Chapter 1 Physical World

CHAPTER NO.1 PHYSICAL WORD

 

1.1 WHAT Is PHYSICS 7

Humans have always been curicus 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 tn 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 Vijnar and the Arabic word im convey similar meaning, namely ‘knowledge’.Science, in a broad sense, is as old as bnmman 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 mpid growth.What is Science and what is the sc-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, expertmenting 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 actence. The scientific method involves several interconnected steps : Systematic observations, controlled experiments, qualitative and 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, theorles must account. for them, ifnecessary, by introducing modifications.Sometimes the modifications may not be drastic

and may lic 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) tmagined by Nicolas Copernicus (1473-1543) had to be replaced by elliptical orbits to fit the data better. Occasionally,however, the existing theory is stmply 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

moat basic features of atomic phenomena.

Shnilarly, the then accepted wave picture of light fafled to explain the photoelectric effect property.

This led to the development ofa radically new theory (Quantum Mechanics) to deal with atomic and molecular phenomena.

 

Just as a new experiment may suggeat 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 fofl, in 1911

by Ermmest 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 Hke Chemistry and Biology. The word Physics comes from a Greek word meaning nature. Its Sanskrit equivalent is Bhaufiki that.

ia 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 taws. The effort is to sce 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.

 

Arelated effort is to derive the properties of a bigger, more complex, system from the properties and interactions of ite 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,intemal energy, entropy, etc. Subsequently, the subjects of kinetic theory and statistical mechanics interpreted these quantities in terms of the properties of the molecular conatituents 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 fet 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

tochales phenomena at the laboratory, terrestrial amd astronomical scales. The microscopic domain includes atomic, molecular and nuclear phenomena’. Classical Physics deals mainly

with macroscopic phenomena and includes

subjects like Mechanics, Electrodynamics,Optics and Thermodynamics. Mechanics founded on Newton’s laws of motion and the law

of gravitation is concerned with the motion (or equilibrium) of particles, rigid and deformable bodies, and general systems of particles. The propulsion of a rocket by a jet of ejecting gases,

propagation of water waves or sound waves in air, the equilfbrium of a bent rod under a lead,etc., are problems ofmechanics. Electrodynamics

deals with electric and magnetic phenomena associated with charged and magnetic bodies.Its basic laws were given by Coulomb, Oersted,Fig. 1.1 Theory and expertment go hand in hand tr ph expertinents of Rutherford gave the nuclear 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 (signa), 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

infernal energy, temperature, entropy, etc., of the system throngh external work and transfer of heat. The efficiency of heat engines and

Tefrigerators, the direction of a physical or chemical proceas, eic., 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 aa you pursue the subject.

 

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**m or even leas) 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

10m. The two length acales differ by a factor of 10® oreven more. The range of time acales can be obtained by dividing the length acales by the speed of light : 108 to 10%. The range of masses goes from, say, 10° kg (mass of an

electron) to 10" kg (mase of known observable universe). Terrestrial phenomena lie somewhere in the middle of this range.

 

Physics is exciting in many ways. To some people the excitement comes from the elegance and unitversality of ita 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 ingenutty 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 Hfe are rather complicated manifestations of the basic laws. Scientists recognised the importance

of extracting the essential featurea of a phenomenon from its less signtficant aspects.It is not practical to take into account all the complexities of a phenomenon in one go. Agood

strategy is to focus first on the casential features,discover the basic principles and then tntroduce

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 reaistance. 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 physica and mathematica. 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, ifwe assume that any two bodies in the universe attract each other with a force proportional to the product of their masees 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.

 

Abypotheais 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 aleo 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 dimenational figures made out of them. But ff you

don't assume ft, you are free to use a different axiom and get a new geometry, as has indeed happened in the past few centuries and decades.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 diacipline 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 mew physics; at other thmes physics gencrates new technology. An example of the latter is the

wireleas communication technology that followed the discovery of the basic laws of electrictty amd 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 tmportant 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 moat 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.

 

Tablel.1 lists some of the great physicists,their major contribution and the country of origin. You will appreciate from this table the

mitlti-cultural, international character of the scientific endeavour. Table 1.2 sts some important technologies and the principles of physics they are basedon. 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.

Physica deals with certain basic rules/laws governing the natural world. What is the

 nature 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 pave an expHcit form for the force for gravitational attraction between two boxlies.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 farces between.bodies, friction fwhich 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

contact with fhitdis, the force due to pressure of a fluid, the force due to surface tension ofa quid,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

shal get familar with some of these forces tn later parts of this course.

 

A great insight of the twentieth century

physics ts that these different forces occurring in different contexts actually arise from only a small number of fundamental forces tn nature.

For example, the clastic spring force arises duc to the net  attraction/repulsion between the

neighbouring atoms of the spring when the spring is clongated/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 1s, however, very complex.

 

At the present stage of our understanding,‘we kmow of four findamental forces in nature,which are described in brief here :

 

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 clectrons 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 ia 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.

 

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 Dut 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 f

 

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 mucleus 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. Anew 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 {107"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 f-decay of a nucleus. In B-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 107°°m.

 

1.4.6 Towards Unification of Forces

We remarked in section 1.1 that unification ia a basic quest in physics. Great advances in physics often amount to unification of different

 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 andmagnetic phenomena are in general inseparable. Maxwell umfied electromagnetism

and optics with the discovery that Hight 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.

 

1.6 NATURE OF PHYSICAL LAWS

Physicists explore the universe. Their

investigations, hased 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. Aremarkable 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 deacribe 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 famillar 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

 inttial 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 syatem. 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 hita

the ground and stays there, the total

mechanical energy is obviously not conserved.The general law of energy conservation, however,js still applicable. The initial potential energy

of the stone gets transformed into other forms of energy : heat and sound. (Ultimately, sound after it 1s 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 kinda of violent phenomena occur in the universe all the time. Yet the total energy of the universe (the moat ideal tsolated

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. Ifthe total binding energy of the reacting molecules {a less than the total binding energy of the product molecules, the difference appears as heat and the reaction fs

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 reactanta ts the

same as the total mass of the products in a chemical reaction. The changes in the binding energy are too amall 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.

 

Ina nuclear process mass gets converted to energy {or vice-versa). This is the energy which is released in a nuclear power generation and nuclear explosions.

 

Energy is a scalar quantity. But all conserved quantities are not necessarily scalars. The total linear momentum and the total angular momentum (both vectors) of an isolated system

are also conserved quantities. 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 tn all domains, even in those where Newton's laws may uot be valid.

 

Besides thetr 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 resilts. For example, we may not know the

complicated forces that act during a collision of two automobiles: yet momentum conservation law enables us tc bypass the complications and predict or nile 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 B-decay, Wolfgang Pauli (1900-1958) correctly predicted in 1931 the existence of a new particle (now called neutrino) emitted in f-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 homogeneomns and there ia 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 Conservation laws in physics Conservation of energy, momentum, angular momentum, charge, ete are considered to he 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 couserved quantities are called spin, baryou

number, strangeness, hypercharge, etc, but you need not worry about them.

 

A conservation law is a hypotheais, 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 ia in conformity with the law verifies or substantiates the law; it does not prove the law. On the other hand, a aingle experiment whose result goea

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,electromagnetiam, optics, atomic and nuclear physics, or any other area.

 

Some students feel that they can prove the conservation of mechanical energy fram a body falling under gravity, by adding the kinetic and potential energies at a point and showing that it

turne out to be constant. As pointed out above,this is only a verification of the law, not its proof.

 

because of differing conditions at different locationa. 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 givea 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 symmetrica play a central role in modem theories

of fundamental forces in nature.

 

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 untvereal and apply in widely different contexts and conditions.

 

2. The scope of physics ts 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 phyaics; 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 im 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, Hnear 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.

 

EXERCI8ES

Note for the student

The exercises given here are meant to enhance your awareness about the isaues surrounding acience, 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 moet profound statements on the nature of ecience have come from

Albert Einstein, one of the greatest scientiste 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 aa 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 beantiful 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 ite potential of becoming a world leader in science.Name some important factors, which in your view have hindered the advancement of

acience in India.

 

1.5 No phyaiciat has ever “seen” an electron. Yet, all physicista believe in the exiatence 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 moatly 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 ita inecrutable ways immortalised his face by imprinting it on the crab shells in that area.

 

(b) After the sea tragedy, Hshermen 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 waa genetically propagated. This is an example of evolution by artificial selection.

[Note ; This interesting tustration taken from Carl Sagan's “The Cosmos’ highlights the fact that often strange and inexplicable facts which on the firat aight appear ‘supernatural actually turn out to have simple ecientific 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 tndustrial 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 m 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 aclence. 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.Gtven 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 succesefully done in India).

 

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

 

(c) Prenatal determination

 

(d) Computers for increase m work efficiency

 

(e) Putting artificial satellites mto orbits around the Earth

 

f) Development of nuclear weapons

 

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

 

h) =Purtfication 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 ia more fmportant to have beauty in the equations of physica than to have them agree with experimenta”’. The great Britiah physicist P. A. M. Dirac held this view.Criticize this statement. Look out for some equations and results in this book which

atrike 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, beaidea Dirac, who have articulated this feeling, are : Einstein, Bohr,

Heisenberg, Chandrasekhar and Feynman. You are urged to make special efforts to get acceaa to the general books and writings by these and other great masters of physica.(See the BibHography 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 tntroverts who never laugh or grin. This image of aclence and scientists is patently false. Scientists, like any other group of humans, have thetr share of humorista, and many have led their lives

with a great sense of fun and adventure, even as they seriously pursued their scientific work. Two great phyaiciate of this genre are Gamow and Feynman. You will enjoy reading their books listed in the Bibliography.