NCERT Physics for UPSC

1. Chapter 11_1: WHAT IS PHYSICS?
 Johannes Kepler (1571-1630) examined the extensive data on planetary motion
collected by Tycho Brahe (1546-1601), the planetary circular orbits in heliocentric
theory (sun at the centre of the solar system) imagined by Nicolas Copernicus (1473–
1543) had to be replaced by elliptical orbits to fit the data better.
 A radically new theory (Quantum Mechanics) to deal with atomic and molecular
phenomena.
 Experiment of scattering of alpha particles by gold foil, in 1911 by Ernest Rutherford
(1871–1937) established the nuclear model of the atom, which then became the basis of
the quantum theory of hydrogen atom given in 1913 by Niels Bohr (1885–1962).
 The concept of antiparticle was first introduced theoretically by Paul Dirac (1902–1984)
in 1930 and confirmed two years later by the experimental discovery of positron
(antielectron) by Carl Anderson.
The word Physics comes from a Greek word meaning nature. Its Sanskrit equivalent is Bhautiki
that is used to refer to the study of the physical world.
Physics as a study of the basic laws of nature and their manifestation in different natural
phenomena.
Two principal thrusts in physics: unification and reduction.
 The same law of gravitation (given by Newton) describes the fall of an apple to the
ground, the motion of the moon around the earth and the motion of planets around the
sun.
 The basic laws of electromagnetism (Maxwell’s equations) govern all electric and
magnetic phenomena. The attempts to unify fundamental forces of nature ref lect this
same quest for unification.
 A related effort is to derive the properties of a bigger, more complex, system from the
properties and interactions of its constituent simpler parts. This approach is called
reductionism and is at the heart of physics.
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 macroscopic phenomena and includes subjects like
Mechanics, Electrodynamics, Optics and Thermodynamics.
 Mechanics 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.
 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.
 Optics deals with the phenomena involving light. The working of telescopes and
microscopes, colours exhibited by thin films, etc., are topics in optics

2.  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.
Recently, the domain intermediate between the macroscopic and the microscopic (the so-called
mesoscopic physics), dealing with a few tens or hundreds of atoms, has emerged as an exciting
field of research.

3. PHYSICS, TECHNOLOGY AND SOCIETY
 A hypothesis, assuming that it is true. It can be verified and substantiated by
experiments and observations.
 An axiom is a self-evident truth while a model is a theory proposed to explain observed
phenomena.
 Euclid’s statement that parallel lines never meet is a hypothesis.
 As late as 1933, the great physicist Ernest Rutherford had dismissed the possibility of
tapping energy from atoms.
 In 1938, Hahn and Meitner discovered the phenomenon of neutron-induced fission of
uranium, which would serve as the basis of nuclear power reactors and nuclear
weapons.
FUNDAMENTAL FORCES IN NATURE: Four fundamental forces in nature.
 Gravitational Force
 Electromagnetic Force
 Strong Nuclear Force
 Weak Nuclear Force
 Towards Unification of Forces

4. Gravitational Force
 The gravitational force is the force of mutual attraction between any two objects by
virtue of their masses. It is a universal force.
 It plays a key role in the large-scale phenomena of the universe, such as formation and
evolution of stars, galaxies and galactic cluster.
Electromagnetic Force
 Electromagnetic force is the force between charged particles. In the simpler case when
charges are at rest, the force is given by Coulomb’s law: attractive for unlike charges and
repulsive for like charges.
 Electric and magnetic effects are, in general, inseparable – hence the name
electromagnetic force.
 Matter, as we know, consists of elementary charged constituents like electrons and
protons.
 Gravity is always attractive, while electromagnetic force can be attractive or repulsive.
Strong Nuclear Force
 The strong nuclear force binds protons and neutrons in a nucleus.
 The strong nuclear force is the strongest of all fundamental forces, about 100 times the
electromagnetic force in strength. Its range is, however, extremely small, of about
nuclear dimensions (10 –15 m). It is responsible for the stability of nuclei.
 Recent developments have, however, indicated that protons and neutrons are built out
of still more elementary constituents called quarks.
Weak Nuclear Force
Nuclear processes such as the β-decay of a nucleus. In β-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.
Towards Unification of Force
 Newton unified terrestrial and celestial domains under a common law of gravitation.
 Oersted and Faraday showed that electric and magnetic phenomena are in general
inseparable.
 Maxwell unified electromagnetism and optics with the discovery that light is an
electromagnetic wave.
 The electromagnetic and the weak nuclear force have now been unified and are seen as
aspects of a single ‘electro-weak’ force.
NATURE OF PHYSICAL LAWS
 For motion under an external conservative force, the total mechanical energy i.e. the
sum of kinetic and potential energy of a body is a constant.

5.  The law of conservation of energy is thought to be valid across all domains of nature,
from the microscopic to the macroscopic. It is routinely applied in the analysis of atomic,
nuclear and elementary particle processes.
 A chemical reaction is basically a rearrangement of atoms among different molecules.
 According to Einstein’s theory, mass m is equivalent to energy E given by the relation E=
mc 2, where c is speed of light in vacuum.
 In a nuclear process mass gets converted to energy (or vice-versa). This is the energy
which is released in a nuclear power generation and nuclear explosions.
 Energy is a scalar quantity. But all conserved quantities are not necessarily scalars. The
total linear momentum and the total angular momentum (both vectors) of an isolated
system are also conserved quantities.
 Using the conservation laws of energy and momentum for β-decay, Wolfgang Pauli
(1900-1958) correctly predicted in 1931 the existence of a new particle (now called
neutrino) emitted in β-decay along with the electron.
 Symmetry of nature with respect to translation (i.e. displacement) in time is equivalent
to the law of conservation of energy.

6. Chapter 7_4: Heat:
 The thermometer that measures our body temperature is called a clinical thermometer.
It has a bulb at one end. This bulb contains mercury. A clinical thermometer reads
temperature from 35°C to 42°C.
 The normal temperature of human body is 37°C.
 The range of a laboratory thermometer is generally from –10°C to 110°C.
 Different types of thermometers are used for different purposes. The maximum and
minimum temperatures of the previous day, reported in weather reports, are measured
by a thermometer called the maximum-minimum thermometer.
 A clinical thermometer has a kink in it. It prevents mercury level from falling on its own.
 Heat flows from a hotter object to a colder object. The process by which heat is
transferred from the hotter end to the colder end of an object is known as conduction.
 In solids, generally, the heat is transferred by the process of conduction.
 The materials which allow heat to pass through them easily are conductors of heat. For
examples, aluminium, iron and copper.
 The materials which do not allow heat to pass through them easily are poor conductors
of heat such as plastic and wood. Poor conductors are known as insulators.
 The water and air are poor conductors of heat.
 When water is heated, the water near the flame gets hot. Hot water rises up. The cold
water from the sides moves down towards the source of heat. This mode of heat transfer
is known as convection.
 The air from the sea is called the sea breeze. The cool air from the land moves towards
the sea. This is called the land breeze.
 From the sun the heat comes to us by another process known as radiation. The transfer
of heat by radiation does not require any medium. It can take place whether a medium is
present or not.
 The Celsius scale was devised by a Swedish astronomer, Anders Celsius in 1742.

7. Chapter 7_13: Motion and Time:
 The distance covered by an object in a unit time as the speed of the object.
 Speed is the total distance covered divided by the total time taken.
 An object moving along a straight line with a constant speed is said to be in uniform
motion.
 One of the most well-known periodic motions is that of a simple pendulum. The to and
fro motion of a simple pendulum is an example of a periodic or an oscillatory motion.
The time taken by the pendulum to complete one oscillation is called its time period.
 The symbols of all units are written in singular.
 The name of famous scientist Galileo (A.D. 1564 –1642)
 One microsecond is one millionth of a second. A nanosecond is one billionth of a second.
Clocks that measure such small time intervals are used for scientific research.
Chapter 9_8 Motion:
 The simplest type of motion is the motion along a straight line.
 The shortest distance measured from the initial to the final position of an object is
known as the displacement
 Thus, two different physical quantities — the distance and the displacement, are used to
describe the overall motion of an object and to locate its final position with reference to
its initial position at a given time
 As the object covers equal distances in equal intervals of time, it is said to be in uniform
motion
 To specify the speed of an object, we require only its magnitude. The speed of an object
need not be constant. In most cases, objects will be in non-uniform motion. Therefore,
we describe the rate of motion of such objects in terms of their average speed.
 The rate of motion of an object can be more comprehensive if we specify its direction of
motion along with its speed. The quantity that specifies both these aspects is called
velocity
 Velocity is the speed of an object moving in a definite direction
 Speed and velocity have the same units, that is, m s –1 or m/s
 During uniform motion of an object along a straight line, the velocity remains constant
with time.

8.  To answer such a question, we have to introduce another physical quantity called
acceleration, which is a measure of the change in the velocity of an object per unit time.
 This kind of motion is known as accelerated motion. The acceleration is taken to be
positive if it is in the direction of velocity and negative when it is opposite to the
direction of velocity. The SI unit of acceleration is m s –2
 When an object moves along a straight line with uniform acceleration, it is possible to
relate its velocity, acceleration during motion and the distance covered by it in a certain
time interval by a set of equations known as the equations of motion.
Chapter 9_9: FORCE AND LAWS OF MOTION

9. Chapter 7_14: Electric Current and its Effects:
 Thomas Alva Edison (A.D. 1847 – 1931) The credit for the invention of the electric
bulb, He made some 1300, inventions including the electric bulb, gramophone, the
motion picture camera and the carbon transmitter, which facilitated the invention of the
telephone.
 Hans Christian Oersted was the first person who noticed the deflection of compass
needle every time the current was passed through the wire.
 These days Miniature circuit breakers (MCBs) are increasingly being used in place of
fuses. These are switches which automatically turn off when current in a circuit exceeds
the safe limit.
Chapter 10_12: Electricity
 The electrons move only if there is a difference of electric pressure – called the potential
difference – along the conductor.
 The SI unit of electric potential difference is volt (V), named after Alessandro Volta
(1745 –1827), an Italian physicist.
 In 1827, a German physicist Georg Simon Ohm (1787–1854) found out the relationship
between the current I, flowing in a metallic wire and the potential difference across its
terminals. He stated that the electric current flowing through a metallic wire is directly
proportional to the potential difference V, across its ends provided its temperature
remains the same. This is called Ohm’s law
 R is a constant for the given metallic wire at a given temperature and is called its
resistance.
 The current through a resistor is inversely proportional to its resistance. If the
resistance is doubled the current gets halved. In many practical cases it is necessary to
increase or decrease the current in an electric circuit. A component used to regulate
current without changing the voltage source is called variable resistance. In an electric
circuit, a device called rheostat is often used to change the resistance in the circuit.
 Motion of electrons through a conductor is retarded by its resistance. A component of a
given size that offers a low resistance is a good conductor. A conductor having some
appreciable resistance is called a resistor. A component of identical size that offers a
higher resistance is a poor conductor. An insulator of the same size offers even higher
resistance

10.  FACTORS ON WHICH THE RESISTANCE OF A CONDUCTOR DEPEND
 The ammeter reading decreases to one-half when the length of the wire is doubled. The
ammeter reading is increased when a thicker wire of the same material and of the same
length is used in the circuit. A change in ammeter reading is observed when a wire of
different material of the same length and the same area of cross-section is used
 Resistance of the conductor depends (i) on its length, (ii) on its area of cross-section,
and (iii) on the nature of its material.
 Resistance of a uniform metallic conductor is directly proportional to its length (l) and
inversely proportional to the area of cross-section (A)
 RESISTANCE OF A SYSTEM OF RESISTORS
 In a series combination of resistors the current is the same in every part of the circuit or
the same current through each resistor.
 The reciprocal of the equivalent resistance of a group of resistances joined in parallel is
equal to the sum of the reciprocals of the individual resistances
 HEATING EFFECT OF ELECTRIC CURRENT
 If the electric circuit is purely resistive, that is, a configuration of resistors only
connected to a battery; the source energy continually gets dissipated entirely in the form
of heat. This is known as the heating effect of electric current. This effect is utilised in
devices such as electric heater, electric iron etc
 This is known as Joule’s law of heating. The law implies that heat produced in a resistor
is (i) directly proportional to the square of current for a given resistance, (ii) directly
proportional to resistance for a given current, and (iii) directly proportional to the time
for which the current flows through the resistor.

11. Chapter 10_13: Magnets:
 MAGNETIC FIELD AND FIELD LINES
 The region surrounding a magnet, in which the force of the magnet can be detected, is
said to have a magnetic field. The lines along which the iron filings align themselves
represent magnetic field lines
 An electric current through a metallic conductor produces a magnetic field around it.
 The magnitude of the magnetic field produced at a given point increases as the current
through the wire increases.
 the magnetic field produced by a given current in the conductor decreases as the
distance from it increases.
 FORCE ON A CURRENT-CARRYING CONDUCTOR IN A MAGNETIC FIELD
 An electric current flowing through a conductor produces a magnetic field. The field so
produced exerts a force on a magnet placed in the vicinity of the conductor. French
scientist Andre Marie Ampere (1775–1836) suggested that the magnet must also exert
an equal and opposite force on the current-carrying conductor.
 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).
 An electric motor is a rotating device that converts electrical energy to mechanical
energy.
 A device that reverses the direction of flow of current through a circuit is called a
commutator. In electric motors, the split ring acts as a commutator.
 The commercial motors use (i) an electromagnet in place of permanent magnet; (ii)
large number of turns of the conducting wire in the current carrying coil; and (iii) a soft
iron core on which the coil is wound. The soft iron core, on which the coil is wound, plus
the coils, is called an armature.
 ELECTROMAGNETIC INDUCTION
 Michael Faraday. In 1831, Faraday made an important breakthrough by discovering how
a moving magnet can be used to generate electric currents.
 Motion of a magnet with respect to the coil produces an induced potential difference,
which sets up an induced electric current in the circuit.
 In an electric generator, mechanical energy is used to rotate a conductor in a magnetic
field to produce electricity.
 The difference between the direct and alternating currents is that the direct current
always flows in one direction, whereas the alternating current reverses its direction
periodically. Most power stations constructed these days produce AC. In India, the AC
changes direction after every 1/100 second, that is, the frequency of AC is 50 Hz. An
important advantage of AC over DC is that electric power can be transmitted over long
distances without much loss of energy.

13. Chapter 7_15: Light:
Class 8_16: LIGHTS:
 The light ray, which strikes any surface, is called the incident ray.
 The ray that comes back from the surface after reflection is known as the reflected ray.
 A ray of light is an idealization. For simplicity, we use the term ray for a narrow beam of
light between the normal and incident ray is called the angle of incidence (∠i)
 The angle between the normal and the reflected ray is known as the angle of reflection
(∠r).
 The angle of incidence is always equal to the angle of reflection. This is known as the law
of reflection.
 The incident ray, the normal at the point of incidence and the reflected ray all lie in the
same plane. This is another law of reflection.
 An image formed by a mirror the left of the object appears on the right and the right
appears on the left. This is known as lateral inversion.
 When all the parallel rays reflected from a plane surface are not parallel, the reflection is
known as diffused or irregular reflection.
 Reflection from a smooth surface like that of a mirror is called regular reflection
 The objects which shine in the light of other objects are called illuminated objects. Ex
The moon.
 The objects which emit their own light are known as luminous objects. Ex such as the
sun, fire, flame of a candle and an electric lamp.
 The periscope makes use of two plane mirrors. Periscopes are used in submarines, tanks
and also by soldiers in bunkers to see things outside.
 Number of images formed by mirrors placed at an angle to one another is used in a
kaleidoscope to make numerous beautiful patterns.

14.  The mirror and water form a prism. This breaks up the light into its colours. Splitting of
light into its colours is known as dispersion of light. Rainbow is a natural phenomenon
showing dispersion.
 The lens focuses light on the back of the eye, on a layer called retina. Retina contains
several nerve cells. Sensations felt by the nerve cells are then transmitted to the brain
through the optic nerve. There are two kinds of cells (i) cones, which are sensitive to
bright light and (ii) rods, which are sensitive to dim light.
 The impression of an image does not vanish immediately from the retina. It persists
there for about 1/16th of a second, a rate faster than 16 per second, then the eye
perceives this object as moving.
 Eyes with eyelids to protect from any object entering the eye. Eyelids also shut out light
when not required.
 Lack of vitamin A in foodstuff is responsible for many eye troubles. Raw carrots, broccoli
and green vegetables (such as spinach) and cod liver oil are rich in vitamin A. Eggs, milk,
curd, cheese, butter and fruits such as papaya and mango are also rich in vitamin A.
 Resources can be of two types: Non-optical aids and optical aids.
 Louis Braille, himself a visually challenged person, developed a system for visually
challenged persons and published it in 1821. The present system was adopted in
1932. There is Braille code for common languages, mathematics and scientific
notation. Many Indian languages can be read using the Braille system. Braille
system has 63 dot patterns or characters.

15. Chapter 10_10: Light
If an opaque object on the path of light becomes very small, light has a tendency to bend
around it and not walk in a straight line – an effect known as the diffraction of light.
 The angle of incidence is equal to the angle of reflection, and
 The incident ray, the normal to the mirror at the point of incidence and the reflected ray,
all lie in the same plane. These laws of reflection are applicable to all types of reflecting
surfaces including spherical surfaces.
 Image formed by a plane mirror is always virtual and erect.
A spherical mirror, whose reflecting surface is curved inwards, that is, faces towards the
centre of the sphere, and is called a concave mirror.
A spherical mirror, whose reflecting surface is curved outwards, is called a convex
mirror.
 The diameter of the reflecting surface of spherical mirror is called its aperture.
 A set of sign conventions called the New Cartesian Sign Convention.
 Magnification produced by a spherical mirror gives the relative extent to which the
image of an object is magnified with respect to the object size.
o A negative sign in the value of the magnification indicates that the image is real.
o A positive sign in the value of the magnification indicates that the image is
virtual.
 In comparing two media, the one with the larger refractive index is optically denser
medium than the other. The other medium of lower refractive index is optically
rarer.
 The speed of light is higher in a rarer medium than a denser medium.
 A transparent material bound by two surfaces, of which one or both surfaces are
spherical, forms a lens. A lens may have two spherical surfaces, bulging outwards.
Such a lens is called a double convex lens. It is simply called a convex lens. Hence
convex lenses are called converging lenses.
 Similarly, a double concave lens is bounded by two spherical surfaces, curved
inwards. Such lenses are called diverging lenses. A double concave lens is simply
called a concave lens.
 The power of a convex lens is positive and that of a concave lens is negative.
 Lenses form images by refracting light.

16. Chapter 8_11: Force and Pressure:
 In science, a push or a pull on an object is called a force
 Forces are due to an Interaction- An interaction of one object with another object
results in a force between the two objects.
 Exploring Forces- Forces applied on an object in the same direction add to one another.
 If the two forces act in the opposite directions on an object, the net force acting on it is
the difference between the two forces.
 The strength of a force is usually expressed by its magnitude.
 A Force can Change the State of Motion- A change in either the speed of an object, or
its direction of motion, or both, is described as a change in its state of motion. Thus, a
force may bring a change in the state of motion of an object
 State of Motion The state of motion of an object is described by its speed and the
direction of motion. The state of rest is considered to be the state of zero speed. An
object may be at rest or in motion; both are its states of motion.

17. Force can Change the Shape of an Object- the application of force on an object may change its
shape
A force
 May make an object move from rest.
 May change the speed of an object if it is moving.
 May change the direction of motion of an object.
 May bring about a change in the shape of an object.
 May cause some or all of these effects.
An object cannot move by itself, it cannot change speed by itself, it cannot change direction by
itself and its shape cannot change by itself.
 Contact Forces- The force resulting due to the action of muscles is known as the
muscular force
 Since muscular force can be applied only when it is in contact with an object, it is also
called a contact force
 Friction- The force responsible for changing the state of motion of objects is the force of
friction.
 The force of friction always acts on all the moving objects and its direction is always
opposite to the direction of motion.
 Non-contact Forces- Attraction or repulsion between objects can also be seen as
another form of pull or push.
 A magnet can exert a force on another magnet without being in contact with it. The force
exerted by a magnet is an example of a non-contact force. Similarly, the force exerted by
a magnet on a piece of iron is also a noncontact force.
 Electrostatic Force- The force exerted by a charged body on another charged or
uncharged body is known as electrostatic force.
 This force comes into play even when the bodies are not in contact. The electrostatic
force, therefore, is another example of a non-contact force.
 Gravitational Force- Objects or things fall towards the earth because it pulls them. This
force is called the force of gravity, or just gravity.
 Gravity is not a property of the earth alone. In fact, every object in the universe, whether
small or large, exerts a force on every other object. This force is known as the
gravitational force.
 Pressure- The force acting on a unit area of a surface is called pressure.
 Pressure = force / area on which it acts.
 At this stage we consider only those forces which act perpendicular to the surface on
which the pressure is to be computed
 Pressure exerted by Liquids and Gases- liquids exert pressure on the walls of the
container, gases too exert pressure on the walls of their container
 The pressure exerted by this air is known as atmospheric pressure.

18. Chapter 9_10: GRAVITATION

19. Chapter 9_11: WORK AND ENERGY
Chapter 8_12: Friction:
 The force of friction always opposes the applied force.
 Spring balance is a device used for measuring the force acting on an object.
 The friction is caused by the interlocking of irregularities in the two surfaces.
 Even those surfaces which appear very smooth have a large number of minute
irregularities on them. Irregularities on the two surfaces lock into one another.
 When we attempt to move any surface, we have to apply a force to overcome
interlocking.
 On rough surfaces, there are a larger number of irregularities. So the force of friction is
greater if a rough surface is involved.

20.  The force required to overcome friction at the instant an object starts moving from rest
is a measure of static friction. On the other hand, the force required to keep the object
moving with the same speed is a measure of sliding friction. Friction can also produce
heat.
 Increasing and Reducing Friction- The substances which reduce friction are called
lubricants. Friction can never be entirely eleminated. No surface is perfectly smooth.
Some irregularities are always there
 When one body rolls over the surface of another body, the resistance to its motion is
called the rolling friction.
 Rolling reduces friction- Since the rolling friction is smaller than the sliding friction,
sliding is replaced in most machines by rolling by the use of ball bearings.
 Fluid Friction- In science, the common name of gases and liquids is fluids. So we can say
that fluids exert force of friction on objects in motion through them.
 The frictional force exerted by fluids is also called drag.
Chapter 8_13: Sound:
 The to and fro or back and forth motion of an object is termed as vibration.
 In humans, the sound is produced by the voice box or the larynx.
 It is at the upper end of the windpipe. Two vocal cords are stretched across the voice
box or larynx in such a way that it leaves a narrow slit between them for the passage of
air.

21.  The vocal cords in men are about 20mm long. In women these are about 5mm shorter.
Children have very short vocal cords. This is the reason why the voices of men, women
and children are different
 The to and fro motion of an object is known as vibration. This motion is also called
oscillatory motion. The number of oscillations per second is called the frequency of
oscillation. Frequency is expressed in hertz. Its symbol is Hz. A frequency of 1 Hz is one
oscillation per second.
 Amplitude and frequency are two important properties of any sound.
 Loudness of sound is proportional to the square of the amplitude of the vibration
producing the sound. For example, if the amplitude becomes twice, the loudness
increases by a factor of 4. The loudness is expressed in a unit called decibel (dB).
 Above 80 dB the noise becomes physically painful.
 The loudness of sound depends on its amplitude. When the amplitude of vibration is
large, the sound produced is loud. When the amplitude is small, the sound produced is
feeble
 The frequency determines the shrillness or pitch of a sound. If the frequency of vibration
is higher we say that the sound is shrill and has a higher pitch. If the frequency of
vibration is lower, we say that the sound has a lower pitch
 Audible and Inaudible Sounds
 The fact is that sounds of frequencies less than about 20 vibrations per second (20 Hz)
cannot be detected by the human ear. Such sounds are called inaudible. On the higher
side, sounds of frequencies higher than about 20,000 vibrations per second (20 kHz) are
also not audible to the human ear. Thus, for human ear, the range of audible
frequencies is roughly from 20 to 20,000 Hz.

22. Chapter 9_12: Sound
 Sound waves are characterised by the motion of particles in the medium and are called
mechanical waves
 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),
 When the vibrating object moves backwards, it creates a region of low pressure called
rarefaction (R)
 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
 Sound is a mechanical wave and needs a material medium like air, water, steel etc. for its
propagation.
 The particles do not move from one place to another but they simply oscillate back and
forth about their position of rest. This is exactly how a sound wave propagates, hence
sound waves are longitudinal waves
 In a transverse wave particles do not oscillate along the line of wave propagation but
oscillate up and down about their mean position as the wave travels
 Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound
 Frequencies higher than 20 kHz are called ultrasonic sound or ultrasound.
• Ultrasonic waves are made to reflect from various parts of the heart and form
the image of the heart. This technique is called ‘echocardiography’.
 Ultrasound scanner is an instrument which uses ultrasonic waves for getting images of
internal organs of the human body. This technique is called ‘ultrasonography’.

23. Chapter 8_14: Chemical Effects of Electric Current:
 Actually, under certain conditions most materials can conduct. That is why it is
preferable to classify materials as good conductors and poor conductors instead of
classifying as conductors and insulators.
 When salt is dissolved in distilled water, we obtain salt solution. This is a conductor of
electricity.
 Small amounts of mineral salts present naturally in water are beneficial for human
health. However, these salts make water conducting. So, we should never handle
electrical appliances with wet hands or while standing on a wet floor.
 In 1800, a British chemist, William Nicholson (1753–1815), had shown that if electrodes
were immersed in water, and a current was passed, bubbles of oxygen and hydrogen
were produced. Oxygen bubbles formed on the electrode connected to the positive
terminal of the battery and hydrogen bubbles formed on the other electrode.
 The passage of an electric current through a conducting solution causes chemical
reactions. As a result, bubbles of a gas may be formed on the electrodes. Deposits of
metal may be seen on electrodes. Changes of colour of solutions may occur.
 Copper gets transferred from one electrode to the other.
 Iron is used in bridges and automobiles to provide strength. However, iron tends to
corrode and rust. So, a coating of zinc is deposited on iron to protect it from corrosion
and formation of rust.