1.1 WHAT IS PHYSICS ?

Humans have always been curious about the world around

them. The night sky with its bright celestial objects has

fascinated humans since time immemorial. The regular

repetitions of the day and night, the annual cycle of seasons,

the eclipses, the tides, the volcanoes, the rainbow have always

been a source of wonder. The world has an astonishing variety

of materials and a bewildering diversity of life and behaviour.

The inquiring and imaginative human mind has responded

to the wonder and awe of nature in different ways. One kind

of response from the earliest times has been to observe the

physical environment carefully, look for any meaningful

patterns and relations in natural phenomena, and build and

use new tools to interact with nature. This human endeavour

led, in course of time, to modern science and technology.

The word Science originates from the Latin verb Scientia

meaning ‘to know’. The Sanskrit word Vijñãn and the Arabic

word Ilm convey similar meaning, namely ‘knowledge’.

Science, in a broad sense, is as old as human species. The

early civilisations of Egypt, India, China, Greece, Mesopotamia

and many others made vital contributions to its progress.

From the sixteenth century onwards, great strides were made

in science in Europe. By the middle of the twentieth century,

science had become a truly international enterprise, with

many cultures and countries contributing to its rapid growth.

What is Science and what is the so-called Scientific

Method? Science is a systematic attempt to understand

natural phenomena in as much detail and depth as possible,

and use the knowledge so gained to predict, modify and

control phenomena. Science is exploring, experimenting and

predicting from what we see around us. The curiosity to learn

about the world, unravelling the secrets of nature is the first

step towards the discovery of science. The scientific method

involves several interconnected steps : Systematic

observations, controlled experiments, qualitative and

CHAPTER ONE

PHYSICAL WORLD

1.1 What is physics ?

1.2 Scope and excitement of

physics

1.3 Physics, technology and

society

1.4 Fundamental forces in

nature

1.5 Nature of physical laws

Summary

Exercises

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PHYSICS2

quantitative reasoning, mathematical

modelling, prediction and verification or

falsification of theories. Speculation and

conjecture also have a place in science; but

ultimately, a scientific theory, to be acceptable,

must be verified by relevant observations or

experiments. There is much philosophical

debate about the nature and method of science

that we need not discuss here.

The interplay of theory and observation (or

experiment) is basic to the progress of science.

Science is ever dynamic. There is no ‘final’

theory in science and no unquestioned

authority among scientists. As observations

improve in detail and precision or experiments

yield new results, theories must account for

them, if necessary, by introducing modifications.

Sometimes the modifications may not be drastic

and may lie within the framework of existing

theory. For example, when 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. Occasionally,

however, the existing theory is simply unable

to explain new observations. This causes a

major upheaval in science. In the beginning of

the twentieth century, it was realised that

Newtonian mechanics, till then a very

successful theory, could not explain some of the

most basic features of atomic phenomena.

Similarly, the then accepted wave picture of light

failed to explain the photoelectric effect properly.

This led to the development of a radically new

theory (Quantum Mechanics) to deal with atomic

and molecular phenomena.

Just as a new experiment may suggest an

alternative theoretical model, a theoretical

advance may suggest what to look for in some

experiments. The result of 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). On the other hand, 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.

Physics is a basic discipline in the category

of Natural Sciences, which also includes other

disciplines like Chemistry and Biology. 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.

A precise definition of this discipline is neither

possible nor necessary. We can broadly describe

physics as a study of the basic laws of nature

and their manifestation in different natural

phenomena. The scope of physics is described

briefly in the next section. Here we remark on

two principal thrusts in physics : unification

and reduction.

In Physics, we attempt to explain diverse

physical phenomena in terms of a few concepts

and laws. The effort is to see the physical world

as manifestation of some universal laws in

different domains and conditions. For example,

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. Similarly, the

basic laws of electromagnetism (Maxwell’s

equations) govern all electric and magnetic

phenomena. The attempts to unify fundamental

forces of nature (section 1.4) reflect 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. For example, the subject

of thermodynamics, developed in the nineteenth

century, deals with bulk systems in terms of

macroscopic quantities such as temperature,

internal energy, entropy, etc. Subsequently, the

subjects of kinetic theory and statistical

mechanics interpreted these quantities in terms

of the properties of the molecular constituents

of the bulk system. In particular, the

temperature was seen to be related to the average

kinetic energy of molecules of the system.

1.2 SCOPE AND EXCITEMENT OF PHYSICS

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

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PHYSICAL WORLD 3

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 problems 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

You can now see that the scope of physics is

truly vast. It covers a tremendous range of

magnitude of physical quantities like length,

mass, time, energy, etc. At one end, it studies

phenomena at the very small scale of length

(10

-14

m or even less) involving electrons, protons,

etc.; at the other end, it deals with astronomical

phenomena at the scale of galaxies or even the

entire universe whose extent is of the order of

m. The two length scales differ by a factor of

or even more. The range of time scales can

be obtained by dividing the length scales by the

speed of light : 10

–22

s to 10

s. The range of

masses goes from, say, 10

–30

kg (mass of an

electron) to 10

kg (mass of known observable

universe). Terrestrial phenomena lie somewhere

in the middle of this range.

Fig. 1.1 Theory and experiment go hand in hand in physics and help each other’s progress. The alpha scattering

experiments of Rutherford gave the nuclear model of the atom.

* 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.

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. 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 problems of mechanics. Electrodynamics

deals with electric and magnetic phenomena

associated with charged and magnetic bodies.

Its basic laws were given by Coulomb, Oersted,

chemical process, etc., are problems of interest

in thermodynamics.

The microscopic domain of physics deals with

the constitution and structure of matter at 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. Classical physics is

inadequate to handle this domain and Quantum

Theory is currently accepted as the proper

framework for explaining microscopic

phenomena. Overall, the edifice of physics is

beautiful and imposing and you will appreciate

it more as you pursue the subject.

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PHYSICS4

Physics is exciting in many ways. To some people

the excitement comes from the elegance and

universality of its basic theories, from the fact that

a few basic concepts and laws can explain

phenomena covering a large range of magnitude

of physical quantities. To some others, the challenge

in carrying out imaginative new experiments to

unlock the secrets of nature, to verify or refute

theories, is thrilling. Applied physics is equally

demanding. Application and exploitation of

physical laws to make useful devices is the most

interesting and exciting part and requires great

ingenuity and persistence of effort.

What lies behind the phenomenal progress

of physics in the last few centuries? Great

progress usually accompanies changes in our

basic perceptions. First, it was realised that for

scientific progress, only qualitative thinking,

though no doubt important, is not enough.

Quantitative measurement is central to the

growth of science, especially physics, because

the laws of nature happen to be expressible in

precise mathematical equations. The second

most important insight was that the basic laws

of physics are universal — the same laws apply

in widely different contexts. Lastly, the strategy

of approximation turned out to be very

successful. Most observed phenomena in daily

life are rather complicated manifestations of the

basic laws. Scientists recognised the importance

of extracting the essential features of a

phenomenon from its less significant aspects.

It is not practical to take into account all the

complexities of a phenomenon in one go. A good

strategy is to focus first on the essential features,

discover the basic principles and then introduce

corrections to build a more refined theory of the

phenomenon. For example, a stone and a feather

dropped from the same height do not reach the

ground at the same time. The reason is that the

essential aspect of the phenomenon, namely free

fall under gravity, is complicated by the

presence of air resistance. To get the law of free

fall under gravity, it is better to create a

situation wherein the air resistance is

negligible. We can, for example, let the stone and

the feather fall through a long evacuated tube.

In that case, the two objects will fall almost at

the same rate, giving the basic law that

acceleration due to gravity is independent of the

mass of the object. With the basic law thus

found, we can go back to the feather, introduce

corrections due to air resistance, modify the

existing theory and try to build a more realistic

Hypothesis, axioms and models

One should not think that everything can be proved

with physics and mathematics. All physics, and also

mathematics, is based on assumptions, each of

which is variously called a hypothesis or axiom or

postulate, etc.

For example, the universal law of gravitation

proposed by Newton is an assumption or hypothesis,

which he proposed out of his ingenuity. Before him,

there were several observations, experiments and

data, on the motion of planets around the sun,

motion of the moon around the earth, pendulums,

bodies falling towards the earth etc. Each of these

required a separate explanation, which was more

or less qualitative. What the universal law of

gravitation says is that, if we assume that any two

bodies in the universe attract each other with a

force proportional to the product of their masses

and inversely proportional to the square of the

distance between them, then we can explain all

these observations in one stroke. It not only explains

these phenomena, it also allows us to predict the

results of future experiments.

A hypothesis is a supposition without assuming

that it is true. It would not be fair to ask anybody

to prove the universal law of gravitation, because

it cannot be proved. 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. But you need not worry at this stage

about the nuances in using these words. For

example, next year you will learn about Bohr’s model

of hydrogen atom, in which Bohr assumed that an

electron in the hydrogen atom follows certain rules

(postutates). Why did he do that? There was a large

amount of spectroscopic data before him which no

other theory could explain. So Bohr said that if we

assume that an atom behaves in such a manner,

we can explain all these things at once.

Einstein’s special theory of relativity is also

based on two postulates, the constancy of the speed

of electromagnetic radiation and the validity of

physical laws in all inertial frame of reference. It

would not be wise to ask somebody to prove that

the speed of light in vacuum is constant,

independent of the source or observer.

In mathematics too, we need axioms and

hypotheses at every stage. Euclid’s statement that

parallel lines never meet, is a hypothesis. This means

that if we assume this statement, we can explain

several properties of straight lines and two or three

dimensional figures made out of them. But if you

don’t assume it, you are free to use a different axiom

and get a new geometry, as has indeed happened in

the past few centuries and decades.

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PHYSICAL WORLD 5

Table 1.1 Some physicists from different countries of the world and their major contributions

theory of objects falling to the earth under

gravity.

1.3 PHYSICS, TECHNOLOGY AND SOCIETY

The connection between physics, technology

and society can be seen in many examples. The

discipline of thermodynamics arose from the

need to understand and improve the working of

heat engines. The steam engine, as we know,

is inseparable from the Industrial Revolution in

England in the eighteenth century, which had

great impact on the course of human

civilisation. Sometimes technology gives rise to

new physics; at other times physics generates

new technology. An example of the latter is the

wireless communication technology that followed

the discovery of the basic laws of electricity and

magnetism in the nineteenth century. The

applications of physics are not always easy to

foresee. As late as 1933, the great physicist

Ernest Rutherford had dismissed the possibility

of tapping energy from atoms. But only a few

years later, 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. Yet another important example of

physics giving rise to technology is the silicon

‘chip’ that triggered the computer revolution in

the last three decades of the twentieth century.

A most significant area to which physics has

and will contribute is the development of

alternative energy resources. The fossil fuels of

the planet are dwindling fast and there is an

urgent need to discover new and affordable

sources of energy. Considerable progress has

already been made in this direction (for

example, in conversion of solar energy,

geothermal energy, etc., into electricity), but

much more is still to be accomplished.

Table1.1 lists some of the great physicists,

their major contribution and the country of

origin. You will appreciate from this table the

multi-cultural, international character of the

scientific endeavour. Table 1.2 lists some

important technologies and the principles of

physics they are based on. Obviously, these

tables are not exhaustive. We urge you to try to

add many names and items to these tables with

the help of your teachers, good books and

websites on science. You will find that this

exercise is very educative and also great fun.

And, assuredly, it will never end. The progress

of science is unstoppable!

Physics is the study of nature and natural

phenomena. Physicists try to discover the rules

that are operating in nature, on the basis of

observations, experimentation and analysis.

Physics deals with certain basic rules/laws

governing the natural world. What is the nature

Name Major contribution/discovery Country of

Origin

Archimedes Principle of buoyancy; Principle of the lever Greece

Galileo Galilei Law of inertia Italy

Christiaan Huygens Wave theory of light Holland

Isaac Newton Universal law of gravitation; Laws of motion; U.K.

Reflecting telescope

Michael Faraday Laws of electromagnetic induction U.K.

James Clerk Maxwell Electromagnetic theory; Light-an U.K.

electromagnetic wave

Heinrich Rudolf Hertz Generation of electromagnetic waves Germany

J.C. Bose Ultra short radio waves India

W.K. Roentgen X-rays Germany

J.J. Thomson Electron U.K.

Marie Sklodowska Curie Discovery of radium and polonium; Studies on Poland

natural radioactivity

Albert Einstein Explanation of photoelectric effect; Germany

Theory of relativity

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PHYSICS6

* Sections 1.4 and 1.5 contain several ideas that you may not grasp fully in your first reading. However, we

advise you to read them carefully to develop a feel for some basic aspects of physics. These are some of the

areas which continue to occupy the physicists today.

of physical laws? We shall now discuss the

nature of fundamental forces and the laws that

govern the diverse phenomena of the physical

world.

1.4 FUNDAMENTAL FORCES IN NATURE*

We all have an intuitive notion of force. In our

experience, force is needed to push, carry or

throw objects, deform or break them. We also

experience the impact of forces on us, like when

a moving object hits us or we are in a merry-go-

round. Going from this intuitive notion to the

proper scientific concept of force is not a trivial

matter. Early thinkers like Aristotle had wrong

ideas about it. The correct notion of force was

arrived at by Isaac Newton in his famous laws of

motion. He also gave an explicit form for the force

for gravitational attraction between two bodies.

We shall learn these matters in subsequent

chapters.

In the macroscopic world, besides the

gravitational force, we encounter several kinds

of forces: muscular force, contact forces between

bodies, friction (which is also a contact force

parallel to the surfaces in contact), the forces

exerted by compressed or elongated springs and

taut strings and ropes (tension), the force of

buoyancy and viscous force when solids are in

Victor Francis Hess Cosmic radiation Austria

R.A. Millikan Measurement of electronic charge U.S.A.

Ernest Rutherford Nuclear model of atom New Zealand

Niels Bohr Quantum model of hydrogen atom Denmark

C.V. Raman Inelastic scattering of light by molecules India

Louis Victor de Borglie Wave nature of matter France

M.N. Saha Thermal ionisation India

S.N. Bose Quantum statistics India

Wolfgang Pauli Exclusion principle Austria

Enrico Fermi Controlled nuclear fission Italy

Werner Heisenberg Quantum mechanics; Uncertainty principle Germany

Paul Dirac Relativistic theory of electron; U.K.

Quantum statistics

Edwin Hubble Expanding universe U.S.A.

Ernest Orlando Lawrence Cyclotron U.S.A.

James Chadwick Neutron U.K.

Hideki Yukawa Theory of nuclear forces Japan

Homi Jehangir Bhabha Cascade process of cosmic radiation India

Lev Davidovich Landau Theory of condensed matter; Liquid helium Russia

S. Chandrasekhar Chandrasekhar limit, structure and evolution India

of stars

John Bardeen Transistors; Theory of super conductivity U.S.A.

C.H. Townes Maser; Laser U.S.A.

Abdus Salam Unification of weak and electromagnetic Pakistan

interactions

Name Major contribution/discovery Country of

Origin

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PHYSICAL WORLD 7

contact with fluids, the force due to pressure of

a fluid, the force due to surface tension of a liquid,

and so on. There are also forces involving charged

and magnetic bodies. In the microscopic domain

again, we have electric and magnetic forces,

nuclear forces involving protons and neutrons,

interatomic and intermolecular forces, etc. We

shall get familiar with some of these forces in later

parts of this course.

A great insight of the twentieth century

physics is that these different forces occurring

in different contexts actually arise from only a

small number of fundamental forces in nature.

For example, the elastic spring force arises due

to the net attraction/repulsion between the

neighbouring atoms of the spring when the

spring is elongated/compressed. This net

attraction/repulsion can be traced to the

(unbalanced) sum of electric forces between the

charged constituents of the atoms.

In principle, this means that the laws for

‘derived’ forces (such as spring force, friction)

are not independent of the laws of fundamental

forces in nature. The origin of these derived

forces is, however, very complex.

At the present stage of our understanding,

we know of four fundamental forces in nature,

which are described in brief here :

Table 1.2 Link between technology and physics

Technology Scientific principle(s)

Steam engine Laws of thermodynamics

Nuclear reactor Controlled nuclear fission

Radio and Television Generation, propagation and detection

of electromagnetic waves

Computers Digital logic

Lasers Light amplification by stimulated emission of

radiation

Production of ultra high magnetic Superconductivity

fields

Rocket propulsion Newton’s laws of motion

Electric generator Faraday’s laws of electromagnetic induction

Hydroelectric power Conversion of gravitational potential energy into

electrical energy

Aeroplane Bernoulli’s principle in fluid dynamics

Particle accelerators Motion of charged particles in electromagnetic

fields

Sonar Reflection of ultrasonic waves

Optical fibres Total internal reflection of light

Non-reflecting coatings Thin film optical interference

Electron microscope Wave nature of electrons

Photocell Photoelectric effect

Fusion test reactor (Tokamak) Magnetic confinement of plasma

Giant Metrewave Radio Detection of cosmic radio waves

Telescope (GMRT)

Bose-Einstein condensate Trapping and cooling of atoms by laser beams and

magnetic fields.

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PHYSICS8

1.4.1 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. 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, motion of the earth and planets

around the sun, and, of course, the motion of

bodies falling to the earth. It plays a key role in

the large-scale phenomena of the universe, such

as formation and evolution of stars, galaxies and

galactic clusters.

1.4.2 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. Charges in motion

produce magnetic effects and a magnetic field

gives rise to a force on a moving charge. Electric

and magnetic effects are, in general,

inseparable – hence the name electromagnetic

force. Like the gravitational force,

electromagnetic force acts over large distances

and does not need any intervening medium. It

is enormously strong compared to gravity. The

electric force between two protons, for example,

is 10

times the gravitational force between

them, for any fixed distance.

Matter, as we know, consists of elementary

charged constituents like electrons and

protons. Since the electromagnetic force is so

much stronger than the gravitational force, it

dominates all phenomena at atomic and

molecular scales. (The other two forces, as we

shall see, operate only at nuclear 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. It underlies the macroscopic forces

like ‘tension’, ‘friction’, ‘normal force’, ‘spring

force’, etc.

Gravity is always attractive, while

electromagnetic force can be attractive or

repulsive. Another way of putting it is that mass

comes only in one variety (there is no negative

mass), but charge comes in two varieties :

positive and negative charge. This is what

makes all the difference. Matter is mostly

electrically neutral (net charge is zero). Thus,

electric force is largely zero and gravitational

force dominates terrestrial phenomena. Electric

force manifests itself in atmosphere where the

atoms are ionised and that leads to lightning.

Albert Einstein (1879-1955)

Albert Einstein, born in Ulm, Germany in 1879, is universally regarded as

one of the greatest physicists of all time. His astonishing scientific career

began with the publication of three path-breaking papers in 1905. In the

first paper, he introduced the notion of light quanta (now called photons)

and used it to explain the features of photoelectric effect that the classical

wave theory of radiation could not account for. In the second paper, he

developed a theory of Brownian motion that was confirmed experimentally

a few years later and provided a convincing evidence of the atomic picture of

matter. The third paper gave birth to the special theory of relativity that

made Einstein a legend in his own life time. In the next decade, he explored the consequences of his

new theory which included, among other things, the mass-energy equivalence enshrined in his famous

equation E = mc

. He also created the general version of relativity (The General Theory of Relativity),

which is the modern theory of gravitation. Some of Einstein’s most significant later contributions are:

the notion of stimulated emission introduced in an alternative derivation of Planck’s blackbody radiation

law, static model of the universe which started modern cosmology, quantum statistics of a gas of

massive bosons, and a critical analysis of the foundations of quantum mechanics. The year 2005 was

declared as International Year of Physics, in recognition of Einstein’s monumental contribution to

physics, in year 1905, describing revolutionary scientific ideas that have since influenced all of modern

physics.

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PHYSICAL WORLD 9

If we reflect a little, the enormous strength

of the electromagnetic force compared to

gravity is evident in our daily life. When we

hold a book in our hand, we are balancing the

gravitational force on the book due to the huge

mass of the earth by the ‘normal force’

provided by our hand. The latter is nothing

but the net electromagnetic force between the

charged constituents of our hand and

the book, at the surface in contact. If

electromagnetic force were not intrinsically so

much stronger than gravity, the hand of the

strongest man would crumble under the

weight of a feather ! Indeed, to be consistent,

in that circumstance, we ourselves would

crumble under our own weight !

1.4.3 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 force of gravity is negligible

compared to the electric force. A new basic force

must, therefore, be invoked. The strong nuclear

force is the strongest of all fundamental forces,

about 100 times the electromagnetic force in

strength. It is charge-independent 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.

1.4.4 Weak Nuclear Force

The weak nuclear force appears only in certain

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

1.4.5 Towards Unification of Forces

We remarked in section 1.1 that unification is a

basic quest in physics. Great advances in

physics often amount to unification of different

Satyendranath Bose (1894-1974)

Satyendranath Bose, born in Calcutta in 1894, is among the great Indian

physicists who made a fundamental contribution to the advance of science

in the twentieth century. An outstanding student throughout, Bose started

his career in 1916 as a lecturer in physics in Calcutta University; five years

later he joined Dacca University. Here in 1924, in a brilliant flash of insight,

Bose gave a new derivation of Planck’s law, treating radiation as a gas of

photons and employing new statistical methods of counting of photon states.

He wrote a short paper on the subject and sent it to Einstein who

immediately recognised its great significance, translated it in German and

forwarded it for publication. Einstein then applied the same method to a

gas of molecules.

The key new conceptual ingredient in Bose’s work was that the particles were regarded as

indistinguishable, a radical departure from the assumption that underlies the classical Maxwell-

Boltzmann statistics. It was soon realised that the new Bose-Einstein statistics was applicable to

particles with integers spins, and a new quantum statistics (Fermi-Dirac statistics) was needed for

particles with half integers spins satisfying Pauli’s exclusion principle. Particles with integers spins

are now known as bosons in honour of Bose.

An important consequence of Bose-Einstein statistics is that a gas of molecules below a certain

temperature will undergo a phase transition to a state where a large fraction of atoms populate the

same lowest energy state. Some seventy years were to pass before the pioneering ideas of Bose, developed

further by Einstein, were dramatically confirmed in the observation of a new state of matter in a dilute

gas of ultra cold alkali atoms - the Bose-Eintein condensate.

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PHYSICS10

theories and domains. Newton unified terrestrial

and celestial domains under a common law of

gravitation. The experimental discoveries of

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. Einstein attempted to

unify gravity and electromagnetism but could

not succeed in this venture. But this did not

deter physicists from zealously pursuing the

goal of unification of forces.

Recent decades have seen much progress on

this front. The electromagnetic and the weak

nuclear force have now been unified and are

seen as aspects of a single ‘electro-weak’ force.

What this unification actually means cannot

be explained here. Attempts have been (and are

being) made to unify the electro-weak and the

strong force and even to unify the gravitational

force with the rest of the fundamental forces.

Many of these ideas are still speculative and

inconclusive. Table 1.4 summarises some of the

milestones in the progress towards unification

of forces in nature.

1.5 NATURE OF PHYSICAL LAWS

Physicists explore the universe. Their

investigations, based on scientific processes,

range from particles that are smaller than

atoms in size to stars that are very far away. In

addition to finding the facts by observation and

experimentation, physicists attempt to discover

the laws that summarise (often as mathematical

equations) these facts.

In any physical phenomenon governed by

different forces, several quantities may change

with time. A remarkable fact is that some special

physical quantities, however, remain constant

in time. They are the conserved quantities of

nature. Understanding these conservation

principles is very important to describe the

observed phenomena quantitatively.

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. The familiar example is the free fall of

an object under gravity. Both the kinetic energy

of the object and its potential energy change

continuously with time, but the sum remains

fixed. If the object is released from rest, the initial

Table 1.4 Progress in unification of different forces/domains in nature

Table 1.3 Fundamental forces of nature

Name Relative Range Operates among

strength

Gravitational force 10

–39

Infinite All objects in the universe

Weak nuclear force 10

–13

Very short, Sub-nuclear Some elementary particles,

size (∼10

–16

m) particularly electron and

neutrino

Electromagnetic force 10

–2

Infinite Charged particles

Strong nuclear force 1 Short, nuclear Nucleons, heavier

size (∼10

–15

m) elementary particles

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potential energy is completely converted into the

kinetic energy of the object just before it hits

the ground. This law restricted for a conservative

force should not be confused with the general

law of conservation of energy of an isolated

system (which is the basis of the First Law of

Thermodynamics).

The concept of energy is central to physics

and the expressions for energy can be written

for every physical system. When all forms of

energy e.g., heat, mechanical energy, electrical

energy etc., are counted, it turns out that energy

is conserved. The general law of conservation of

energy is true for all forces and for any kind of

transformation between different forms of

energy. In the falling object example, if you

include the effect of air resistance during the

fall and see the situation after the object hits

the ground and stays there, the total

mechanical energy is obviously not conserved.

The general law of energy conservation, however,

is still applicable. The initial potential energy

of the stone gets transformed into other forms

of energy : heat and sound. (Ultimately, sound

after it is absorbed becomes heat.) The total

energy of the system (stone plus the

surroundings) remains unchanged.

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. At

the other end, all kinds of violent phenomena

occur in the universe all the time. Yet the total

energy of the universe (the most ideal isolated

system possible!) is believed to remain

unchanged.

Until the advent of Einstein’s theory of

relativity, the law of conservation of mass was

regarded as another basic conservation law of

nature, since matter was thought to be

indestructible. It was (and still is) an important

principle used, for example, in the analysis of

chemical reactions. A chemical reaction is

basically a rearrangement of atoms among

different molecules. If the total binding energy

of the reacting molecules is less than the total

binding energy of the product molecules, the

difference appears as heat and the reaction is

exothermic. The opposite is true for energy

absorbing (endothermic) reactions. However,

since the atoms are merely rearranged but not

destroyed, the total mass of the reactants is the

same as the total mass of the products in a

chemical reaction. The changes in the binding

energy are too small to be measured as changes

in mass.

According to Einstein’s theory, mass m is

equivalent to energy E given by the relation

E = mc

, 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.

Sir C.V. Raman (1888-1970)

Chandrashekhara Venkata Raman was born on 07 Nov 1888 in Thiruvanaikkaval.

He finished his schooling by the age of eleven. He graduated from Presidency

College, Madras. After finishing his education he joined financial services of the

Indian Government.

While in Kolkata, he started working on his area of interest at Indian Asso-

ciation for Cultivation of Science founded by Dr. Mahendra Lal Sirkar, during his

evening hours. His area of interest included vibrations, variety of musical instru-

ments, ultrasonics, diffraction and so on.

In 1917 he was offered Professorship at Calcutta University. In 1924 he was

elected ‘Fellow’ of the Royal Society of London and received Nobel prize in Physics

in 1930 for his discovery, now known as Raman Effect.

The Raman Effect deals with scattering of light by molecules of a medium

when they are excited to vibrational energy levels. This work opened totally new

avenues for research for years to come.

He spent his later years at Bangalore, first at Indian Institute of Science and then at Raman Re-

search Institute. His work has inspired generation of young students.

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PHYSICS12

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. These laws can

be derived from Newton’s laws of motion in

mechanics. But their validity goes beyond

mechanics. They are the basic conservation

laws of nature in all domains, even in those

where Newton’s laws may not be valid.

Besides their great simplicity and generality,

the conservation laws of nature are very useful

in practice too. It often happens that we cannot

solve the full dynamics of a complex problem

involving different particles and forces. The

conservation laws can still provide useful

results. For example, we may not know the

complicated forces that act during a collision

of two automobiles; yet momentum

conservation law enables us to bypass the

complications and predict or rule out possible

outcomes of the collision. In nuclear and

elementary particle phenomena also, the

conservation laws are important tools of

analysis. Indeed, 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.

Conservation laws have a deep connection

with symmetries of nature that you will explore

in more advanced courses in physics. For

example, an important observation is that the

laws of nature do not change with time! If you

perform an experiment in your laboratory today

and repeat the same experiment (on the same

objects under identical conditions) after a year,

the results are bound to be the same. It turns

out that this symmetry of nature with respect to

translation (i.e. displacement) in time is

equivalent to the law of conservation of energy.

Likewise, space is homogeneous and there is no

(intrinsically) preferred location in the universe.

To put it more clearly, the laws of nature are the

same everywhere in the universe. (Caution : the

phenomena may differ from place to place

because of differing conditions at different

locations. For example, the acceleration due to

gravity at the moon is one-sixth that at the earth,

but the law of gravitation is the same both on

the moon and the earth.) This symmetry of the

laws of nature with respect to translation in

space gives rise to conservation of linear

momentum. In the same way isotropy of space

(no intrinsically preferred direction in space)

underlies the law of conservation of angular

momentum*. The conservation laws of charge and

other attributes of elementary particles can also

be related to certain abstract symmetries.

Symmetries of space and time and other abstract

symmetries play a central role in modern theories

of fundamental forces in nature.

* See Chapter 7

Conservation laws in physics

Conservation of energy, momentum, angular

momentum, charge, etc are considered to be

fundamental laws in physics. At this moment,

there are many such conservation laws. Apart from

the above four, there are others which mostly deal

with quantities which have been introduced in

nuclear and particle physics. Some of the

conserved quantities are called spin, baryon

number, strangeness, hypercharge, etc, but you

need not worry about them.

A conservation law is a hypothesis, based on

observations and experiments. It is important to

remember that a conservation law cannot be

proved. It can be verified, or disproved, by

experiments. An experiment whose result is in

conformity with the law verifies or substantiates

the law; it does not prove the law. On the other

hand, a single experiment whose result goes

against the law is enough to disprove it.

It would be wrong to ask somebody to prove

the law of conservation of energy. This law is an

outcome of our experience over several centuries,

and it has been found to be valid in all

experiments, in mechanics, thermodynamics,

electromagnetism, optics, atomic and nuclear

physics, or any other area.

Some students feel that they can prove the

conservation of mechanical energy from a body

falling under gravity, by adding the kinetic and

potential energies at a point and showing that it

turns out to be constant. As pointed out above,

this is only a verification of the law, not its proof.

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SUMMARY

1. Physics deals with the study of the basic laws of nature and their manifestation in

different phenomena. The basic laws of physics are universal and apply in widely different

contexts and conditions.

2. The scope of physics is wide, covering a tremendous range of magnitude of physical

quantities.

3. Physics and technology are related to each other. Sometimes technology gives rise to

new physics; at other times physics generates new technology. Both have direct impact

on society.

4. There are four fundamental forces in nature that govern the diverse phenomena of the

macroscopic and the microscopic world. These are the ‘gravitational force’, the

‘electromagnetic force’, the ‘strong nuclear force’, and the ‘weak nuclear force’. Unification

of different forces/domains in nature is a basic quest in physics.

5. The physical quantities that remain unchanged in a process are called conserved

quantities. Some of the general conservation laws in nature include the laws of

conservation of mass, energy, linear momentum, angular momentum, charge, parity,

etc. Some conservation laws are true for one fundamental force but not for the other.

6. Conservation laws have a deep connection with symmetries of nature. Symmetries of

space and time, and other types of symmetries play a central role in modern theories of

fundamental forces in nature.

EXERCISES

Note for the student

The exercises given here are meant to enhance your awareness about the issues surrounding

science, technology and society and to encourage you to think and formulate your views

about them. The questions may not have clear-cut ‘objective’ answers.

Note for the teacher

The exercises given here are not for the purpose of a formal examination.

1.1 Some of the most profound statements on the nature of science have come from

Albert Einstein, one of the greatest scientists of all time. What do you think did

Einstein mean when he said : “The most incomprehensible thing about the world is

that it is comprehensible”?

1.2 “Every great physical theory starts as a heresy and ends as a dogma”. Give some

examples from the history of science of the validity of this incisive remark.

1.3 “Politics is the art of the possible”. Similarly, “Science is the art of the soluble”.

Explain this beautiful aphorism on the nature and practice of science.

1.4 Though India now has a large base in science and technology, which is fast expanding,

it is still a long way from realising its potential of becoming a world leader in science.

Name some important factors, which in your view have hindered the advancement of

science in India.

1.5 No physicist has ever “seen” an electron. Yet, all physicists believe in the existence of

electrons. An intelligent but superstitious man advances this analogy to argue that

‘ghosts’ exist even though no one has ‘seen’ one. How will you refute his argument?

1.6 The shells of crabs found around a particular coastal location in Japan seem mostly

to resemble the legendary face of a Samurai. Given below are two explanations of this

observed fact. Which of these strikes you as a scientific explanation ?

(a) A tragic sea accident several centuries ago drowned a young Samurai. As a tribute

to his bravery, nature through its inscrutable ways immortalised his face by

imprinting it on the crab shells in that area.

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(b) After the sea tragedy, fishermen in that area, in a gesture of honour to their

dead hero, let free any crab shell caught by them which accidentally had a shape

resembling the face of a Samurai. Consequently, the particular shape of the

crab shell survived longer and therefore in course of time the shape was genetically

propagated. This is an example of evolution by artificial selection.

[Note : This interesting illustration taken from Carl Sagan’s ‘The Cosmos’ highlights

the fact that often strange and inexplicable facts which on the first sight appear

‘supernatural’ actually turn out to have simple scientific explanations. Try to think

out other examples of this kind].

1.7 The industrial revolution in England and Western Europe more than two centuries

ago was triggered by some key scientific and technological advances. What were these

advances ?

1.8 It is often said that the world is witnessing now a second industrial revolution, which

will transform the society as radically as did the first. List some key contemporary areas

of science and technology, which are responsible for this revolution.

1.9 Write in about 1000 words a fiction piece based on your speculation on the science

and technology of the twenty-second century.

1.10 Attempt to formulate your ‘moral’ views on the practice of science. Imagine yourself

stumbling upon a discovery, which has great academic interest but is certain to have

nothing but dangerous consequences for the human society. How, if at all, will you

resolve your dilemma ?

1.11 Science, like any knowledge, can be put to good or bad use, depending on the user.

Given below are some of the applications of science. Formulate your views on whether

the particular application is good, bad or something that cannot be so clearly

categorised :

(a) Mass vaccination against small pox to curb and finally eradicate this disease

from the population. (This has already been successfully done in India).

(b) Television for eradication of illiteracy and for mass communication of news and

ideas.

(d) Computers for increase in work efficiency

(e) Putting artificial satellites into orbits around the Earth

(f ) Development of nuclear weapons

(g) Development of new and powerful techniques of chemical and biological warfare).

(h) Purification of water for drinking

(i) Plastic surgery

(j ) Cloning

1.12 India has had a long and unbroken tradition of great scholarship — in mathematics,

astronomy, linguistics, logic and ethics. Yet, in parallel with this, several superstitious

and obscurantistic attitudes and practices flourished in our society and unfortunately

continue even today — among many educated people too. How will you use your

knowledge of science to develop strategies to counter these attitudes ?

1.13 Though the law gives women equal status in India, many people hold unscientific

views on a woman’s innate nature, capacity and intelligence, and in practice give

them a secondary status and role. Demolish this view using scientific arguments, and

by quoting examples of great women in science and other spheres; and persuade yourself

and others that, given equal opportunity, women are on par with men.

1.14 “It is more important to have beauty in the equations of physics than to have them

agree with experiments”. The great British physicist P. A. M. Dirac held this view.

Criticize this statement. Look out for some equations and results in this book which

strike you as beautiful.

1.15 Though the statement quoted above may be disputed, most physicists do have a feeling

that the great laws of physics are at once simple and beautiful. Some of the notable

physicists, besides Dirac, who have articulated this feeling, are : Einstein, Bohr,

Heisenberg, Chandrasekhar and Feynman. You are urged to make special efforts to get

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access to the general books and writings by these and other great masters of physics.

(See the Bibliography at the end of this book.) Their writings are truly inspiring!

1.16 Textbooks on science may give you a wrong impression that studying science is dry

and all too serious and that scientists are absent-minded introverts who never laugh

or grin. This image of science and scientists is patently false. Scientists, like any

other group of humans, have their share of humorists, and many have led their lives

with a great sense of fun and adventure, even as they seriously pursued their scientific

work. Two great physicists of this genre are Gamow and Feynman. You will enjoy

reading their books listed in the Bibliography.

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