
(i) It is directly proportional to the square of amplitude.
(ii) It is inversely proportional to the square of distance.
(iii) It is directly proportional to the surface area of vibrating
(iv) It is directly proportional to the density of the medium.
(v) The more the resonant bodies nearby, the more will be
The unit of loudness is phon.
The unit of intensity level is decibel.
Intensity is a measurable quantity whereas loudness is
L = K log I, where L = Loudness
I = Intensity
K = Constant
Intensity is proportional to (i) square of amplitude, (ii)
square of frequency and (iii) density of air.
Source of sound Intensity (db)
Ordinary conversation 40-60
Hot discussion 70-80
Heavy motor vehicle, 90-95
The Doppler effect is the change in frequency of a wave
(sound or light) due to the motion of the source or observer.
The frequency (and hence pitch) of a sound appears to be
higher when the source approaches the listener and lower
when the source recedes from him. It is due to the Doppler
30 l Physics
effect that the whistle of a train appears shriller when it
approaches a listener than when it moves away from him.
Speed guns (or radar sets), used by police to measure
the speed of vehicles, use Doppler effect. A radar set sends
out a radio pulse and waits for the reflection. Then it measures
the Doppler shift in the signal and uses the shift to determine
The Doppler effect is very useful in astronomy. It can
be used to find out whether a star is approaching towards us
or receding away from us. When a star is receding from us
the light emitted from the star appears more red (red light is of
lower frequency than other colours). Thus, the fact that the
light emitted by the stars of distant galaxies suffers a red shift
when observed from the earth means that these galaxies are
receding from our galaxy. This is the principle evidence in
favour of the hypothesis of expanding universe.
Doppler effect can also be used to detect or even
measure the rotation of a star, e.g. the sun.
The effect can be used to track a moving object, such
as a satellite, from a reference point on the earth. The method
is remarkably accurate; changes in the position of a satellite
10m away can be determined to a fraction of a centimetre.
gathering and focussing mechanism of light by the eye-lens
are well described by the wave phenomena. But the light
absorption by the rods and cones (of the retina) requires the
photon phenomena of light.
Speed of light
Firstly Romer (an astronomer) obtained the value of the
speed of light with the help of the motion of the satellite of
the planet Jupiter. In different media the speeds of light are
different. It depends upon the refractive index of the medium.
The medium which has a larger refractive index has a smaller
speed of light.
The refractive index or index of refraction (μ)
= speed of light in medium
speed of light in vacuum
The speed of light is maximum in vacuum and it is 3 × 108
metre/sec (1,86,310 miles/sec or 2,99,776 km/sec). In any
denser medium the speed of light is always smaller than that
of vacuum. In air the value of the speed of light is 0.03%
lesser than the value of the speed of vacuum; in water it is
smaller than 25% and in glass it is lesser than 35%.
Values of the speeds of light in various media
Medium Speed of light (m/s)
Vacuum 3 × 108
Water 2.25 × 108
Oil of turpentine 2.04 × 108
Glass 2 × 108
Rock salt 1.96 × 108
Nylon 1.96 × 108
Reflection of light
The bouncing back of light when it strikes a smooth or
polished surface is called reflection of light.
Light is a transverse, electromagnetic wave that can be seen
by humans. The wave nature of light was first illustrated
through experiments on diffraction and interference. Like all
electromagnetic waves, light can travel through a vacuum.
The transverse nature of light can be demonstrated through
polarisation. The human eye has the sensitivity to detect
electromagnetic waves within a small range of the
electromagnetic spectrum. Electromagnetic radiation
belonging to this region of the spectrum (wavelength of
about 400 nm to 750 nm) is called light.
The speed of light is finite and measurable. Its presently
accepted value in vacuum is c = 2.99792458 × 108 m s–1. For
many purposes, it suffices to take c = 3 × 108 m s–1. The speed
of light in vacuum is the highest speed attainable in nature.
A light wave can be considered to travel from one point
to another, along a straight line joining them. The path is
called a ray of light, and a bundle of such rays constitutes a
beam of light.
The scientific study of the behaviour of light is called
Dual (Wave-Particle) Nature of Light
The wave nature of light shows up the phenomena of
interference, diffraction and polarisation. This phenomena
shows that light is an electromagnetic wave consisting of
electric and magnetic fields with continuous distribution of
energy over the region of space over which the wave is
On the other hand, photoelectric effect and Compton
effect, which involve energy and momentum transfer and
radiation, show the particle nature of light. (Particles exist
in the form of photon.)
For example: In the familiar phenomenon of seeing an
object by our eye, both descriptions are important. The
Physics l 31
1. The image is as far behind the mirror as the object is in
2. The line joining the image to the object is perpendicular
to the mirror.
3. The image is of the same size as the object, virtual and
4. The image has right-left reversal.
5. If a plane mirror is rotated in the plane of incidence by
an angle , then the image rotates by an angle 2, the
reflected ray rotates by an angle , while the incident
ray remains fixed.
6. If the object is displaced through a distance x away
from the mirror, the image is also displaced by a distance
x away from the mirror.
7. If a mirror moves through a distance x towards or away
from an object, then its image moves by a distance x
towards or away from the mirror.
8. If a person moves with a velocity v towards or away
from the mirror, then his image appears to move with a
velocity 2v towards or away from the mirror to him, but
if a stationary observer observes the image then it
appears to move with a velocity v.
9. If the mirror moves with a velocity v towards or away
from a stationary observer, then the image will appear to
move with a velocity 2v towards or away from the object.
Note: If an object is placed between two mirrors lying parallel
to each other then infinite number of images are formed.
Application of plane mirrors
Plane mirrors have many applications. Some of these are:
1. The most common use is that of a looking mirror in the
bathroom or dressing room.
2. It is used in showrooms to increase the effective length
of the room.
3. It is used in the optician’s room to increase the effective
length of the room.
4. Large plane mirrors are mounted at the corners of a blind
curve on highways to enable drivers to see the
5. A plane mirror is used below the pointers of instruments
such as voltmeter, ammeter and galvanometer to remove
6. It is used in instruments such as periscope, sextant,
photocopier, overhead projector, solar cooker and
A periscope is an optical instrument which enables us to see
objects over or round an obstacle. It can be used to view
objects from above the head of people in a crowd. Its most
common use is in the submarine.
The light ray that strikes the surface of the mirror is
called incident ray.
The light ray that bounces off from the surface of the
mirror is called reflected ray.
The normal is a line perpendicular to the mirror surface
where the reflection occurs.
The angle between the incident ray and the normal is
called the angle of incidence (i).
The angle between the reflected ray and the normal is
called the angle of reflection (r).
The image formed by reflection in a plane mirror is
1. laterally inverted
2. same size as the object
5. as far behind the mirror as the object is in front of it.
Note: Real image: Image that can be seen on a screen.
Virtual image: Image that cannot be seen on a screen.
Laws of Reflection
The reflecting surfaces obey the following two laws called
the laws of reflection:
1. The angle of incidence (i) is equal to the angle of
2. The incident ray, the reflected ray and the normal at the
point of incidence all lie in the same plane.
Application of Reflection of Light
The reflection of light is useful in the functioning of modernday
equipment like in the operation of laser printers. In fact,
the reflection of laser light is used widely, in applications
ranging from measuring the speeds of automobiles to reading
price information from bar codes at the supermarket.
One surface of the mirror is plane and another surface has a
sharp metallic polish, which is pasted. This is done to avoid
polish decay. The backside of the mirror with silver or mercury
layers (metallic polish) works as reflective surface. The object
and image both are located at equal distance. In a plane
mirror, the image formed is always imaginary and equal to
the size of the object laterally inverted.
Characteristics of image formed by a plane mirror
It is worthwhile to remember the following in case of a plane
32 l Physics
Images by a Concave mirror
Position Position Nature Use
of object of image of image
At infinity At the principal Real, inverted To collect heat
focus or in the focal and extremely radiations in solar
plane diminished in size devices
Beyond the centre of Between the principal Real, inverted and ————
curvature focus and centre of curvature diminished
At the centre of At the centre of Real, inverted and Reflecting mirror for
curvature curvature equal to the size of projector lamps
Between focus and Beyond centre of Real, inverted and In flood lights
the centre of curvature bigger than object
At the principal focus At infinity Real, inverted and In torches, head lights
Between the pole and the Behind the mirror Virtual, erect and Shaving mirror, dentist
principle focus magnified
Note: A convex mirror always forms a virtual and erect image no matter at what place in front of the mirror the object is placed.
The image is always situated between the pole and the focus, irrespective of the position of the object in front of the
For a Convex Mirror
Position of object Position of image Nature of image Uses
At infinity Appears at the Virtual, erect and Used as a rear view mirror
principal focus extremely diminished and reflectors in street lights
Between infinity and the Appears between the Virtual, erect and Used as a rear view mirror
pole principal focus and the diminished and reflectors in street lights
Uses of Periscope
A periscope is used to view objects
(i) from above the head of people in a crowd.
(ii) on the other side of a high wall.
(iii) from a trench in a war.
(iv) from a submarine.
The mirror constructed in a spherical plane is called a
spherical mirror. One side of the mirror has a layer of mercury
or coating of lead oxide.
Spherical mirrors are of two types: If the inside or the
concave side of the spherical surface is polished, it is a
concave mirror or a converging mirror. If the outside or the
convex surface is polished, it is a convex mirror or a
Both concave and convex mirrors are constructed from
the same spherical glass. The centre of the glass sphere is
called the centre of curvature (C) and the middle point (O) of
the spherical mirror is called the pole. The line passing
through the centre of curvature and the pole is called the
principal axis. The middle point of the straight line drawn
from the pole to the radius of curvature is called focus (F).
The focal distance (f) =
radius of curvature
The focal distances (f) for both concave and convex
mirrors are evaluated by the following formula
1 where u = Object distance
v = Image distance
f = Focal length of the mirror
Physics l 33
Uses of Spherical Mirrors
1. To collect heat radiation in solar devices: Heat radiation
from the sun coming from infinity is brought to focus
by the concave mirror in its focal plane or at the focus.
2. Reflecting mirror for projector lamps: The object is
placed at the centre of curvature to obtain an image of
the same size.
3. In floodlights: The source of light is placed just beyond
the centre of curvature. This illuminates a certain section
of the ground.
4. In torches and headlights: The source of light is placed
at the focus to obtain a parallel beam of light.
5. Shaving mirror or dentist's mirror: It produces an
erect virtual and highly magnified image of an object
placed between its pole and focus.
6. It is used in the ophthalmoscope by doctors to
concentrate light on a small region which is to be
1. Rear view mirror: It is used in automobiles. This is due
to the reason that a convex mirror provides a wider field
of view than a plane or concave mirror. It produces an
erect, diminished and virtual image. It does not give the
exact distance of the vehicle coming from behind.
2. Street light reflector: It is used as a reflector in street
lamps so as to diverge light over a large area.
3. Security mirror: It is used as a security mirror in shops
and on roads at sharp bends and concealed entrances.
Refraction of light
It is the phenomenon of light in which a ray of light bends
from its path when it travels from one medium to another.
When a ray of light travels from an optically rarer medium
into an optically denser medium, it bends towards the normal
i.e., i > r.
When a ray of light travels from a denser medium to a
rarer medium, it bends away from the normal that is i > r.
If the ray is incident normally on the surface separating
the two media, it goes undeviated i.e, i = 0, r = 0.
Cause of refraction: Light has different speeds in
Laws of Refraction and Refractive Index
Incident ray, the refracted ray and the normal at the point of
incidence lie in the same plane. The ratio of the sine of angle
of incidence to the sine of angle of refraction is a constant
value which is known as refractive index for the pair of media.
It is also equal to the inverse ratio of the two indices of the
two media, i.e.,
1 is the refractive index of the 2nd medium
with respect to the 1st medium.
This law is also known as Snell’s law. Refractive index has
no unit as it is a ratio. It is equal to unity for vacuum.
Speed of light in medium
Speed of light in vacuum
When a ray of light goes from a rarer medium to a
When a ray of light goes from a denser medium to a
The refractive index of a medium depends on
(i) Nature of the two media in contact
(ii) Temperature of the medium
(iii) Wavelength of light
The refractive index of the two media and the
wavelength of light in those media are related as:
11 = 22
Snell’s law and hence laws of refraction fail if
i = r = 0, i.e. when a ray of light falls normally on the
Critical angle is the angle of incidence in the denser medium
for which angle of refraction in the rarer medium is 90°, when
the ray of light is travelling from the denser to the rarer medium.
Critical Angles of Some Transparent Media
Substance Refractive Critical
medium index angle
Water 1.33 48.75°
Crown glass 1.52 41.14°
Dense flint glass 1.62 37.31°
Diamond 2.42 24.41°
Applications of Refraction of Light
1. Twinkling of stars at night: The light from stars suffers
a series of refractions due to atmospheric layers of
different densities and different refractive indices.
34 l Physics
Denser layer Earth Observer
Due to this reason, a ray of light starting from a star S
bends more and more towards the normal before reaching
the observer on the earth. But the observer sees the star
in the direction of the ray reaching finally in the eye, so
star appears to him at S1 instead of S. On account of
change in temperature and density the apparent position
of the start changes continuously, which gives the
twinkling effect to the star.
2. A harpoon used to kill a fish is aimed at a point below the
apparent position of the fish because the fish appears to
be raised up due to refraction of light from denser medium
(water) to rarer medium (air) at the surface separating the
3. When a coin is kept inside a water container, the
phenomenon of refraction takes place and the coin kept
in the container seems upwardly uplifted.
4. A fish inside water seems lifted up from its original
5. A straight rod partially sunk inside water seems to be
6. When the sun is below the horizon before sunrise and
after sunset, the region around the sun appears red due
7. Water in a pond appears to be only three-quarters of its
Total Internal Reflection
When a ray of light travels from a denser to a rarer medium
and the angle of incidence in the denser medium for that pair
of media is more than the critical angle, the ray of light, instead
of going into the rarer medium, comes back in the denser
medium and follows the laws of reflection, i.e., i = r. In this
case, no ray of light is refracted or absorbed. It is totally
reflected and the image formed is very bright. That’s why it is
known as total internal reflection.
Due to total internal reflection, diamonds sparkle, mirage
occurs in hot countries and looming occurs in cold countries.
Applications of total internal reflection
On hot summer days, the air near the ground becomes hotter
than the air at higher levels. The refractive index of air
increases with its density. Hot air is less dense and has
smaller refractive index than cool air. If the air currents are
small, that is, the air is still, the optical density at different
layers of air increases with height. As a result, light from a
tall object such as a tree, passes through a medium whose
refractive index decreases towards the ground. Thus, a ray
of light from such an object successively bends away from
the normal and undergoes total internal reflection, if the angle
of incidence for the air near the ground exceeds the critical
angle. When shown in to a distant observer, the light appears
to be coming from somewhere below the ground. The
observer naturally assumes that light is being reflected from
the ground, say, by a pool of water near the tall object. Such
inverted images of distant tall objects cause an optical illusion
to the observer. This phenomenon is called mirage.
Mirage is especially common in hot deserts. It is noticed
that while moving in a bus or a car during a hot summer day,
a distant patch of road, especially on a highway, appears to
be wet. But, you do not find any evidence of wetness when
you reach that spot. This is also due to mirage.
Diamonds are known for their spectacular brilliance. Their
brilliance is mainly due to the total internal reflection of light
inside them. The critical angle for diamond-air interface
(24.4°) is very small, therefore once light enters a diamond,
it is very likely to undergo total internal reflection inside it.
Diamonds found in nature rarely exhibit the brilliance for
which they are known. It is the technical skill of a diamond
cutter which makes diamonds to sparkle so brilliantly. By
cutting the diamond suitably, multiple total internal reflections
can be made to occur.
Prisms designed to bend light by 90º or by 180º make use of
total internal reflection. Such a prism is also used to invert
images without changing their size.
Optical fibres too make use of the phenomenon of total
internal reflection. They are fabricated with high-quality
composite glass/quartz fibres. When a signal in the form of
light is directed at one end of the fibre at a suitable angle, it
undergoes repeated total internal reflections along the length
of the fibre and finally comes out at the other end.
Since light undergoes total internal reflection at each
stage, there is no appreciable loss in the intensity of the
light signal. Optical fibres are fabricated such that light
reflected at one side of inner surface strikes the other at an
Physics l 35
angle larger than the critical angle. Even if the fibre is bent,
light can easily travel along its length. Thus, an optical fibre
can be used to act as an optical pipe. A bundle of optical
fibres can be put to several uses. Optical fibres are extensively
used for transmitting and receiving electrical signals, which
are converted to light by suitable transducers. Obviously,
optical fibres can also be used for transmission of optical
signals. For example, these are used as a ‘light pipe’ to
facilitate visual examination of internal organs like
oesophagus, stomach and intestines.
A commonly available decorative lamp has fine plastic
fibres with their free ends forming a fountain-like structure.
The other end of the fibres is fixed over an electric lamp.
When the lamp is switched on, the light travels from the
bottom of each fibre and appears at the tip of its free end as
a dot of light.
The fibres in such decorative lamps are optical fibres.
The main requirement in fabricating optical fibres is that
there should be very little absorption of light as it travels for
long distances inside them. This has been achieved by
purification and special preparation of materials such as
quartz. In silica glass fibres, it is possible to transmit more
than 95% of the light over a fibre length of 1 km. (Compare
with what you expect for a block of ordinary window glass 1
Nowadays optical fibres are extensively used for
transmitting audio and video signals through long distances
like telephone and other transmitting cables.
Terms related to lens
Optical centre: It is the mid point in a lens and any light
ray which passes through it doesn’t deviate.
Focus: The point at which the light rays coming parallel
to the principal axis meet or appear to meet after refraction is
Focal length: The distance between the optical centre
and the focus is called focal length.
Image formation by convex lens: The nature of image
formed, its size and position etc. depend on the distance of
the object kept from the focus.
Note: 1. The focal length of a mirror does not depend upon
the nature of the medium in which it is placed whereas
the total length of a lens depends upon the medium
in which it is placed. Thus, there will be no change
in the focal length of the concave mirror whereas
the focal length of the convex lens will change.
2. The type of lens (converging and diverging lens)
changes, if it is placed in a medium having a higher
refractive index than that of the material of the lens.
Refraction through a lens
Lens is a portion of a transparent refracting medium
bounded by one or two spherical surfaces, i.e. convex
lens (converging lens) and concave lens (diverging lens).
The power of a lens is a measure of deviation produced
by it in the path of rays refracted through it. Thus, power
of a lens in dioptre (D) is the reciprocal of its focal length
The power of a convex lens is taken as positive, i.e. a
person suffering from long-sightedness (hypermetropia)
uses lens of positive power in his/her spectacles.
The power of a concave lens is taken as negative, i.e. a
person using spectacles for short-sightedness (myopia)
has negative power of spectacles.
The least distance of distinct vision is the distance of
the nearest point up to which the normal human eye can
see distinctly without strain.
The power of accommodation is the ability of the eyelens
to focus the objects at different distances by
changing its focal length.
Persistence of vision is the time up to which the
impression of an image persists upon the retina (1/16 of
a second) after the object has been removed.
Simple Microscope: A simple microscope is based on
the principle that a convex lens produces an erect, virtual
and highly magnified image of the object when it is placed
between the principal focus and the optical centre of the
lens. It consists of a single converging or convex lens of
short focal length.
36 l Physics
Formation of images by a convex lens and a concave lens for different positions of the object
In case of convex lenses
S. No. Position of object Figure Position of image Nature of image Uses
1. Between the optical On the same side Virtual, erect Magnifying
centre and the as that of the object and magnified glass, eye-lenses,
principal focus spectacles for longsightedness
2. At the principal At infinity Real, inverted Spotlights
focus and extremely
3. Between F and 2F Beyond 2F Real, inverted Projector,
and bigger than microscope
4. At 2F At 2F Real, inverted Photocopier
and equal to
the size of the object
5. Beyond 2F Between F Real, inverted In a camera,
and 2F and diminished in eye while reading
6. At infinity At the principal Real, inverted Telescope, camera
axis and extremely lens, burning glass
In case of concave lenses
1. At infinity Appears at the Virtual, erect Spectacles for shortprincipal
focus and extremely sightedness
2. Between infinity Appears between Virtual, erect Spectacles for shortand
the optical the principal focus and diminished sightedness
centre and the lens or myopia
Physics l 37
Dispersion of light
The process of splitting of white light (polychromatic light)
into its constituent colours is known as dispersion.
When different colours or wavelengths are present in
the incident light, different colours are deviated to different
extents in a medium other than air or vacuum. In vacuum, all
colours move with the same speed and so appear white, but
when they are made to pass through prisms or lenses, the
violet colour, whose refractive index is the maximum, is
deviated the most, and the red colour, whose refractive index
is the least, is deviated to the least extent. The deviations of
other colours lie between those of red and violet.
Dispersion of white light through a prism
Violet is deviated the most while red is deviated the least.
A band of colours obtained on the screen when a
polychromatic (white) light splits into its constituent colours
is called a spectrum.
Wavelength is the main characteristic of colour. The
light of a certain colour has a fixed wavelength. The portion
of the spectrum ranging from 8000Å to 4000 Å is visible to
human eye and is called the visible spectrum. In the visible
spectrum, the wavelength of red colour is the maximum and
that of violet the minimum. The frequency of red colour is
the minimum and that of violet the maximum. In vacuum, all
colours have the same speed, but the velocity of visible
colours changes in different mediums. In glass, red colour
has higher speed than violet.
Examples of Visible Spectrum
(i) Rainbow: The seven-coloured spectrum which is
formed opposite the sun after rain.
(ii) Colour of an object changes on heating: The colour
of iron ball is red at low temperature. When it is further
heated its colour changes from red to orange to yellow. At
very high temperature it glows with bluish white light,
because its colours shift from red to violet side of the
spectrum as temperature increases.
The portion of the spectrum which extends on either side of
the visible spectrum is called the invisible spectrum. These
do not produce any sensation of vision but their presence
can be detected with suitable instruments. They are: (a)
Infrared spectrum (b) Ultraviolet spectrum.
(a) Infrared radiations are long-wavelength radiations
having wavelength 8000Å–400,000Å (where 1Å = 10–10 m).
They cause heating effect. They do not affect ordinary
(b) Ultraviolet radiations have short wavelengths
ranging from 4000Å–100Å. They strongly affect
photographic plates. They produce fluorescence. Their
penetration power is large.
Scattering of Light
The deflection of light by fine particles of solid, liquid or
gaseous matter from the main direction of beam is called
scattering. The scattering of light by the earth’s atmosphere
depends upon the wavelength of light and the size of the
particles. For large-sized particles, reflection and refraction
are more prominent. For particles whose size is much smaller
than the wavelength of light, scattering is more pronounced.
According to the fourth-power law of Rayleigh, the intensity
of scattered radiation is inversely proportional to the fourth
power of wavelength. This means that short wavelength will
be scattered more
I 1 . Very fine particles scatter blue
light while particles of large size scatter red light more. The
blue colour of the sky is due to the scattering by air molecules
and the sun appears red is because of the removal of blue
from the direct beam. Chalk dust particles are whitish because
their size is quite large and reflection is more prominent. The
scattered light appears to be white. It is for this reason that
clouds are white.
Rainbow is an example of dispersion of light. The rainbow
is formed due to dispersion of light by water drops hanging
in the atmosphere after the rainfall. The sunrays are incident
on the raindrops and get dispersed and are then totally
reflected internally and transmitted to form a rainbow.
Colour of Objects
We see objects because of the light they reflect. Most of the
objects around us reflect only a part of the light that is incident
upon them and it is the reflected part which gives the objects
When a rose is viewed in white light, its petals appear
red, and the leaves appear green, because the petals reflect
the red part of the white light and the leaves reflect the green
part. The remaining colours are absorbed. When the same
rose is viewed in green light, the petals will appear black and
the leaves green. In blue or yellow light, both the petals and
the leaves will appear black.
38 l Physics
This page appears white because it reflects all the
colours and the print appears black because it absorbs all
Mixing Coloured Light: White light can be produced
by a mixture of red, green and blue lights. In fact all colours
can be produced by a suitable mixture of these three colours.
Red, green and blue are therefore called primary colours.
Others, such as yellow, are secondary colours.
Thus, Red + Green = Yellow
Red + Blue = Magenta
Green + Blue = Cyan
Green + Magenta = White
Red + Cyan = White
Blue + Yellow = White
Two colours which give white light when put together
are called complementary colours. Blue and yellow are
By adding various amounts of red, green and blue, we
can produce any colour in the spectrum. For this reason,
they are called the additive primary colours. Yellow, magenta
and cyan, which are obtained by mixing two additive primary
colours, are called subtractive primary colours. The colours
obtained by mixing two subtractive colours are:
Cyan (Peacock Blue) + Yellow = Green
Magenta + Yellow = Red
Cyan + Magenta = Blue
Interference of light
The superposition of two (or more) waves of the same kind
that pass the same point in space at the same time is called
At some points the intensity is maximum and the
interference at these points is called constructive
interference. At some other points the intensity is minimum
(possibly even zero) and the interference at these points is
called destructive interference.
Interference is the most fundamental characteristic of a
wave and there is no loss of energy in it; there is only
redistribution of energy from maxima to minima. The
phenomenon of interference was firstly demonstrated by
Thomas Young in his experiment called Young’s double slit
Examples related to interference
(i) The kerosene oil spread on the water surface seems to
be coloured due to interference of light.
(ii) The soap bubbles have a brilliant colour in sunlight due
to interference of light.
Diffraction of light
Diffraction is a general characteristic exhibited by all types of
waves, be it sound waves, light waves, water waves or matter
waves. Since the wavelength of light is much smaller than the
dimensions of most obstacles, we do not encounter diffraction
effects of light in everyday observations. Indeed the colours
that we see when a CD is viewed is due to diffraction effects.
When a beam of light passes through a narrow slit or an
aperture, it spreads out to a certain extent into the region of
geometrical shadow. This is an example of diffraction, i.e. of
the failure of light to travel in a straight line. If one uses
monochromatic light for diffraction, bright and dark bands
are observed in the region of geometrical shadow. With white
light, coloured bands are observed. Diffraction is a particular
case of interference and is due to the wave nature of light.
A diffraction grating is a device used to disperse a
beam of light for producing its spectrum. Gratings may be
prepared by ruling equidistant parallel lines onto a glass
(transmission grating) or metal surface (reflection grating).
Note: In interference and diffraction, light energy is
redistributed. If it reduces in one region, producing a
dark fringe, it increases in another region, producing
a bright fringe. There is no gain or loss of energy,
which is consistent with the principle of conservation
Polarisation is a feature of transverse waves only. Longitudinal
waves can never be polarised. A wave is plane-polarised if all
the vibrations in the wave are in a single plane, which contains
the direction of propagation of the wave. Suppose we have
a rope and make waves down it, we could make waves in any
direction we liked. But if we made waves through a narrow
vertical slit, we would find that the waves would only pass
through if they were vertical. This would be a vertically
Light waves are easily polarised using polaroid filters.
Light waves, like all electromagnetic waves, consist of an
electric field component perpendicular to a magnetic field
component, which are always in phase. We normally consider
only the electric field component in polarisation, because the
electrical effects are those that dominate.
A light wave that is vibrating in more than one plane is
referred to as unpolarised light. Light emitted by the sun, by
a lamp in the classroom, or by a candle flame is unpolarised
It is possible to transform unpolarised light into polarised
light. Polarised light waves are light waves in which the
vibrations occur in a single plane. There are a variety of
methods of polarising light. For example,
1. Polarisation by Transmission
2. Polarisation by Reflection
3. Polarisation by Refraction
4. Polarisation by Scattering
Physics l 39
Light waves are transverse in nature; i.e., the electric field
associated with a propagating light wave is always at right
angles to the direction of propagation of the wave. This can
be easily demonstrated using a simple polaroid. You must
have seen thin plastic-like sheets, which are called polaroids.
A polaroid consists of long-chain molecules aligned in a
particular direction. The electric vectors (associated with
the propagating light wave) along the direction of the aligned
molecules get absorbed.
Thus, if an unpolarised light wave is incident on such a
polaroid then the light wave will get linearly polarised with
the electric vector oscillating along a direction perpendicular
to the aligned molecules.
Applications of Polarisation
Polarisation has a wealth of other applications besides
their use in glare-reducing sunglasses.
In industry, polaroid filters are used to perform stress
analysis tests on transparent plastics.
Polarisation is also used in the entertainment industry
to produce and show 3-D movies.
Polaroids are used in headlights and windscreens of
cars to cut off the dazzling light of a car approaching
from the opposite side.
Coherence and incoherence
Coherent radiation originates from a single oscillator, or
a group of oscillators in perfect synchronisation (phaselocked),
e.g., microwave ovens, radars, lasers, radio
towers (i.e., artificial sources).
Incoherent radiation originates from independent
oscillators that are not phase-locked. Natural radiation
Light Amplification by Stimulated
Emission of Radiation (LASER)
A laser is an optical device that produces an intense beam of
coherent monochromatic light. A laser is not a source of
energy. It is simply a converter of energy, taking advantage
of stimulating emission to concentrate a certain fraction of
energy (commonly 1%) into radiation of a single frequency,
moving in a single direction.
1. Eye surgeons use lasers to ‘weld’ detached retinas back
into place without making incision.
2. Laser beams have been used to measure the exact
distance between the earth and the moon and to provide
information on continental drift.
3. The detection and measuring of pollutants in vehicular
exhaust gases is accomplished with lasers.
4. Communications can be carried in a laser beam directed
through space, through atmosphere, or through optical
fibres that can bend like cables.
5. A laser beam is used as a non-wearing ‘optical’ needle
for video and phonograph records, as a knife to rapidly
and accurately cut cloth in garment factories, as a tool
for meat inspection, and for fingerprint detection.
6. Police use special guns emitting short bursts of infrared
laser lights to measure the speed of vehicles. A laser
speed gun measures the round-trip time for light to reach
a vehicle and reflect back. If the gun takes a large number
(say 1000) of readings per second, it can compare the
change in distance between readings and calculate the
speed of vehicles.
Human eye consists of a natural convex lens which forms
real, inverted and diminished image on the retina. Ciliary
muscles can change the focal length of the human eye. For a
grown-up person, the separation between the retina and the
lens is about 2.5 cm, which is the image distance.
A normal eye can see up to maximum distance of infinity
and the minimum distance of clear vision is about 25 cm. For
seeing at maximum distance of infinity, eye muscles are fully
relaxed having maximum focal length and for seeing at
minimum distance, muscles are strained, having minimum focal
Some Common Defects
(A) Shortsightedness or Myopia: A person suffering from
this defect cannot see a distant object but near objects
are clearly seen. In this, fmax is less than the distance
from the lens to the retina and hence the image of a
distant object is formed short of retina (see figure a).
For remedy, a divergent lens is given to a myopic
person (see figure c). This lens forms the image of a
distant object at a distance x, which is maximum distance
of clear vision.
(a) (b) (c)
(B) Farsightedness or Hypermetropia: A person suffering
from this defect cannot see a near object (see figure
(a)). For remedy, a convergent lens is used (see figure
If y is the minimum distance up to which the eye can
clearly see, the converging lens should form the image
of an object at distance y = 25 cm.
(a) (b) (c)
40 l Physics
(C) Presbyopia: In this defect, both far-off and nearer
objects are not clearly seen. This is corrected by using
(D) Astigmatism: Such a person cannot see all directions
equally well. This is corrected by using cylindrical
It is an optical instrument used to see the small objects which
cannot be seen with the naked eye.
Simple Microscope: It consists of a single convex lens
of small focal length. Hence, a magnifying glass may be called
a simple microscope. It forms the final image magnified and
In general, for a microscope or for a telescope, remember
that when the final image is formed at infinity (also called
normal adjustment) our eyes are most relaxed and
magnification is minimum. But if the final image is formed at
the least distance of distinct vision (D = 25 cm), our eyes are
most strained and magnification is maximum.
Magnifying Power: It is the ratio of the angle subtended
by the image at the eye to the angle subtended by the object
at the eye when both are placed at the least distance of distinct
Compound Microscope: It consists of two convex lenses
placed co-axially and the distance between them can be
changed. The lens towards the object is called objective (O)
and that near the eye is called eyepiece (E). The objective has
small focal length and small aperture. The eyepiece has large
aperture and large focal length compared to the objective.
Telescope is an optical instrument to observe a distant object
clearly. Basically, they are of two types, refracting and
Astronomical Telescope: It consists of two converging
lenses placed co-axially. The objective has large aperture and
large focal length. The eyepiece has smaller aperture and
smaller focal length. The separator between the two lenses
can be changed.
Magnifying Power (M): It is the ratio of the angle
subtended by the image at the eye when it is at the least
distance of distinct vision to the angle subtended by the
object at the eye.
Terrestrial Telescope: It is used to see the objects on
the earth. It forms the final image erect.
It consists of three converging lenses, namely, objective
(O), eyepiece (E) and field lens (F). The focal plane of the
objective is at a distance of 2f from the field lens so that A'B'
= AB (see fig above).
Galilean Telescope: It is a terrestrial telescope having
very small field of view due to the concave lens used as an
Lens Camera: Basically, a camera consists of lightproof
box with a lens system in front and photographic film at the
back. The lens system, which converges light onto the film,
consists of a number of lenses. The purpose of using more
than one lens is to minimise defects or aberrations of the
image. Objects at different distances are focused on the film
by moving the lens system. Like the pupil in the eye, a camera
also has an opening or aperture whose diameter can be varied
by the camera iris. There is a shutter placed between the lens
system and the film. When a photograph is taken, the shutter
opens and closes rapidly. The time for which the shutter
remains open can be adjusted.
Example: We may have felt a shock when we touched a metal
door knob after sliding across a car seat.
Charge: Charge is the property associated with matter
due to which it produces and experiences electrical and
There are two types of electric charges termed as positive
and negative charge.
Unlike charges attract while like charges repel each other.
The word electricity comes from the Greek word electron,
which means “amber”.
Amber is petrified tree resin currently used in jewellery
and if amber rod is rubbed with a piece of cloth, it attracts
small pieces of dry leaves or paper. This is known as static
electricity or amber effect.
The branch of Physics that deals with the interaction
between stationary charges is called Electrostatics.
Physics l 41
Charge is conserved in nature. Charge is neither created
nor destroyed; it is merely transferred from one body to other.
Charging by Friction,
Conduction and Induction
Franklin is the smallest unit of charge while Faraday is
Friction causes one object to gain electrons from another
Charging by conduction occurs when excess charges in
one object are transferred through contact to another
Charging by induction occurs when a charged object
influences the charge distribution in another object.
Note: Benjamin Franklin was the first to assign positive
and negative signs of charge.
The existence of two types of charges was discovered
Modern concept or principle of electricity : According
to this theory, when two bodies are rubbed, the electrons of
the outermost orbit of the atoms of one body transfer to the
atoms of another body, and thus, the first body has lesser
number of electrons than the second body. Thereby, the first
body becomes positively charged and the second body
becomes negatively charged. This is the latest concept of
Electric Field: If an isolated charge is kept anywhere,
the region or space around it, up to which if any other charged
particle is brought, experiences a force. This region or space
is called electric field.
The concept of potential difference
The positive charge flows from a body at a higher potential
to a body at a lower potential and the negative charge flows
from a body at a lower potential to a body at a higher potential.
The flow of charge continues until the potential between
the two bodies becomes equal.
The electric potential difference between two
conductors is defined as the amount of work done in moving
a unit positive charge from one conductor to the other.
Mathematically, suppose the charge is q; then the
electric potential difference between the two conductors is
Since work (W) is measured in joule and charge (q) in
coulomb, the electric potential difference is measured in joule
per coulomb (JC-1). This unit occurs so often in our study of
electricity that it has been named as volt, in honour of the
scientist Alessandro Volta (the inventor of the Voltaic cell).
1volt 1 joule
Potential difference is a scalar quantity.
Electromotive force (emf)
Electromotive force is the work done in establishing the flow
of unit charge in the circuit. This requires an arrangement.
Such an arrangement is called a source of emf.
The electric cell is an equipment which maintains a potential
difference between any two points of the conducting wire so
that the flow of electric current is continuously sustained. In
electric cells, the chemical energy which is produced by
various chemical reactions transforms into electrical energy.
There are two metallic rods in every electric cell which are
called electrodes and which have opposite nature. The metallic
rod which is positively charged is called anode and the
collected ions are called anions, while the metallic rod which
is negatively charged is called cathode, and the collected
Electric current is defined as the amount of charge per unit
time that crosses the surface. Current is measured in ampere
(A). Charge is measured in coulomb (C).
1 ampere current flows when 1 coulomb of charge flows
for 1 sec. or 6.25 × 1018 electrons flow in 1 second through a
Q = n × e (number of electrons × charge present on 1
Current always flows from positive potential towards
negative in the circuit.
42 l Physics
ions are called cations. These metallic rods (electrodes) are
kept inside the solvent called electrolyte.
Usually electric cells are of two types :
(i) Primary cell
(ii) Secondary cell
(i) Primary cell: In a primary cell, chemical energy is directly
converted into electrical energy, and when all the chemical
energy is exhausted (used up), the cell becomes dead. The
Voltaic cell, Leclanche cell, Dry cell, Daniel's cell etc. are the
examples of primary cell.
(ii) Secondary cell: In a secondary cell, first the electrical
energy is converted into chemical energy. Then this chemical
energy is converted into electrical energy. The entire process
is completed by charging, and during its use, it is discharged,
then it is again charged, and this is the process (way) of its
functioning. That's why all the secondary cells are
rechargeable. The process of recharging is done through an
external source of electrical energy. The battery or cell
attached in motor vehicle, motor bike, emergency light etc.
are the examples of secondary cell.
Voltaic cell: The Voltaic cell was invented by Professor
Alessandro Volta in 1799. In this cell a zinc rod and a copper
rod are kept inside the glass container of sulphuric acid
In this cell the copper rod acts like an anode and the zinc rod
acts like a cathode. The value of emf in this cell is 1.08 volt.
Leclanche cell: In this cell a saturated solution of
NH4Cl is taken in a glass container, in which the zinc rod acts
like a cathode and the carbon rod kept in a mixture of
manganese dioxide (MnO2) acts like an anode.
The value of emf is 1.5 volt. This type of cell is used where
electric current is not regularly available. Such cells are mainly
used in electric alarm, siren, telephone etc.
Dry cell: In this cell the electrolyte used is not in the
form of solution but it remains in the dry form. In it there is a
zinc container (vessel) in which manganese dioxide (MnO2),
ammonium chloride (NH4Cl) and carbon are kept and in the
middle of this mixture a carbon rod is kept. Here, the carbon
rod acts like an anode while the zinc container acts like a
cathode. The dense paste of ammonium chloride is kept inside
the mixture of MnO2 and carbon. The value of emf this cell is
also 1.5 volt. Such cells are used frequently in torch,
transistor, radio etc.
Electromotive force of cells
Voltaic cell 1.08 volt
Daniel’s cell 1.08 volt
Dry cell 1.50 volt
Leclanche cell 1.50 volt
Lead container cell 2.00 volt
Battery of six cells 12.00 volt
It is the obstruction to the flow of electrons. The more the
resistance in the circuit, the less will be the current flowing
through the circuit. Increase in resistance causes electrical
energy to get converted into heat energy.
R length (l)
R area of cross section (a)
A long and thin wire offers more resistance than a short
and thick wire. It is measured in ohm ().
The resistance of a conductor is said to be one ohm if
under a potential difference of one volt a current of one
ampere flows through the conductor.
Materials chosen for making
(1) Connecting wires: Materials having a flow value of
resistivity, e.g., copper, aluminium.
(2) Resistance boxes: Materials having a high value of
resistivity but whose resistance does not increase with
increase in temperature, e.g., constantan, manganin,
(3) Resistors and filaments: Materials having large value
of resistivity, not pure metals but alloys, e.g., nichrome,
(4) Fuse wire: Material having low melting point and high
resistivity, e.g., solder, an alloy of lead and tin.
In the electrical case, resistance (R) is defined as the
ratio of the voltage V applied across a piece of material to the
current I through the material, or R = V/I. When only a small
current results from a large voltage, there is a high resistance
to the moving charge. For many materials (e.g., metals), the
ratio V/I is the same for a given piece of material over a wide
range of voltages and currents. In such a case, the resistance
Physics l 43
is a constant and the relation R = V/I is referred to as Ohm’s
law, after the German physicist George Simon Ohm (1789-
1854), who discovered it.
The above relation is an empirical formula which relates
the current flowing through a conductor to the potential
difference applied across it.
It states that “Physical conditions remaining the same,
the electric current flowing through a conductor is directly
proportional to the potential difference across the two ends
of the conductor.”
V I or
V = IR
where R is a “constant” and is called the resistance of
The SI unit of resistance is volt per ampere, which is
called ohm and is represented by the Greek capital letter
The internal resistance of a cell is defined as the opposition
of electrolyte to the flow of current through it. It is always in
series in a circuit. If a number of cells are in series, then the
internal resistances of all the cells are added according to
series combination. But if a number of cells are connected in
parallel then internal resistance is also calculated according
to reciprocal law of parallel combination.
P 1 2 r3
The internal resistance of a secondary cell is always
less than that of a primary cell.
Electric current, resistance and potential difference
in series and in parallel circuits:
For a series combination of resistors
(i) The current is the same in every part of the series
(ii) The equivalent resistance is the sum of the
individual resistances, including the internal
resistance of the cell in the circuit.
(iii) Current in the circuit is independent of the relative
positions of the resistors in series.
(iv) The equivalent resistance is greater than any
individual resistance in the series combination.
(v) The potential drop across each resistor is
proportional to its resistance.
(vi) In series combination of resistors, we have
RS = R1 + R2 + R3 + …
In parallel combination of resistors
(i) Total current through the combination is the sum of
the individual currents through the various
(ii) The potential difference across all resistors is the
(iii) The current through each branch is inversely
proportional to the resistance of that branch.
(iv)The reciprocal of the equivalent resistance equals
the sum of the reciprocals of the individual
(v) If two resistors R1 and R2 are in parallel, then the
current I1 and I2 in them will be distributed as:
1 R R
and I R I
I R I
and I V
Where I is the total current in the circuit.
P 1 2 R3
About combination of resistors, we have:
(i) Using n conductors of equal resistances, the number
of combinations one can have at a time is 2n-1.
(ii) If resistances are different, then the number of
combinations is 2n.
(iii) For n equal resistances, 2
The resistivity of a material is the resistance of a
conductor of this material of unit length and unit crosssectional
Resistivity is independent of the length and crosssectional
area of the conductor and depends upon the
nature of the material and the temperature.
The connecting wires are usually made of a material
having a low value of resistivity, e.g., copper, aluminium.
Resistors are made of wires having a large value of
resistivity. Usually, they are made of alloys, e.g.,
nichrome, manganin and constantan.
The resistance and resistivity of a conductor is directly
proportional to its absolute temperature.
With rise in temperature, the resistance and resistivity
(i) Conductors increase
(ii) Alloys increase
(iii) Semiconductors decrease
(iv) Electrolytes decrease
Resistance of certain alloys does not increase much or
remains almost the same with rise in temperature, e.g.,
44 l Physics
manganin, constantan, German sub silver etc. That’s
why they are used for making resistance boxes.
Factors affecting the resistance of a conductor
The electrical resistance of a conductor depends upon the
Effect of length of conductor: On increasing the length
of a wire, its resistance increases, and on decreasing the
length of a wire, the resistance will reduce. Actually, the
resistance of a wire is directly proportional to its length.
Effect of area of cross-section of conductor: It has been
found that the resistance of a conductor is inversely
proportional to the area of the cross-section of the
conductor which is used in the circuit.
Effect of nature of material of conductor: The electrical
resistance of a conductor depends on the nature of the
material of which it is made.
Effect of temperature: The resistance of conductors of
pure metals increases on increasing the temperature and
decreases on decreasing the temperature.
Domestic electric circuits series or parallel: When
designing an electric circuit, we should consider whether a
series or parallel circuit is better for the intended use. For
example, if we want to connect a large number of electric
bulbs for decorating buildings and trees as during festivals
such as Diwali or marriage functions, then the series circuit is
better because all bulbs connected in series can be controlled
with just one switch. A series circuit is also safer because the
flow of current in it smaller. But there is a problem with this
circuit. This is because if one bulb gets fused, then the circuit
breaks and all the bulbs are turned off. An electrician has to
spend a lot of time in locating the fused bulb form among
hundreds of bulbs, so as to replace it and restore the lighting.
The parallel electric circuit is better for connecting bulbs
in house because then we can have separate switches for
each bulb and hence operate it separately. In addition to
having ease of operation, parallel domestic circuits have many
other advantages over the series circuits.
Electric power: We know that the rate of doing work is
known as power. So electric power is the electrical work done
per unit. That is
Power = Work done/time taken
Or P = W/t
Unit of power: The SI unit of electric power is watt, denoted
by the letter W. The power of 1 watt is the rate of working of
1 joule per second. That is
1 Watt = 1 joule/1 second
Heating effect of current: When an electric current is
passed through a high-resistance wire like nichrome wire, the
resistance wire becomes hot and produces heat. This is known
as the heating effect of current. The role of resistances in
circuits is the same as that of friction in machines.
Since a conductor, say a resistance wire, offers resistance
to the flow of the current, work must be done by a current
continuously to keep itself flowing. We will calculate the
work done by a current I when it is passing through a
resistance R for time t. Now when an electric charge Q moves
against a potential difference V, the amount of work done is
W = Q × V
From the definition of current, we have
I = Q /t or Q = I t
And from Ohm’s law, we have V/I = R
or potential difference , V = I × R
Now putting Q = I × t and V = I × R,
we have W = I2 × R × t
or Heat produced, H = I2 × R × t joules
It is clear that the heat produced in a wire is directly
Square of current
Resistance of wire
Time for which current is passed
Applications of the heating effect of current
The important applications of the heating effect of electric
current are the following:
The heating effect of current is utilised in the working of
electrical heating appliances such as electric iron, kettle,
toaster, oven, room heater and water geyser.
The heating effect of current is utilised in electric bulb
for producing light.
The heating effect of current is utilised in electric fuse
for protecting household wiring and appliances.
1. Electric Bulb: It is based on the principle that when a
current is passed through the resistor filament, it gets
very hot and emits light. To retain such a high
temperature, the filament must be made of a metal of
high remelting point. Generally, a filament is made of a
thin wire of tungsten enclosed in a glass bulb containing
some inert gases like nitrogen and argon. Tungsten
has a high melting point of nearly 3380 °C. The inert
gases prolong the life of the filament. Due to high
resistance of the filament, a large amount of heat is
produced which raises the temperature and hence it
begins to emit light at high temperature much below its
melting point. For every watt of electrical power
consumed, the luminous intensity of bulb is nearly 1
2. Electric Fuse: It is a protective device which prevents
excessive current. It is always connected in series with
an electric circuit or an electric appliance. It consists of
a piece of wire generally made of lead-tin alloy having
very low melting point and high resistance. If a current
Physics l 45
larger than the specified value flows, the fuse wire melts
and breaks the circuit and hence the electric appliance
is saved from damage. The melting point and the
resistance of fuse wires are different for different electric
When an electric heater of 2kW is operated at 220V, a
current of A
= 9.1A will be obtained in the circuit.
So, a fuse wire of 10A should be used.
3. Tube light: Basically it is a long tube of glass and the
inside wall of the tube is coated with a thin layer of
fluorescent material. The tube glass has mercury along
with some inert gas like argon inside it. The two ends
of the tube have two terminals on which a thin layer of
barium oxide is coated. Whenever these two terminals
of the tube are activated to pass an electric current, the
electrons are emitted, which are directly respondent to
ionise the gas present inside the tube. Consequently,
through ionisation of the gas, ions generate a flow of
current inside the tube. The mercury confined inside
the tube gets sufficient thermal energy and it (mercury)
starts to vaporise and finally, due to the electron
emission, UV rays are emitted. When these UV rays are
incident on the inside wall of the tube on which
fluorescent material is coated, they are absorbed by
the wall and visible rays, or light of lower frequency,
seem to appear.
The fluorescent material coated is used in such a way
that light produced from the tube light appears similar
to a white visible sunlight. In the tube light internal
energy is produced in smaller amount, so 60% to 70%
electrical energy transforms into light energy. That's
why the power of tube light is sharper than that of an
ordinary bulb. A 40-watt tube light provides 6 to 8
times more light than an ordinary bulb of 40 watts.
Measurement of Voltage,
Current and Resistance
Galvanometer: It is a very sensitive instrument to detect
very small current, as small as 10-3 A. The deflection in the
instrument is directly proportional to the current passing
Ammeter: It measures current and hence is always
connected in series to the element through which the current
has to be measured so that the total current of the element
goes into the ammeter. Ideally the resistance of an ammeter
will be zero so that it doesn't introduce any resistance into
the main circuit and the main circuit is not affected. (An ideal
ammeter has been impossible so far.) In practice, the resistance
of an ammeter is approximately 1 , so the current measured
is always slightly less than what is flowing into the main
Voltmeter: It's a device to measure potential difference
across a circuit element and hence is always connected in
parallel to that element. Ideally the resistance of a voltmeter
is infinite so that it doesn't draw any main-circuit current and
the potential difference across the circuit element remains to
its true value. (The ideal voltmeter has alternatively been
realised in practice in the form of potentiometer.)
Types of current
There are two types of electric current: Direct Current (DC)
and Alternating Current (AC).
a) If the magnitude and direction of current does not vary
with time, it is known as direct current (DC).
b) If a current is periodic, i.e. its magnitude varies periodically
and its polarity reverses after each half cycle, it is known
as alternating current (AC).
An alternating current is the one which has the following
l Its magnitude is not constant but is constantly varying.
l Its direction also reverses after every half a cycle.
l In one cycle of A.C. the current rises from zero to
maximum and then decreases to zero and again rises to
maximum in the opposite direction before again
l The frequency of an A.C. is the number of cycles of the
A.C. produced in one second. In our houses, we use 50
cycles of A.C. in 1 second.
l Alternating current and alternating emf are those whose
magnitude and direction vary periodically with time.
l The simplest types of alternating current and alternating
emf have a sinusoidal variation, given respectively by
i=i0sint and = 0sin t
where i0 and 0 are called peak values of current and
voltage respectively and is the angular frequency.
l The time taken by an alternating current to go through
one cycle of change is called its period (T) and T=2/
l The number of cycles per second of an alternating
current is called its frequency, n =1/T= /2
l The phase of an alternating current at any instant
represents the fraction of the time period that has elapsed
since the current last passed through the zero position
of reference. Phase can also be expressed in terms of
angle in radians.
l An alternating current or emf varies periodically from a
maximum in one direction through zero to a maximum in
the opposite direction, and so on. The maximum value
of the current or emf in either direction is called the peak
46 l Physics
Comparison between AC and DC
(a) The generation of AC is more economical than that of
(b) AC voltages can be easily stepped up or stepped down
(c) AC can be transmitted to longer distances with less
loss of energy.
(d) AC can be easily converted into DC by using rectifiers.
(e) The phenomenon of electrolysis cannot be performed
through an AC, so for the extraction of metals in
metallurgy and other works in the industrial workshops,
only DC, and not AC, can be used.
(f) In electroplating, AC is not used; it can be done by
using DC only.
(g) Unlike DC, AC cannot be stored in an accumulator cell.
(h) Electromagnets can only be prepared through DC.
In India, the AC changes direction after every 1/100
second, that is, the frequency of AC is 50 Hz.
Disadvantages of AC
(a) AC is more fatal and dangerous than DC.
(b) AC always flows on the outer layer of the conductor
(skin effect) and hence requires stranded wires.
(c) AC cannot be used in electrolysis, e.g. electroplating
Closed and Open Circuits
Open circuit: A circuit is said to be open if no current passes
through the circuit. The resistance of such a circuit is infinite.
This happens if the key is not plugged.
Closed circuit: A circuit is said to be closed if current
passes through the circuit. The resistance of such a circuit
is finite. This happens if the key is plugged.
Domestic Electric Circuits
In our homes, we receive supply of electric power through a
main supply (also called mains), either supported through
overhead electric poles or by underground cables. One of
the wires in this supply, usually with red insulation cover, is
called live wire (or positive). Another wire, with black
insulation, is called neutral wire (or negative). In our country,
the potential difference between the two is 220 V.
At the metre-board in the house, these wires pass into
an electricity meter through a main fuse. Through the main
switch they are connected to the line wires in the house.
These wires supply electricity to separate circuits within the
house. Often, two separate circuits are used, one of 15 A
current rating for appliances with higher power ratings such
as geysers, air coolers, etc. The other circuit is of 5 A current
rating for bulbs, fans, etc. The earth wire, which has insulation
of green colour, is usually connected to a metal plate deep in
the earth near the house. This is used as a safety measure,
especially for those appliances that have a metallic body, for
example, electric press, toaster, table fan, refrigerator, etc.
The metallic body is connected to the earth wire, which
provides a low-resistance conducting path for the current.
Thus, it ensures that any leakage of current to the metallic
body of the appliance keeps its potential to that of the earth,
and the user may not get a severe electric shock.
In each separate circuit, different appliances can be
connected across the live and neutral wires. Each appliance
has a separate switch to ‘ON’/‘OFF’ the flow of current
through it. In order that each appliance has equal potential
difference, they are connected parallel to each other. Electric
fuse is an important component of all domestic circuits.
A fuse in a circuit prevents damage to the appliances
and the circuit due to overloading. Overloading can occur
when the live wire and the neutral wire come into direct
contact. (This occurs when the insulation of wires is damaged
or there is a fault in the appliance.) In such a situation, the
current in the circuit abruptly increases. This is called shortcircuiting.
The use of an electric fuse prevents the electric
circuit and the appliance from a possible damage by stopping
the flow of unduly high electric current. The Joule heating
that takes place in the fuse melts it to break the electric
Overloading can also occur due to an accidental hike in
the supply voltage. Sometimes overloading is caused by
connecting too many appliances to a single socket.
Based up on electrical properties, crystalline solids are
classified as (i) conductors, (ii) semi-conductors and (iii)
Insulators and Conductors
1. Conductors: These are the substances which easily
allow the passage of electricity through them. These
materials have a large number of free electrons (–1028 m–
3) and very small resistivity (–10–8 m).
The resistivity of an ideal conductor is zero and it
increases with the rise in temperature in metals (copper,
aluminium, silver, gold etc) and decreases in non-metals
(graphite). In case of alloys of metals such as nichrome,
manganin or constantan, resistivity is more than that of
metals but varies slowly with temperature.
2. Insulators: These are the substances which have
practically no free electrons and have very high
resistivity (–1016 m). The resistivity of an ideal
insulator is infinity and decreases with the rise in
temperature. Mica, rubber, glass and porcelain are some
examples of insulators.
A semiconductor is a material that has a resistivity value in
between that of a conductor and an insulator.
The conductivity of a semiconductor material can be
Physics l 47
varied under an external electric field. Devices made from
semiconductor materials are the foundation of modern
electronics, including radio, computer, telephone, and many
other devices. Semiconductor devices include transistor,
many kinds of diodes including the light-emitting diode,
silicon-controlled rectifier, and digital and analog integrated
circuits. Solar photovoltaic panels are large semiconductor
devices that directly convert light energy into electrical
In a metallic conductor, current is carried by the flow of
electrons. In semiconductors, current can be carried either
by the flow of electrons or by the flow of positively charged
holes in the electron structure of the material. Silicon is used
to create most semiconductors commercially. So many other
materials are used, including germanium and gallium arsenide.
In semiconductor production, doping intentionally introduces
impurities into an extremely pure (intrinsic) semiconductor
for the purpose of modulating its electrical properties. The
impurities are dependent upon the type of semiconductor.
Intrinsic semiconductors are those in which impurities
are not present and are therefore called pure semiconductors.
Extrinsic semiconductors are those in which impurities
are present in large quantities.
Based on the impurities present in the extrinsic
semiconductors, they are classified into two categories:
1. n-type semiconductors and
2. p-type semiconductors
When a pentavalent substance (Group V elements) like
Phosphorus, Arsenic or Antimony is added in sufficient
quantities to the pure form of Si or Ge crystal, it is said to be
l In n-type semiconductor, electrons are majority carriers
and holes are minority carriers (ne>nh).
When a trivalent substance (Group III elements) like Boron,
Aluminium, Gallium or Indium is added in sufficient quantities
(1 in 10 or less) to the pure form of Si or Ge crystal, it is said to
l In p-type semiconductor, holes are majority carriers and
electrons are minority carriers (nh>ne).
p-n junction: When a semiconducting material such as
silicon or germanium is doped with impurity in such a way
that one side has a large number of acceptor impurities and
the other side has a large number of donor impurities, the
resulting semiconductor is called p-n junction.
Forward biased: In a p-n junction diode, if p-region is
connected to +ve terminal (relatively higher potential) of the
battery and n-region is connected to –ve terminal (relatively
lower potential) of the battery then it is said to be forward
Reverse biased: In a p-n junction diode, if p-region is
connected to –ve terminal (relatively low potential) of the
battery and n-region is connected to the +ve terminal
(relatively high potential) of the battery then it is said to be
Semiconductor devices are electronic components that exploit
the electronic properties of semiconductor materials,
principally silicon, germanium, and gallium arsenide, as well
as organic semiconductors. Semiconductor devices have
replaced thermionic devices (vacuum tubes) in most
Semiconductor devices are small in size, consume low
power, operate at low voltages and have long life and high
The Cathode Ray Tube (CRT) used in television and
computer monitor that works on the principle of vaccum tube
is being replaced by LCD (Liquid Crystal Display) monitors
with supporting solid-state electronics.
The best examples of the semiconductor devices are:
Diode and Transistor.
Diodes are made from a single piece of semiconductor material
which has a positive P-region at one end and a negative Nregion
at the other, and has a resistivity somewhere between
that of a conductor and an insulator.
Application of Diode as a device
PN diode as a rectifier
Zener diode as a voltage regulator
Photodiodes used for detecting optical signals (photo
Light emitting diodes (LED) which convert electrical
energy into light
Photovoltaic devices which convert optical radiation
into electricity (solar cells)
GUNN Diode as a sensor and measuring instrument
Light Emitting Diodes: Light emitting diodes or LEDs
are among the most widely used of all the types of diodes
available. They are the most visible type of diode that emits a
fairly narrow bandwidth of visible coloured light, invisible
infrared or laser type light when a forward current is passed
through them. A light emitting diode or LED, as it is more
commonly called, is basically just a specialized type of P-N
junction diode made from a very thin layer of fairly heavily
doped semiconductor material.
Unlike normal diodes, which are made for direction or
power rectification and which are generally made either from
Germanium or Silicon semiconductor material, light-emitting
diodes are made from compound-type semiconductor
materials such as Gallium Arsenide (GaAs), Gallium Phosphide
(GaP), Gallium Arsenide Phosphide (GaAsP), Silicon Carbide
48 l Physics
(SiC) or Gallium Indium Nitride (GaInN).
LEDs have the following advantages over conventional
incandescent low-power lamps:
(i) Low operational voltage and less power
(ii) Fast action and no warm-up time required
(iii) The bandwidth of emitted light is 100 Å to 500 Å or in
other words it is nearly (but not exactly)
(iv) Long life and ruggedness
(v)Fast on-off switching capability
Liquid Crystal Display
An LCD or Liquid Crystal Display is a flat, thin display
device consisting of any number of pixels aligned in front of
a reflector or source of light. The LCD has been widely hailed
as a prized invention as it is relatively cheap and it consumes
less power to function than competing technologies, making
it almost indispensable in battery-powered electron devices.
Types of LCD
LCDs are broadly classified as either transmissive or
reflective, depending upon the position of their source of
light. A transmissive LCD is illuminated by a light source
from the base and is viewed from the front.
Such LCDs are used in applications where high luminal
levels are required, such as computer displays, personal digital
assistance televisions, and mobile phones.
On the other hand, reflective LCDs, usually found in
digital displays of watches and calculators, are illuminated
by an external light, which in turn is reflected back by a
diffusing reflector located behind the display. As the light
has to pass twice through the liquid crystal layer, it is
attenuated twice and hence reflective LCDs produce darker
blacks than their transmissive counterparts. But, since the
same attenuating phenomenon, to an extent, happens in the
translucent part of the liquid crystal layer as well, the contrast
of the display image will be less than a transmissive LCD.
In terms of power consumption, reflective LCDs, due to
the absence of an artificial light source, are more powerefficient
than their transmissive counterparts.
There are now LCDs, which combine the basic features
of both transmissive and reflective LCDs. They are called
transflective LCDs and they operate transmissively or
reflectively depending upon the ambient light conditions.
A transistor is formed by sandwiching a thin layer of a ptype
semiconductor between two layers of n-type
semiconductors or by sandwiching a thin layer of an n-type
semiconductor between two layers of p-type semiconductors.
Transistor means “transfer of resistance” and is invented
by John Bardeen, WH Brattain and William Shockley in 1948.
Transistors are of two types:
i) n-p-n; ii) p-n-p
A transistor mainly consists of three sections:
i) emitter, ii) base, iii) collector.
A transistor has three doped regions forming two p-n
junctions between them. There are two types of transistors:
(i) n-p-n transistor: Here two segments of n-type
semiconductor (emitter and collector) are separated by a
segment of p-type semiconductor (base).
(ii) p-n-p transistor: Here two segments of p-type
semiconductor (termed as emitter and collector) are separated
by a segment of n-type semiconductor (termed as base).
1. Transistor acts as an amplifier.
An electronic device which can raise the magnitude of
current or of voltage input signals is called as amplifier.
2. Transistors are used in electronic circuits called
3. Transistors are also used in stabilized power supplies.
4. Transistors form important components of micro
electronic systems called ICs (Integrated Circuits) or
ancient times. A thin long piece of a magnet, when suspended
freely, pointed in the north-south direction. The name
lodestone (or loadstone) is given to a naturally occurring ore
of iron magnetite. The technological exploitation of this
property is generally credited to the Chinese texts dating 400
BC that mention the use of magnetic needles for navigation
Magnetic phenomena are universal in nature. Vast, distant
galaxies, tiny invisible atoms, men and beasts all are permeated
through and through with a host of magnetic fields from a
variety of sources. The earth’s magnetism pre-dates human
evolution. The word magnet is derived from the name of an
island in Greece called Magnesia, where magnetic-ore
deposits were found as early as 600 BC.
The directional property of magnets was known since
Physics l 49
A substance which attracts substances like iron, nickel, cobalt,
etc. is called a magnet.
l When a magnet is freely suspended or pivoted, it comes
to rest, showing north and south directions.
l Like poles repel and unlike poles attract each other .
l A magnet attracts substances like iron, nickel, cobalt
l A magnet imparts its properties to other magnetic
Magnets are of two types. They are:
(i) Natural magnets
(ii) Artificial magnets
l Magnets which are available in nature are called natural
l Magnetite is a natural magnet.
l It is also called lodestone.
l It is the magnetic oxide of iron.
l Its formula is Fe3O4.
Natural magnets have no regular shape. They have less
Magnets which are made by artificial methods are called
artificial magnets; e.g. bar magnets, cylindrical magnets,
horseshoe magnets, Robinson magnets, pot-shaped
l Artificial magnets have regular shape. Their magnetic
power is more.
l Horseshoe magnets are used in cycle dynamos.
l Pot-shaped magnets are used in loud speakers.
l Magnets are also used in magneto-therapy to cure some
l The two poles of the magnet are generally of equal
strength and lie just below the ends.
l The straight line joining the two poles of a magnet is
called axial line.
l The line passing through the midpoint and normal to the
axial line is called equatorial line.
l The straight line joining the poles of the magnet is known
as magnetic length.
l Magnetic length is about 5/6 times or 83.3% of the
Magnetic lines of force: A line of force in a magnetic
field is the path or curve along which a free unit North Pole
Magnetic Field and
Magnetic Field Lines
Magnetic field: The region or space around a magnet through
which any other magnetic material experiences a force of
attraction or repulsion is called magnetic field.
Characteristics of magnetic lines of force
l Magnetic lines of force start from North Pole and ends
on the South Pole outside the magnet.
l Inside the magnet, magnetic lines of force run from South
Pole to North Pole.
l They are closed loops.
l No two magnetic lines of force intersect each other.
l They have a tendency to repel each other laterally (they
have lateral elongation).
l They contract longitudinally.
l The tangent drawn to the magnetic line of force at any
point gives the direction of magnetic field at that point.
l In a uniform magnetic field, lines of force will be straight
and parallel lines.
l The number of lines of force at a region represents the
intensity of magnetic field at that region, i.e., if the field
is strong, the lines of force are crowded, whereas in weak
fields they are spaced apart.
Magnetic flux: The number of lines of force passing
through any area in a magnetic field is known as magnetic
Magnetic field induction
l The number of magnetic lines of induction passing
through unit area normal to the surface is called magnetic
flux density or magnetic field induction (B).
l The unit of B is tesla (T).
l The ratio of magnitude of intensity of magnetization (I)
in a material to that of magnetizing field (H) is called
magnetic susceptibility of that material.
l The intensity of magnetization inducted in a material by
unit magnetizing field is known as magnetic
l has no units and no dimensions.
Note: The intensity of the magnetic field or magnetising field
strength (H): ‘H’ is an auxiliary field which is measured
as the ratio of magnetic induction to the permeability of
the medium at the given point.
Absolute magnetic permeability (m)
l The ratio of magnitude of magnetic induction to
magnetising field is defined as magnetic permeability.
50 l Physics
l Magnetic permeability of a medium is the extent to which
magnetic lines of force can enter a medium. It is the
characteristic property of the magnetic material.
l Magnetic permeability represents the amplification of
magnetising field in that material.
l is always positive and is different for different materials.
l The value of depends on magnetising field.
The Earth’s Magnetism
Our earth behaves as if it were a powerful magnet. Within it
the south pole is towards the earth’s north pole and the north
pole is towards the earth’s south pole. It is supported by the
(i) A freely suspended magnetic needle stays in north-south
direction. If a magnetic needle is suspended so that it is
free to move in the horizontal plane, then its north pole
rests pointing north and the south pole pointing south.
(ii) On drawing the lines of force of a magnet, we get neutral
points, where the magnetic field due to the magnet is
exactly neutralised by the earth’s magnetic field. Had
there been no earth’s magnetic field, neutral points would
not have been available.
(iii) An iron piece buried in earth becomes a magnet. If we
bury an iron rod in the earth in the direction in which a
freely-suspended magnetic needle stays, then after some
time the rod becomes a magnet.
Magnetism in medicine
An electric current always produces a magnetic field. Even
weak ion currents that travel along the nerve cells in our
body produce magnetic fields. When we touch something,
our nerves carry an electric impulse to the muscles we need
to use. This impulse produces a temporary magnetic field.
These fields are very weak and are about one-billionth of the
earth’s magnetic field. Two main organs in the human body
where the magnetic field produced is significant, are the
heart and the brain. The magnetic field inside the body forms
the basis of obtaining the images of different body parts.
This is done using a technique called Magnetic Resonance
Imaging (MRI). The analysis of these images helps in medical
diagnosis. Magnetism has, thus, got important uses in
Magnetic Classification of Substances
On the basis of magnetic behaviour of different materials,
they are divided into categories:
(1) diamagnetic substances,
(2) paramagnetic substances and
(3) ferromagnetic substances
Diamagnetic substances are those which have tendency to
move from stronger to the weaker part of the external magnetic
field. In other words, unlike the way a magnet attracts metals
like iron, it would repel a diamagnetic substance.
Some diamagnetic materials are bismuth, copper, lead,
silicon, gold, zinc, air, hydrogen, nitrogen (at STP), water and
sodium chloride. Diamagnetism is present in all the
substances. However, the effect is so weak in most cases
that it gets shifted by other effects like paramagnetism,
The most exotic diamagnetic materials are
superconductors. These are metals cooled to very low
temperatures which exhibit both perfect conductivity and
perfect diamagnetism. Superconducting magnets can be
gainfully exploited in a variety of situations, for example, for
running magnetically levitated superfast trains.
Note: Diamagnetism is universal. It is present in all materials.
But it is weak and hard to detect if the substance is
para- or ferromagnetic.
Paramagnetic substances are those which get weakly
magnetised when placed in an external magnetic field. They
have tendency to move from a region of weak magnetic field
to strong magnetic field, i.e., they get weakly attracted to a
magnet. The individual atoms (or ions or molecules) of a
paramagnetic material possess a permanent magnetic dipole
moment of their own.
Some paramagnetic materials are aluminium, sodium,
calcium, oxygen (at STP) and copper chloride. Experimentally,
one finds that the magnetisation of a paramagnetic material is
inversely proportional to the absolute temperature T.
Ferromagnetic substances are those which get strongly
magnetised when placed in an external magnetic field. They
have a strong tendency to move from a region of weak
magnetic field to strong magnetic field, i.e., they get strongly
attracted to a magnet. The individual atoms (or ions or
molecules) in a ferromagnetic material possess a dipole
moment as in a paramagnetic material. However, they interact
with one another in such a way that they spontaneously
align themselves in a common direction over a macroscopic
volume called domain.
Alnico, an alloy of iron, aluminium, nickel, cobalt and
copper, is one such material There are a number of elements,
which are ferromagnetic: iron, cobalt, nickel, gadolinium, etc.
The ferromagnetic property depends on temperature. At
high enough temperature, a ferromagnet becomes a
Physics l 51
The temperature of transition from ferromagnetism to
paramagnetism is called the Curie temperature (TC).
1. The phenomenon of production of a magnetic field
around a current-carrying conductor is called magnetic
effect of currents. The direction of magnetic field is
found by Snow rule or Ampere's swimming rule.
2. When the current flows through a straight conductor,
the conductor behaves like a magnet.
3. If the direction of current flowing through coil is anticlockwise,
then north polarity is created, and vice versa.
A coil of many circular turns of insulted copper wire wrapped
closely in the shape of a cylinder is called a solenoid.
The intensity of the magnetic field of a solenoid can be
(i) by increasing the number of turns on the solenoid,
(ii) by increasing the strength of current flowing through
the solenoid, and
(iii) by placing soft iron core along the axis of the
Electromagnet: A coil wound over a soft iron piece is
usually called an electromagnet. The advantage of such a
magnet is that when the current is switched off, the soft iron
core loses most of its magnetism.
Electromagnets are used in electrical applicances such
as electric bell, electric fan, etc. They are used for magnetic
separation and for lifting heavy iron loads. A permanent
magnet is made from steel. Once magnetised, it cannot lose
its magnetism easily.
Electric motor: An electric motor is a device which
converts electrical energy into mechanical energy. It is
based on the principle that when a current-carrying
conductor capable of moving freely is placed in a magnetic
field, it experiences a force and begins to move in a direction
given by Fleming's left hand rule.
It does not work on the principle of electromagnetic
Electric motor is used as an important component in
electric fans, refrigerators, mixers, washing machines,
computers, MP3 players, etc.
The speed of rotation of an electric motor can be
increased by (i) increasing the number of turns in the coil, (ii)
increasing the area of cross-section of the coil, (iii) increasing
the current flowing into the coil, (iv) increasing the strength
of the radial magnetic field and (v) using soft iron core.
l The phenomenon of electromagnetic induction is the
production of induced current in a coil placed in a region
where the magnetic field changes with time. The magnetic
field may change due to a relative motion between the
coil and a magnet placed near to the coil. If the coil is
placed near a current-carrying conductor, the magnetic
field may change either due to a change in the current
through the conductor or due to the relative motion
between the coil and the conductor. The direction of the
induced current is given by Fleming’s right-hand rule.
l In the phenomenon of electromagnetic induction,
mechanical and magnetic energy are converted into
Faraday's laws of electromagnetic induction are:
(i) Whenever there is a change in magnetic flux within a
conductor, an induced emf is set up in it which gives rise to
induced current. The direction of induced current is always
opposite to the cause which produces it. (Lenz's law)
(ii) The magnitude of the emf induced is directly
proportional to the rate of change of magnetic field, the
number of turns in the coil and the area of cross-section of
The direction of induced emf is given by Fleming's right
hand rule. Fleming's right hand rule states that if the thumb,
the middle finger and the forefinger of the right hand are
stretched mutually perpendicular to each other and if the
forefinger indicates the direction of the magnetic field and
the thumb indicates the direction of motion of conductor
then the middle finger will indicate the direction of induced
Transformer: A transformer is a device used to change
the AC voltage. It converts a low voltage at a high current to
a high voltage at a low current, and vice versa.
A transformer is of two types:
(i) Step-up and (ii) Step-down
A step-up transformer increases the emf whereas the
step-down transformer decreases the emf. Transformer
works on the principle of mutual induction, i.e., whenever
the magnetic flux linked with a coil changes, an induced emf
is produced in the neighbouring coil.
The energy loss in a transformer takes place in the
(i) Increasing the current through it
(ii) Heating in the coils
(iii) Eddy currents in the core
(iv) Hysteresis loss in the core
Eddy currents are induced circular currents in the soft
iron core itself, tending to oppose the change in the magnetic
flux through it. Eddy currents produce a heating effect and
lower the efficiency of the transformer.
The ratio of the turns
is called the transformation
ratio where NS is the number of turns on output or secondary
winding and NP is the number of turns on the input or primary
52 l Physics
winding. For a step-up transformer,
> 1 and for a stepdown
In a step-up transformer, the primary coil consists of
smaller number of turns of thin wire. In a step-down
transformer, the primary coil consists of a large number of
turns of thin wire and the secondary coil consists of a smaller
number of turns of thick wire.
Transformer is used in a) power station, b) television, c)
telephone, d) telegraph and e) radio.
Based on the phenomenon of electromagnetic induction,
the experiments studied above generate induced current,
which is usually very small. This principle is also employed
to produce large currents for use in homes and industry. In
an electric generator, mechanical energy is used to rotate a
conductor in a magnetic field to produce electricity.
A generator of dynamo is a device which converts
mechanical energy into electrical energy. It is based on the
principle of electromagnetic induction.
Emission of electrons
There are four types of emission of electrons:
(i) Thermionic emission
(ii) Photoelectric emission
(iii) Field emission and
(iv) Secondary emission
The emission of electrons from a metal surface when
heat is supplied to it is called thermionic emission.
The phenomenon due to which the surface of a metal
ejects free electrons by absorbing some kind of energy is
called photoelectric emission.
The minimum energy required to emit electron from the metal
surface is called work function or threshold energy.
The number of electrons emitted per second depends
on: (i) Material of the surface, (ii) Temperature of the surface,
(iii) Its surface area.
Thermionic emitters should have the following
properties: (i) High melting point, (ii) Low work function, (iii)
Mechanically rugged, (iv) Low vapour pressure.
Some practical thermionic emitters are: (i) Pure
tungsten, (ii) Thoriated tungsten, (iii) Alkali metal oxide
Work function is measured in electron volt (eV).
1eV = 1.6 × 10–19 C × 1V or 1 eV = 1.6 × 10-19 J.
The work function of tungsten = 4.52 eV and that of
thoriated tungsten = 2.6 eV.
The minimum frequency at which a given metal can emit
photoelectrons is called threshold frequency.
Photoelectric effect is the phenomenon of emission of
electrons by metals when illuminated by light of suitable
The minimum energy needed by an electron to come out
from a metal surface is called the work function of the metal.
Energy greater than the work function (f) required for electron
emission from the metal surface can be supplied by suitably
heating or applying strong electric field or irradiating it with
light of suitable frequency.
Certain metals respond to ultraviolet light while others
are sensitive even to the visible light. Photoelectric effect
involves conversion of light energy into electrical energy. It
follows the law of conservation of energy. The photoelectric
emission is an instantaneous process and possesses certain
Photoelectric current depends on (i) the intensity of
incident light, (ii) the potential difference applied between
the two electrodes, and (iii) the nature of the emitting
The device which converts light energy into electric
energy is called photoelectric cell.
A photoelectric cell uses the photoelectric effect. It
converts light energy into electric energy. A photocell
consists of a cathode coated with an alkali metal. Opposite
to it a collector is placed. These are arranged in an evacuated
When light falls on alkali metal photoelectrons are
liberated. They are attracted by the collector due to the
Physics l 53
positive potential on it and current flows through the circuit.
The changes in current are according to the changes in the
light falling on alkali metal.
Uses of photocells:
l In automatic switching on and off of street lights
l In photometry, they are used to compare the illuminating
powers of two sources.
l They are used in fire alarms and burglar’s alarms.
l In meteorology, they are used to record the intensity of
l The photocells inserted in the street light electric circuit
are used to switch on and off the street lighting system
automatically at dusk and dawn.
l Photocells are used in the control of a counting device,
which records every interruption of the light beam. So
photocells help count the persons entering a temple or
l They are used to reproduce sound in cinematograph
and in the television camera for scanning and telecasting
l They are also used in automatic opening and closure of
l They are used in solar arrays to generate electricity.
l They are used in controlling the temperature of furnaces.
l They are used in industries for detecting minor flaws of
holes in metal sheets.
l They are used for detection of traffic law defaulters.
l The temperature of celestial bodies is measured and
their spectra are studied by photocells.
Natural radioactivity is defined as the spontaneous
disintegration of a nucleus with the emission of certain
particles and radiations.
Radioactivity is unaltered by: (i) Strongest chemical or
physical treatments, (ii) Excessive heating or cooling, (iii)
Strong electric or magnetic fields.
In radioactivity, , and rays are emitted.
In radioactivity, the nucleus which breaks is called the
parent nucleus and the one which is formed as a result of
decay is called the daughter nucleus.
and are positively and negatively charged particles
respectively but -rays is an electro-magnetic wave
During and particles emissions, the emitting nucleus
undergoes a change in its atomic number and mass number,
but during -emission no such change takes place.
Alpha and beta particles are deflected by electric and
Radioactivity is used to cure diseases like leukemia and
cancer by radiation therapy. The radio isotopes are used as
fertilisers for plants. They are used to study wear and tear of
piston rings and gears in engine. They are used to provide
The isotope of carbon-14 has been used for carbon
dating to estimate the age of rocks and trees.
Properties of radiations: Some properties of a,
and are as shown in the table below:
S.No. Property Alpha Particles Beta Particles Gamma Rays
1. Nature These are helium nuclei These are fast moving These are electromagnetic
2. Charge +3.2 × 10-19 C (+2e) –1.6 × 10–19 C (e) Zero
3. Rest mass 6.6 × 10–27 kg (4m) 9.1 × 10–31 kg Zero
4. Penetrating power Minimum, can hardly 100 times that of alpha, 100 times that of beta, can
penetrate can travel 10 cm through penetrate through several
matter cm of lead
5. Ionising power 100 times that of beta 100 times that of gamma Minimum
6. Effect of electric Are deflected by electric Are deflected by electric Undeflected by electric
and magnetic fields and magnetic fields and magnetic fields and magnetic fields
7. Photographic Can blacken a photo- Can blacken a photo- Can blacken a photoeffect
graphic plate graphic plate graphic plate
8. Fluorescence Can cause fluorescence Can cause fluorescence Can cause fluorescence
9. Burning effect Causes burning effect Harmful to mankind Very harmful
10. Speed Speed of 15,000 km per About 33% to 99% of Travel with the velocity
second the velocity of light of light
54 l Physics
Fission is the splitting up of the nucleus of a heavy atom into
two roughly equal fragments, accompanied by the release of
energy. For example, uranium-235 splits up when it captures a
slow neutron according to the fission reaction
U n Ba Kr 31 n
In this reaction, the total mass on the left-hand side is
more than the total mass on the right-hand side. This excess
mass is converted into energy in accordance with Einstein's
mass-energy relation, E = mc2. The energy released in the
fission of one nucleus of uranium-235 is nearly 200 million
electron volts (1 electron volt = 1.6 × 10-19 joules). This is an
enormous amount of energy. The energy produced on
complete fission of just one gram of uranium-235 is equivalent
to that from an electric power plant operating at one megawatt
for nearly one day.
If the neutrons produced in the fission reaction are slowed
down, they may produce further fission and, thus, start a
chain reaction. However, if the uranium-235 lump is small,
many neutrons escape from its surface without producing
fission and, therefore, a chain reaction does not develop.
The size of the material that sustains a chain reaction is called
the critical size, the mass of which is called critical mass. If
the mass of fissile material is greater than the critical mass,
the chain reaction takes place so fast that an explosion occurs.
Atomic Bomb: In an atomic bomb, two subcritical masses of
uranium-235 (or plutonium 239) are brought together in less
than a microsecond. Since the combined mass exceeds the
critical mass, a violent explosion takes place. In such
explosions, temperatures as high as 107° C or even more are
produced. Tremendous air blasts and intense radioactivity
cause destruction. It is interesting to note that uranium-235
used in the Hiroshima blast was only of the size of a cricket
Enriched Uranium: For an atomic bomb, fissile uranium-235
is needed. Natural uranium contains only 0.7% of uranium-
235. The rest of it is uranium-238, which is not fissile. Therefore,
uranium-235 has to be separated from natural uranium as far
as possible. Uranium with an abundance of the uranium-235
isotope is known as enriched uranium.
For nuclear reactors, enriched uranium having nearly
6% U-235 is required. However, for nuclear bombs, highly
enriched uranium (HEU) containing nearly 90% U-235 is used.
Nuclear Reactor: A nuclear reactor is a device in which fission
occurs at a controlled rate. Common features of a nuclear
(i) Nuclear fuel, generally uranium, that has been somewhat
enriched in uranium-235 isotope
(ii) A moderator to slow down fast neutrons. Usually,
graphite or heavy water is used as moderator.
(iii) A control device to control the flow of neutrons by
absorbing some of them. Generally, boron or cadmium
rods, that can be moved in or out of the reactor, are used
for this purpose.
When proper adjustments are made in a reactor such that
every fission reaction leads to, on an average, one further
reaction, the reactor is said to have become 'critical' and is
ready to produce controlled energy.
In several countries, including India, nuclear reactors are being
used to produce electricity. Besides, reactors are used to
produce radioisotopes. Reactors are also used to convert
uranium-238 into plutonium-239, which is fissile and used for
Breeder Reactor: A reactor that produces more fissionable
material than it burns is called a breeder reactor. These reactors
fuelled initially with 238U 239Pu or 232Th 233U operate
subsequently with the addition of 235U or 232Th, which are
much more abundant than the only naturally occurring
fissionable material, 235U.
Nuclear Fusion: The combining of the nuclei of light atoms
to form heavier nuclei with the release of energy is termed
nuclear fusion. Nuclear fusion takes place in the sun and
other stars and is one of the important sources of stellar
energy. A typical fusion reaction is
H H He 1n energy
As in a fusion reaction, here also, the surplus mass is
converted into energy. An extremely high temperature, such
as that in the sun, is required for fusion to take place. On the
earth, fusion reaction occurs during the explosion of a
hydrogen bomb, which requires an atomic bomb for its
Research is currently going on to evolve the technique of
controlled fusion. Efforts are being made to achieve fusion of
the hydrogen isotope using laser beams.
Uncontrolled fusion reactions
Hydrogen bomb: It is based on the phenomenon of nuclear
fusion and was made in 1952 by American scientists. The
central core of a hydrogen bomb is a uranium (or plutonium)
fission bomb which is surrounded by a compound of heavy
hydrogen, like lithium hydride (LiH2). When the fission bomb
is exploded, it produces such a high temperature and pressure
that the heavy hydrogen nuclei come extremely close and
fuse together, liberating huge energy.
1H3 + 1H2 2He4 + 0n1 + 17.6 MeV
The sun has continuously been emitting light and heat at a
very high rate for crores of years. The emission of such high
energy by chemical reactions is impossible. The reason is
that even if the sun was made entirely of carbon, its complete
combustion would have supplied energy at such a high rate
only for a few thousand years.