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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