NCERT Textbook - Physical World Class 11 Notes | EduRev

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Class 11 : NCERT Textbook - Physical World Class 11 Notes | EduRev

 Page 1


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 Vijnan 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
© NCERT
not to be republished
Page 2


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 Vijnan 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
© NCERT
not to be republished
PHYSICS 2
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
© NCERT
not to be republished
Page 3


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 Vijnan 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
© NCERT
not to be republished
PHYSICS 2
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
© NCERT
not to be republished
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
10
26 
m.  The two length scales differ by a factor of
10
40
 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
18 
s.  The range of
masses goes from, say, 10
?30
 kg  (mass of an
electron) to 10
55 
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.
© NCERT
not to be republished
Page 4


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 Vijnan 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
© NCERT
not to be republished
PHYSICS 2
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
© NCERT
not to be republished
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
10
26 
m.  The two length scales differ by a factor of
10
40
 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
18 
s.  The range of
masses goes from, say, 10
?30
 kg  (mass of an
electron) to 10
55 
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.
© NCERT
not to be republished
PHYSICS 4
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.
© NCERT
not to be republished
Page 5


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 Vijnan 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
© NCERT
not to be republished
PHYSICS 2
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
© NCERT
not to be republished
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
10
26 
m.  The two length scales differ by a factor of
10
40
 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
18 
s.  The range of
masses goes from, say, 10
?30
 kg  (mass of an
electron) to 10
55 
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.
© NCERT
not to be republished
PHYSICS 4
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.
© NCERT
not to be republished
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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