Chapter 1 - Physical World PDF

R SI PHYSICAL WORLD IT 1.1 WHAT IS SCIENC...

R SI PHYSICAL WORLD IT 1.1 WHAT IS SCIENCE? Scientific attitude. The tremendous growth of 1. What is science ? science has taken place only due to the use of scientific attitude and scientific method. The scientific attitude Science. It is a systematised and organised knowledge requires a flexible, open-minded approach towards solving about the various natural phenomena which is obtained by problems in which other important points of view are not careful experimentation, keen observation and accurate rea- H neglected without any reason. First a solution. is soning. The Sanskrit word Shastra and Arabic word Ilm suggested for a problem. This -solution is tried. If it also have a similar meaning i.e., organised knowledge. works satisfactorily, it is adopted and otherwise it is 2. What are physical and biological sciences. ? replaced by a better solution to the same problem. Two types of sciences. The knowledge of science 4. What is scientific method ? Mention the various O can be divided into two broad categories: steps involved in it.. 1. Biological sciences. The sciences which deal with the Scientific method. The step by step approach used by behaviour of living things are called biological sciences. a scientist in studying natural phenomena and establishing These sciences include Botany, Zoology, Orinthology, laws which govern these phenomena is called scientific Anthropology, Entomology, Forensic Science, etc. method. M 2. Physical sciences. The two main physical Generally, it involves the following steps: sciences are physics and chemistry. Chemistry is the 1. Taking a large number of systematic observations study of every substance, its structure, its composition and by doing controlled experiments. changes in which it takes part. Physics is the study of the natural world which deals with the concepts of space, time, 2. Studying these observations and making quali- motion, matter, energy, radiation, etc. Other physical tative and quantitative reasoning. sciences include Geology, Geography, Astronomy, 3. Suggesting mathematical models to account for Astrology, Oceanology, etc. the observed behaviour. 4. Predicting new phenomena on the basis of 1.2 SCIENTIFIC ATTITUDE AND suggested model. SCIENTIFIC METHOD 5. Modifying the theory, if necessary, in the light 3. What is scientific attitude ? of the fresh evidences. - --------  1.2 PHYSICS-XI 1.3 SCIENTIFIC THEORY 2. Sometimes a theoretical advancement predicts a 5. What is scientific theory ? new experiment. For example, the concept of antiparticle was introduced theoretically by Scientific theory. Theory is the name given to a set of Paul Dirac in 1930 and was confirmed two years limited number of laws in terms of which the behaviour of a later by the experimental discovery of positron physical system can be explained. A good theory should (anti-electron) by Carl Anderson.. not only explain the already existing phenomenon but it should be able to predict and explain the new 1.4 WHAT IS PHYSICS? phenomenon. 8. What is physics ? What is the origin of the word 6. When does a scientific theory need modification or physics ? What is its Sanskrit equivalent? R replacement by a new theory ? Briefly explain giving Physics. The word physics originates from a Greek suitable examples. word which means nature. This word was introduced Need for modification of a scientific theory. In by ancient scientist Aristotle in the year 350 B.c. The science, no theory is final. A theory must be able to Sanskrit equivalent of physics is bhauitaka which refers explain what may be observed from time to time. to the physical world. SI Following two types of problems may ari~e : Physics is the branch of science that deals with the study 1. When any new observation shows some devia- of basic laws of nature and their manifestation in various tion from the existing theory, the theory has to be duly natural phenomena. It is concerned with the interaction modified. For example, when Johann Kepler examined of matter with matter or energy. It deals with the the extensive data on planetary motion collected by various features of the natural world such as space, Tycho Brahe, the planetary circular orbits in helio- time, matter, motion, energy, radiation, etc. Physics is centric theory (sun at the centre of the sol at: system) the most fundamental of all sciences as it is concerned imagined by Nicholas Coperanicus had to be replaced with the study of various natural phenomena. IT by elliptical orbits to fit the data better. 1.5 TWO BASIC QUESTS IN PHYSICS 2. When even the modification of a theory fails to account for the new observations, the theory has to be 9. What are the two principal thrusts in physics ? Give suitable examples for each. replaced by a new theory. For example, it was realised in the beginning of twentieth century that Newtonian Two basic quests in physics. The two principal mechanics could not explain some basic features of thrusts in physics are unification and reductionism. H atomic phenomena. Also, the wave theory of light 1. Unification. In physics, attempt is made to failed to explain photoelectric effect. This resulted in explain various physical phenomena in terms of just the development of an entirely new theory, called Quan- few concepts and laws. We try to see the physical tum Mechanics, to deal with microscopic phenomena. world as manifestation of some universal laws. For 7. Theory and experiments go hand in hand in example, the same (Newton's) law of gravitation can be used to describe the motion of a body falling O physics and help each 'other's progress. Give two suitable examples in support of your answer. towards the earth, motion of the moon around the earth and motion of planets around the sun. Attempts Or are being made to unify fundamental forces of nature With the help of suitable examples, briefly explain the in the persuit of unification. interplay between theoretical models and experiments. 2. Reductionism. Another attempt made in physics M Interplay between theoretical models and is to explain a macroscopic system in terms of its experiments. The interplay of theory and experiment microscopic constituents. This persuit is called red- (or observation) is important for the progress of uctionism. For example, thermodynamics was developed science. to explain the macroscopic properties such tempe- This is obvious from the following examples: rature, internal energy, entropy, etc., of the bulk systems. 1. Sometimes a new experiment suggests an alter- Later on, these properties were explained in terms of native theoretical model. For example, alpha molecules in kinetic theory and statistical mechanics. particle scattering experiments in 1911 by Ernest 1.6 SCOPE OF PHYSICS Rutherford established the nuclear model of the atom, which then became the basis of quantum Introduction. The scope of physics is very wide. theory of hydrogen atom given by Niels Bohr Every event, which occurs around us in our daily life, in 1913. is governed by one or the other principle of physics. ~'------  PHYSICAL WORLD 1.3 One way of getting the idea of the scope of physics is to covers wide ranges of mass, length, time, energy, etc. look at its various sub-disciplines. Another way is look The mass of an object can vary from 10- 30 kg (mass of at the vast range of magnitude of the physical an electron) to 1055 kg (mass of the universe). The quantities it deals with. length of an object can vary from 10- 14 m (size of a / 10. Briefly explain the main disciplines and sub- nucleus) to 1026 m (size of the universe). Time interval. disciplines in physics. can vary from 10- 22 s (time taken by light to cross a' nuclear distance) to 1018 s (life of the sun). The ~., Branches of physics. Physics has two main everyday phenomena lie in the middle ranges..* domains of study-macroscopic and microscopic. Classical physics deals with macroscopic phenomena which may be at the laboratory, terrestrial and astro- For Your Knowledge R nomical scales. It includes branches like mechanics,.\. The scope of physics (or science in general) is very optics, thermodynamics and electrodynamics. Quantum wide. It has been quite aptly described by some mechanics deals with microscopic phenomena at the scientists as follows : / minute scales of atoms, molecules and nuclei. ~ Science is a method for describing, creating and understanding human experience. Main sub-disciplines in physics: SI R. Bruce Lindsay 1. Mechanics. It deals with the equilibrium or motion of material bodies at low speeds. It is based on laws of ~ Science is the ever unfinished quest to discover all gravitation. The propulsion of rocket, equilibrium of facts, the relationship between things and the rod bent under a load, propagation of water waves or laws by which the world runs. Gerold Holton sound waves in air, etc., are studied in mechanics. ~ The two processes that of Science and Art are not very different. Both Science and Art, formed in the 2. Optics. It deals with the nature and propagation of course of centuries a human language by which we light. It deals with the formation of images by mirrors can speak about the more real part of reality. and lenses, colours in thin films, etc. Heisenberg IT 3. Thermodynamics. It deals with a macroscopic system in equilibrium and is concerned with the changes ~ The task of science is both to extend the range of in internal energy, temperature, entropy, etc., of the our experience and to reduce it to order. system through external work and heat. Here we study Niels Bohr the efficiency of heat engines and refrigerators. ~ The most incomprehensible thing about the world 4. Electrodynamics. It deals with electric and magnetic is that it is comprehensible. Albert Einstein phenomena associated with charged and magnetic bodies. It ~ Nature is pleased with simplicity and effects, not H is based on laws given by Coulomb, Oersted, Ampere the pump of superfluous causes. Isaac Newton and Faraday, which were later on unified by Maxwell. ~ We know very little and yet it is astonishing that It deals with problems like motion of current-carrying we know so much and still more astonishing that so conductor in a magnetic field, propagation of radiowaves little knowledge can give us so much power. through the atmosphere, etc. Bertrand Russel O 5. Quantum mechanics. It deals with the mechanical behaviour of sub-microscopic particles like atoms and 1.7 EXCITEMENT OF PHYSICS nuclei and their interaction with projectiles like 12. Physics is a science of excitement. How? electrons, photons and other elementary particles. Excitement of physics. The study of physics is not 6. Relativity. It is theory of invariance in nature. It only educative but also exciting in many ways as follows: M deals with the motion of the particles having speeds comparable to the speed of light. 1. Inspite of the wide range and complexity of the.;f.\. An exciting field of research these days is a new physical phenomena, it is quite exciting that these phenomena can be analysed and understood in terms of few universal laws and principles. domain, called mesoscopic physics, which is 2. Some other people get excitement in carrying intermediate between the microscopic and macro- out new challenging experiments to unfold the scopic domains and deals with a few tens or hundreds mysteries of nature and in verifying or falsi- of atoms. fying the existing theories.. 11. The scope of physics is truly vast. Elaborate. 3. Applied physicists get great satisfaction when The scope of physics is truly vast. We can see in they develop technologies for the welfare of the another way. that the scope of physics is truly vast. It human beings just by using basic laws of physics.  1.4 PHYSICS-XI 13. What are the reasons behind the large scale planets and other heavenly bodies in the sky. progress of physics in the last few centuries ? (ii) Radiotelescopes have helped to discover quasers, Reasons behind the phenomenal growth of pulsars, etc., and can be used to see upto the farthest physics in the last few centuries : limits of the universe. (iii) Dopler's effect predicted the 1. Precise and accurate measurements are central big-bang theory of the universe. to the growth of physics because most of laws 5. Physics in relation to geology. Diffraction of nature can be expressed in mathematical techniques help to study the crystal structure of various forms. rocks. Radioactivity is used to estimate the age of rocks 2. The basic laws of physics are universal. The and fossils. R same laws can be applied in widely different 6. Physics in relation to seismology. The move- domains and conditions. ment of the earth crust and types of waves so 3. Because of ability of the scientists to separate generated help a lot in the study of earthquake and its the important and essential features from the effect. less important ones, it is easier to understand 7. Physics in relation to meteorology. By studying SI even a highly complex phenomenon. variation of pressure with temperature, we can forecast 1.8 PHYSICS IN RELATION TO weather. OTHER SCIENCES 1.9 PHYSICS IN RELATION TO SOCIETY 14. How is the study of physics useful to the study of other sciences ? 15. The fate of a society is linked to the develop- Physics in relation to other sciences. Physics is ments in physics. Explain. the most fundamental discipline of all sciences. It has Physics in relation to society. The fate of a played a key role in the development of all other society is closely linked to physics. Whatever is dis- IT sciences. covered in physics, it immediately affects the society. 1. Physics in relation to mathematics. Physics is a For example: quantitative science. Mathematics provides the necessary 1. The developments of telephone, telegraph, telex signs and tools which the physicists use. It has played enable us to quickly exchange messages between an important role in the development of theoretical far off places. physics. Had Newton not invented calculus, he would H 2. The discoveries of radio and television have not have been able to discover the universal law of made possible the instantaneous commu- gravitation. nication with other parts of the world. 2. Physics in relation to chemistry. In physics, we 3. The launching of satellites into space has study the structure of atom, radioactivity, X-ray revolutionised the concept of communication. diffraction, etc. Such studies have enabled chemists to O arrange elements in the periodic table on the basis of 4. The development of alternative sources of their atomic numbers. This has further helped to know energy is of great importance to the mankind. the nature of valency and chemical bonding and to 5. Microelectronics, lasers, computers, super- understand the complex chemical structures. conductivity and nuclear energy have entirely 3. Physics in relation to biological sciences. The changed the thinking and the living style of the M developments in life sciences owe a great deal to human beings. physics. (i) Optical microscopes are extensively used in The modem science in general and physics in the study of biology. (ii) With the help of an electron particular has the potentiality to eradicate poverty and microscope, one can study the structure of cell. (iii) The hunger from the surface of the earth and to usher in an X-rays and neutron diffraction techniques have helped age of prosperity: On the other hand, a wrong use of in understanding the structure of nucleic acids, which discoveries of physics may bring about a disaster. The helped to control vital life processes. (iv) Radioisotopes knowledge of physics can be applied alike, to the are used in radiation therapy for the cure of deadly creation of heaven on the earth or to the' total diseases like cancer. annihilation. Our wiseness lies in applying physics to 4. Physics in relation to Astronomy. (i) Astrono- solve the pressing problems the society faces and not mical telescopes are used to study the motion of to annihilate it.  PHYSICAL WORLD 1.5 t,able 1.1 Some great phYSiCists and their discoveries S. No. I Name of Scientist Country Discovery S. No. I Name of Scientist. Country Discovery l. Abdus Salam America Unification of 25. Einstein, Albert Germany Theory of relati- (Pakistan born) weak and electromagnetic I I vity, mass-energy equivalence,. interactions I photoelectric 2. Alfred Noble Sweden Dynamite effect Anderson C.D. America Positron 26. Edison, A America ' Electric bulb, 3. telegraphy R 4. Antony Hewert England Pulsars 27. Faraday, England Laws of electro- 5. Appleton, E. England Exploration of Michael magnetic ionosphere induction, laws of 6. Ampere France Magnetism is due electrolysis, to electric current designed electric 7. Archimedes Greece Principle of motor SI buoyancy, 28. Fermi, Enrico Italy Controlled Principle of the nuclear fission lever 29. Gabor, D. America Holography 8. Bardeen, John America Transistor, superconductivity 30. Galileo, Galilei Italy Law of inertia 9. Bequerrel, France Discovery of 3l. Grahm Bel America Telephone Henry A spontaneous 32. Hertz, Heinrich Germany ; Electromagnetic radioactivity Rudolf waves 10. Bhabha, Homi India Cosmic ray 33. Hess, V.F. Austria Cosmic rays Jehangir showers 34. Huygens, Holland Wave theory of IT 11. Bose J.c. India Ultrashort Christiaan light radiowaves 35. Hubble, Edwin America Expanding 12. Bose, S.N. India Bose-Einstein universe statistics. Uncertainty 36. Heisenberg, Germany 13. Bohr, Niels Denmark Quantum model Wemes principle, Quantum of hydrogen atom mechanics 14. Bragg, W.H. England Crystal structure 37. Kamerling Dutch Low temperature H by X-rays Onnes and liquid helium 15. Bragg, W.L. England Crystal structure 38. Kelvin (William England I Second law of by X-rays Thomson) thermodynamics, 16. Barkla, England Number of thermodynamic Charles G. electrons in an scale of atom temperature O 17. Cavendish England Experimental deter- 39. Landau, Lev Russia Theory of cond- mination of 'G' Davidovich i ensed matter, j Liquid helium 18. Cerenkov, P.A. Russia Cerenkov 40. Lawrence, E.O. America ' Cyclotron radiations 4l. Lippman, G. France Colour 19. Chadwick, England Neutron James photography I M Compton, AH. America , Compton effect 42. Maxwell, James England Electromagnetic 20. Cleric theory, Light as an 2l. Coulomb, France Coulomb's law electromagnetic Charles A de wave 22. Curie, Marie Poland Studies on natural 43. Michelson, AA America Precision optical Sklodowaska radioactivity, I instruments Discovery of , radium and 44. Marconi Italy I Wireless telegraphy polonium 45. Millikan, R.A America ' Measurement of. 23. de-Broglie, France Wave nature of charge on an Louis Victor matter electron 24. Dirac, Paul England Relativistic theory 46. Newton, Isaac England I Law of gravitation, of electron, r I Laws of motion, Quantum statics Reflecting telescope  1.6 PHYSICS-XI S. Name of Discovery 1. Electromagnetic waves are used in radio, Country No. 47. I Scientist Oersted France I Magnetic effect of current television, radar and wireless communication. 2. Newton's concept of gravitation is used in geostationary satellites which help us in fore- 48. Planck, Max Germany Quantum theory casting weather and in geophysical survey. 49. I Pauli, W. America I of radiation Exclusion principle for 3. X-rays are used in radiotherapy, in detecting fractures or dislocations in bones, in studying filling electrons in crystal systems, in engineering and industry. orbitals 4. The study of thermodynamics has helped to SO. Raman, C.V. India Inelastic scattering R I of light (Raman design heat engines which have revolutionised I I I 51. Ramachandran, j India G.N. I I cffect) Triple helical structure of the industry. 5. The study of electricity has led to the develop- ment of electric appliances like electric motors proteins and generators which are the backbones of '1 molecules SI industry. 52. Robert Boyle 1 England Boyle's.law 53. Robert Hooke England Elasticity of 6. Nuclear power stations based on nuclear fission solids, balance constitute one of the major sources of energy. wheel of a watch 54. II Rutherford, Ernest England I Nuclear model of atom 7. The concepts of modem electronics find exten- sive use in telephone exchanges, robots, etc. 8. The discovery of silicon chips has brought a 55. Roentgen, W.K. Germany I X-rays revolution in computer industry. 56. S. America Structure and Chandershekhar (India evolution of stars, 9. Geothermal energy, i.e., the heat in the depth of born) I Chandrashekhar IT the earth is being used these days. The tidal limit energy in the oceans and solar energy too can be 57. SchrOdinger, E. Germany Wave mechanics converted into other forms of energy and used. 58. I Saha, M.N. India Thermal ionisation 10. Radioactive isotopes are now being widely used 59. Thomson, J.J. England I Electron in medicine, agriculture and industry. 60. Thomson, G.P. England I Diffraction of electrons by These technologies have made our lives comfor- H I crystals table and materially prosperous. 61. 62. Townes, C.H. Van der Walls, I America Dutch I Maser, Laser Expansion of. Sometimes physics generates new technology. At other times technology gives rise to new physics. Both J.D. I gases and liquids have a direct impact on society. 63. Von Laue, Max Germany Diffraction of X-rays in crystals l:.,able 1.2 Some important technologies and O 64. Volta Italy Discovered first battery. their links with physics 65. Well, W. Germany Laws of radiation of heat Technology Scientific Principle(s) 66. Watt, James England I Steam Engine Steam engine Laws o( thermodyna.~cs __ 67. Yukawa, I Japan I Meson theory of Nuclear reactor - - Controlled nuclear fission M - Hedeki nuclear forces Radio and Generation, propagation and detec- Television tion of ~~ctromagn~tic waves - 1.10.. PHYSICS IN RELATION TO TECHNOLOGY Wireless telegraphy Propagation of electromagnetic waves 16. Technological advancements owe a great deal to S?~uters Digital logic ~ electronic circuits _ the developments in physics. Briefly explain. Lasers Light amplification by stimulated Physics in relation to technology. The applica- emission of radiation (population tions of physics have played a key role in the develop- inversion) ment of technology. Today we see the applications of Production of ultra Superconductivity physics in every walk of life. Some of the major high magnetic fields technologies based on the applications of physics are as follows: Rocket propulsion Newton's laws of motion  PHYSICAL WORLD 1.7 Technology Scientific Principle(s) law of gravitation, the gravitational attraction between Electric generator Faraday's laws of electromagnetic two bodies of masses 11"J. and "2 and separated by induction distance r is given by Hydroelectric Conversion of gravitational power potential energy into electrical energy Aeroplane Bernoulli's principle in fluid where G is the universal gravitational constant. dynamics __ Important properties of gravitational force: Particle accelerator / Motion of charged particles in 1. It is a universal attractive force. Cyclotron electromagnetic fields 2. It is directly proportional to the product of the R Sonar Reflection of ultrasonic waves masses of the two bodies. Optical fibres _ Total internal reflection of light 3. It obeys inverse square law. Non-reflecting Thin film optical interference coatings _ _ _ _ _ 4. It is a long range force and does not need any Electron microscope ':\':ave nature of electrons intervening medium for its operation. SI Photocell Photoelectric effect S. Gravitational force between two bodies does not depend upon the presence of other bodies. Fusion test reactor Magnetic confinement of plasma (Tokamak) 6. It is the weakest force known in nature. Giant Metrewave Detection of cosmic radio waves 7. It is a central force (i.e., it acts along the line Radio Telescope joining the centres of the two bodies). (GMRT) 8. It is a conservative force (i.e., work done in Bose-Einstein Trapping and cooling of atoms by laser beams and magnetic fields moving a body against the gravitational force is condensate path independent). IT 1.11 FUNDAMENTAL FORCES IN NATURE 9. Gravitational force between two bodies is thought to be caused by an exchange of a 17. Name the four basic forces in nature. Arrange particle called graviton. them in the order of their increasing strengths. Examples of gravitational force: Fundamental forces in nature. In the macroscopic 1. All bodies fall because of the gravitational force world, we observe several kinds of forces : muscular of attraction exerted on them by the earth. H force, contact forces of support and friction, forces exerted by springs and strings, viscous forces, electric 2. Gravitational force governs the motion of the forces, magnetic forces, etc. All these forces between moon and the artificial satellites around the earth ; macroscopic objects arise from two fundamental forces: and the motion of the planets around the sun. 1. Gravitational force 2. Electromagnetic force 3. Gravitation plays a key role in the formation and evolution of stars, galaxies and galactic clusters. O In the microscopic world, in addition to the above two forces, two more basic forces are required to 1.13 THE ELECTROMAGNETIC FORCE account for the various atomic and nuclear processes. 19. What is electromagnetic force ? Mention its These are important properties. Give some examples of the electro- 1. Strong nuclear force 2. Weak nuclear force magnetic force. M The ratio of the strength of the four fundamental Electromagnetic force. The force acting between two forces in nature is electric charges at rest is called electrostatic force. According Fe : Fw : FE : Fs = 1: 10 25 : 10 36 : 10 38 to Coulomb's law, the magnitude of the electrostatic force.F between two point charges ql and q2 separated 1.12 THE GRAVITATIONAL FORCE by distance r in vacuum is given by 18. What is gravitational force? Mention its important F __ 1_ qlq2 properties. Give some examples of gravitational force. - 41t EO' ,z Gravitational force. It is the force of mutual where EO is the permittivity of vacuum. The force acting attraction between two bodies by virtue of their masses. It is between two magnetic poles is called magnetic force. In fact, a universal force. Every body attracts every other body electrostatic and magnetic forces are closely inter- of the universe with this force. According to Newton's related. For example, a moving charge produces a  1.8 PHYSICS-XI magnetic field. Also, a magnetic field exerts a force on Generally, these forces act normal to the surface of a moving charge. This force depends both on the contact and are of pushing or repelling nature. For magnitude and direction of the velocity of the electric example, a book lying on a table pushes it downwards charge. Thus the electrostatic and magnetic forces are while the table pushes the book upwards. inseparable and are considered as the two facets of a (ii) Force of friction. Sometimes, the electro- general force known as electromagnetic jorce. magnetic contact force between two bodies may have a Important properties of electromagnetic force: component acting parallel to the surface of contact. This ,'1. Electromagnetic force may be attractive or component is called friction. When bodies are placed with their smooth surfaces in contact, they provide repulsive. Like charges repel each other and only a small parallel component of contact force and unlike charges attract each other. hence friction between them is negligibly small. For R 2. It obeys inverse square law. example, when we climb up a tree, its rough trunk 3. It is a long range jorce and does not require any provides sufficient frictional force parallel to the surface intervening medium for its operation. of the tree which helps us cling on to the trunk. It is 4. It is a central jorce. difficult to climb up a smooth metallic lamp post because it does not provide enough parallel frictional force. SI 5. It is a conservative jorce. 6. It is 10 36 times stronger than the 'gravitational I (iii) Eldas(tiCforce in da)~pring. Wh en a fSPlrin.g is e ongate or compresse ,It exerts a orcef 0 e asticity force. - which arises due to the net attraction (or repulsion) 7. It is caused by the exchange of photons (y) between the neighbouring atoms of the spring. This between two charged particles. net attraction (or repulsion) is the (unbalanced) sum of Examples of electromagnetic force: the electrostatic forces between the electrons and nuclei 1. When a spring is compressed/elongated, it exerts of these atoms. a force of elasticity due to the net repulsion / (iv) Van der Walls' force. The Van der Walls' force IT attraction between its neighbouring atoms. This between two neutral molecules of a gas is not a funda- net repulsion or attraction is the sum of the mental force but a derived force. It is the net residual electrostatic forces between the electrons and force obtained by summing up the fundamental nuclei of the atoms. electrostatic forces between the various electrons and nuclei of the two molecules. 2. The Van der Walls' force between two neutral molecules of a gas is net sum of the electrostatic (v) Force in a taut string/rope. Consider a block forces between the electrons and nuclei of the tied to the lower end of a string suspended from a rigid H two molecules. support. The string is in a state of tension. The electrons and protons of the lower end of the string exert 1.14 V ELECTROMAGNETIC NATURE OF electrostatic forces on the electrons and protons of the SOME MACROSCOPIC FORCES block. The resultant of these forces balances the weight of the block. Generally, a string under tension exerts an Introduction. The macroscopic forces of our daily electromagnetic force of pulling nature on the two O life such as tension, friction, contact force, spring force, bodies attached to its two ends. etc., are derived from the fundamental electrostatic 21. Electromagneticjorce is enormously stronger than force only. All matter consists of charged particles like the gravitational jorce. Give an example from daily life electrons and protons. The strong electromagnetic to illustrate it. force between these particles is responsible for the M structure of atoms and molecules, rate of chemical Electromagnetic force is much stronger than the reactions, and the mechanical, thermal and electrical gravitational force. When we hold a book in our hand, the earth exerts a very large gravitational force properties of materials. on the book due to its huge mass. This force is balanced 20. Discuss the electromagnetic nature of (i) contact by the normal force exerted by our hand. But the latter force between two bodies (ii) force of friction (iii) elastic force is the net electromagnetic force between the force in a spring (iv) Van der'Walls' force between gas charged constituents of our hand and the book at the molecules and (v) force in a taut string/rope. surface of contact. Clearly, electromagnetic force is (i) Contact force between two bodies. When we much stronger than the gravitational force.. place two bodies in contact with each other, their 22. Although gravitational jorce is incomparably atoms come close to each other at the surface of weaker than the electromagnetic jorce, yet it governs the contact. Large electromagnetic forces begin to act large scale motion both on terrestrial and astronomical between the charged constituents of these atoms. scales. How ?  PHYSICAL WORLD 1.9 Gravitational force governs the large scale 2. Radioactivity occurs in heavier nuclei because---' motion. Mass is only of one type i.e., there are no of insufficient nuclear force between their positive and negative masses. So the gravitational protons and neutrons. force is always attractive. But electric charges may be 3. The concept of nuclear force is useful in positive and negative. Consequently, the electro- obtaining nuclear energy via the processes of magnetic forces may be both attractive and repulsive. nuclear fission and fusion. Between two neutral objects, the gravitational force Electrons do not experience the strong goes on adding over all pairs of particles of the two nuclear force. objects while the electromagnetic forces being equally attractive and repulsive, add up to zero. That is why 1.16 THE WEAK NUCLEAR FORCE R the large scale motion in the universe is controlled by 24. What is weak nuclear force ? Explain with the the weakest gravitational force. help of an example. Give important properties of weak nuclear force. 1.15 THE STRONG NUCLEAR FORCE Weak nuclear force. It is the force that appears only 23. What is strong nuclear force ? Mention its between elemeniaru particles involved in a nuclear process SI important properties. Give some examples of this force. such as the I3-decayof a nucleus. In a I3-decay, the nucleus Strong nuclear force.The strong attractive force emits an electron and an uncharged particle called which binds together the protons 'and neutrons in a nucleus neutrino. The electron and neutrino interact with each is called strong nuclear force. This force cannot be other through the weak nuclear force. The weak electrostatic force because positively charged protons nuclear force is much stronger than the gravitational strongly repel each other at such small separations of force, but much weaker than strong 'nuclear and the order of 10 -15 m. Also the gravitational attraction electromagnetic forces. This is obvious from the fact between two protons being much weaker, cannot that the decay of an elementary particle caused by overcome this electrostatic repulsion. So a new attractive weak nuclear force (e.g., the decay of a pion to a muon IT force must be acting between the nucleons (protons and a neutrino) is much slower than the decays caused and neutrons). This strong nuclear force is strongest of by strong nuclear or electromagnetic forces. all the fundamental forces, about 100 times stronger Important properties of weak nuclear force: than the electromagnetic force. 1. Any process involving neutrino and antineu- Important properties of strong nuclear force: trino is governed by weak nuclear force beacuase these particles can experience only weak interac- 1. It is the strongest interaction known in nature, H tion and not the strong nuclear interaction. which is about 10 38 times stronger than the 2. Weak nuclear force is 1025 times stronger than gravitational force. the gravitational interaction. 2. It is a short range force that operates only over 3. It operates only through a range of nuclear size the size of the nucleus (= 10- 15 m). ("" 10-15 m). 3. It is basically an attractive force, but becomes O 4. The messenger particles that transmit the weak repulsive when the distance between the force between elementa7 particles are the nucleons becomes less than 0.5 fermi (1 fermi massive vector bosons (W , Z). =10-15 m). 4. It varies inversely with some higher power (> 2) of distance...., Neutrino is an elementary particle with zero rest mass M 5. It is a non-central and non-conservative force. and zero charge. 6. It has charge independent character i.e., nuclear..., It will be more correct to say that /3- decay ejects an forces between proton-proton, proton-neutron electron and an antineutrino while /3+decay ejects a positron (antielectron) and' a neutrino. and neutron-neutron are almost equally strong...., Elementary particles are grouped as baryons, 7. It is caused by the exchange of particles, called mesons, leptons. Nucleons (protons and neutrons) Il-mesons, and their higher counterparts are called baryons. The Examples of nuclear force: pions, kaons, etc., are mesons. Baryons and mesons 1. Nuclear forces bind together the protons and are together called hadrons which can interact through strong nuclear force. Electrons, neutrinos neutrons in the nuclei, So they are responsible and their higher mass counterparts are called leptons. for stability of nuclei and hence of the atoms and Leptons do not experience the strong force, only the ultimately of all matter that exists in the universe. weak force is dominant between them.  1.10 PHYSICS-XI 1.17.. BASIC MECHANISM OF THE l:,able 1.3 Some of the important aspeds of FUNDAMENTAL FORCES fundamental forces in nature 25. Briefly explain the mechanism that gives rise to lunda- Particles the various fundamental forces between elementary mental Relative Range on which Messenger particles. Give the names of the exchange particles for strength particle force force acts each of these forces. ' Gravi- 1 Infinite All Gravitons Fundamental forces arise due to exchange of particles. tational particles force Each fundamental force between two elementary particles -- arises from the exchange of its characteristic particles called Weak 1025 Very short, Elemen- Vector nuclear within tary bosons the quanta of that force or the messenger particles. To R force nuclear size particles '-understand this mechanism, consider interaction bet- - l::10-15m - ween two persons A and R Suppose A throws a ball Electro- 1036 Infinite Charged Photons and B catches it. By momentum conservation, A suffers magnetic particles a recoil when he throws the ball. Similarly, B suffers a force - --38 - - - - -- recoil when he catches the ball. Also the recoil force Strong 10 Very short, Nucleons Mesons SI nuclear within changes the momentum of each person. force nuclear size The exchange particles for various fundamental (""to-15m) forces are as follows: 1. Electromagnetic force between two charged For lour Knowledge - particles (like electrons) arises from the exchange of.\. The strong nuclear force is not a true' fundamental photons (y) between them, as shown in Fig. l.1(a)..force. The nucleons themselves are builtof subunits These photons are emitted by one electron and called quarks. The quark-quark force; which binds absorbed by the other within a very short time. As the nucleons together is now regarded as fundamental IT these, photons cannot be detected, so they are called force. It is caused by the exchange of massless virtual photons. particles called gIuons. 2. Weak nuclear force during the ~ -decay of a nucleus arises from the exchange of massive' particles, 1.18.. UNIFICATION OF FORCES called vector bosons (W ± , Z) between the elementary 26. What is the basic quest of modern physicists ? particles. As shown in Fig. 1.1(b), the charge of vector Mention the various significant attempts made towards H boson (W -) is exchanged in the ~ -decay of a neutron. the unification of forces in a chronical order. 3. Strong nuclear force between two nucleons arises Basic quest of modern physicists is the from the exchange of mesons between them. For unification of forces. The great advacements in example, negatively charged pior. (1t-) is exchanged in ' physics are the result of unification of different the nuclear force inyolved in np" pn scattering, "as theories and domains. Different attempts made by the O shown in Fig. l.1(c). distinguished physicists from time to time in persuing !.-----0::::::::- the goal of unification of forces are listed in Table 1.4. ~-H:- n W. v,. l:,able 1.4 Some of the milestones towards the uniffcation of forces M (a) Electromagnetic (b) Weaknuclear Physidst( s) Achievement In Year unification ,,~. ~-- Development of nuclear weapons. rockets and satellites. (g) Development of new and powerful techniques if 2. Fabrication of superconductors near the room chemical and biological warfare. temperature will help to. transmit electric po.wer (h) Purification of water for drinking. without any wastage of energy. ' (i) Plastic surgery. 3. The extensive use of computers will increase work (j) Cloning. R efficiency. 4. Biotechnology. Ans. (a) Good, mass vaccination protects us from this 1.9. Write in about 1000 words afiction piece based on your speculation on the science and technology of the twenty-second drea'ded disease. century. (b) Good, it educates,' entertains and creates awareness amongst the people. SI Ans. These days, we go.to.distant places by means of a car or an aircraft fuelled by petrol. In the twenty-second ,(c) It cannot be clearly catego.rised as it may be century, we may plan for a journey to.a distant star located misused by people, -t-, hundreds of light years away from the earth by means of a (d) Good, computers work very fast and with great spaceship, without taking care of any fuel needs. accuracy. The spaceship is sent into.space by firing a rocket engine (e) Good, artificial satellites are useful for studying from a launching pad. As it enters the region of magnetic universe, as communication means and in field in space, it is propelled by electricity generated due forecasting weather. to. electromagnetic induction. The current induced is fed if> Bad, nuclear weapo.ns are weapo.ns of mas's destruction, IT to. the electric motors via superconducting wires. This avoids wastage of electric energy in the form of heat. (g) Bad, these techniques are the weapo.ns of mass Now suppo.se the spaceship enters a region in space destruction, where the temperature is very high. The connecting wires (h) Good, otherwise the polluted water may cause at once lose the superconducting properties. This may many diseases. ' cause a panic in the spaceship as no. po.wer is available. (i) Good, it helps to.remove deformations in the body, Another spaceship containing both matter and antimatter (]) Good. H in separate chambers may come to. its rescue. Hence the spaceship may continue its journey to.distant star without 1.12. India has had a long and unbroken tradition of great bothering about any fuel crisis. scholarship - in mathematics, astronomy, linguistics,' logic and ethics. Yet, in parallel with this, several superstitious and -.1.10. Attempt to Jc1ffiiUfate' your.'moral views on the r obscurantistic attitudes and practices flourished in our soc,iety, ·practic~ence.. Imagi~e, yourself stumbling upon a and unfortunately continue even today - among many educated. O discouerv, which has' great academic interest but is certain to people too. How will you use your knOwledge' of. science to have nothing but dangerous consequences for the human develop str~tegies tocounter these attitudes?.' society. How, if at all, will' you resolve your dilemma ?. Ans, Educating the common man is the only way..to. Ans. Science is the search for eternal truth. It is the get rid of superstitious and obscurantistic attitudes and moral duty of a scientist to.expose the truth. If there is any practices flourished in our society. The phenomena which danger to.the mankind from a discovery, he should try to. people attribute to. evil spirits and- gods should be M build a public opinion against the misuse of discovery. explained scientifically through mass media such as , Moveover, he should develop.the means to. prevent its radio, television, newspapers,' cinema, etc. The school misuse. It is equa:lly possible that this' discovery may students must be explained the day to. day phenomena pro.ve to.be of immense 'importance to.the mankind later on, occurring around them in an effective scientific manner.. 1.11. Science, like any knowledge, can be put to good or bad use, 1.13. Though the law gives women equal status in' India; depending on the user. Given below are some of the applications of many people hold unscientific views on a woman's innate science. Formulate your views on whether the particular application nature, capacity and intelligence, and in pr.actice give- them a' is good, bad or something that cannot be so clearly categorised : secondary status and role. Demolish this "view using scientific: (a) Mass vaccination against small pox to curb and finally arguments, and by quoting examples of great women in sciimce eradicate this disease from the population. (This has and other spheres; and persuade yourself and others that, given already been successfully done in India). equal opportunity, women are on par with men. (b) Television for eradication of illiteracy 'and for mass Ails. The development of human mind depends communication of news and ideas. mainly on the nutrition content of prenatal and postnatal  1.16 PHYSICS-XI diet and the environment around it. The gender factor has Bohr, Heisenberg, Chandrasekhar and Feynman. You are urged no role in the development of human mind. When given to make special efforts to get access to the general books and equal opportunities, mental development of females will writings by these and other great masters of physics. Their be as rapid as that of males. Madam Marie Curie won the writings are truly inspiring. Noble prize twice in science which is a rare gesture. Mrs. Ans. There is no doubt that the great laws of physics Indira Gandhi and Mrs. Margret Thatcher have flourished are at once simple and beautiful. The Einstein's mass- excellently well in politics. energy equivalence relation : E = mc2 has tremendous 1.14. "It is more important to have beauty in the equations impact on various physical phenomena and on human of physics than to have them agree with experiments ". The great lives yet it is so simple. Bohr's quantum condition : British physicist P. A. M. Dirac held this view. Criticize this L = nh / 21t and Planck's quantum condition : E = hv are R statement. Look out for some equations and results in this book quite simple, yet quite important laws of physics. which strike you as beautiful. Heisenberg's uncertainty principle : ax· Sp ~ h / 21t is a Ans. Dirac was his genious best when he pronounced simple law but changed the thinking of physicists this statement. A physical equation must be simple and drastically. These are just few examples of the beauty of hence beautiful. It will automatically agree with the physics laws which inspired the physicists and opened up new vistas in physics. SI experimental r~sults. For example, consider Einstein mass-energy relationship: E = mc2 1.16. Textbooks on science may give you a wrong This simple equation not only governs energy impression that studying science is dry and all too serious and generation in the sun and the other stars but it is also that scientists are absent-minded introverts who never laugh or grin. This image of science and scientists is patently false. mainly responsible for energy generation on the earth Scientists, like any other group of humans, have their share of from the processes of nuclear fission and nuclear fusion. humorists, and many have led their lives with a great sense of However, it is not always true. Although some equa- [un and adventure, even as they seriously pursued their tions of Quantum Mechanics and Theory of Relativity are scientific work. Two great physicists of this genre are Gamow highly cumbersome and difficult to understand, yet they and Feynman. You will enjoy reading their books. IT agree with the experiments. Ans. True, the scientists like any other group of 1.15. Though the statement quoted above may be disputed, humans have their share of humorists, and many have led most physicists do have a feeling that the great laws of physics their lives with a great sense of fun and adventure inspite are at once simple and beautiful. Some of the notable physicists, of pursuing their scientific work seriously. Two such great besides Dirac, who have articulated this feeling, are: Einstein, physicists were Gamow and Feynman. H Text Based Exercises 1 Mark Each O 1. What is science? 14. Some exciting and educative physical phenomena 2. What is the origin of the word science? and experience lead to the development of a 3. What is meant by the sanskrit word Shastra and theory. Give two such examples. Arbic word Ilm ? 15. Name two Indian born physicists who have been 4. What is the basic aim of science? awarded Noble Price in physics. (Chandigarh02] 5. What is the difference between physical and 16. Name the scientist who replaced circular orbits by M biological sciences ? elliptical orbits in the heliocentric theory of the sun. 6. What is scientific attitude? 17. Name the scientist who discovered X -rays. 7. What is a theory ? 18. Who discovered electrons? 8. What is physics? 19. Name the scientist who first proposed the wave 9. What is the origin of the word physics? theory of light. 10. What are the two basic quests in physics? 20. Name the famous scientist who discovered the law n. Name the domain of physics that deals with of gravitation. phenomena intermediate between macroscopic. 21. Name the physicist who first proposed the nuclear and microscopic domains. model of the atom. 12. Physics has a very limited scope and is only the 22. Who gave the quantum model of hydorgen atom? pastime of a few blessed ones. Is it true? 23. Name the scientists responsible for the develop- 13. Stateone law that holds good in all natural processes. ment of Quantum Mechanics.  PHYSICAL WORLD 1.17 24. Name the physicist who first proposed the concept 48. Name the four fundamental forces in nature. of antiparticle. 49. Name the forces having the longest and shortest 25. Which physicist first confirmed the existence of ranges of operation.. positrons experimentally? 50. Arrange the weak-nuclear force, electromagnetic 26. Name the physicist who first measured the charge force and gravitational force in the decreasing on an electron experimentally. order of the strengths. 27. Name the physicist associated with wave- particle 51. Give the ratio of the strengths of the four funda- duality. mental forces in nature. 28. Who first discovered neutrons? 52. What is the nature of the intermolecular Van der 29. Name the scientist who received Nobel Prize twice Walls' forces ? R in physics. 53. How many times is the strong nuclear forcestronger 30. Name the physicist who first gave the exchange than the electromagnetic force ? theory of nuclear forces. 54. Is strong nuclear force a true fundamental force? 31. Name the scientist who was first awarded two 55. Which force mainly governs the structure of atoms Nobel Prizes. and molecules? SI 32. What were the important contributions of Madame 56. Which fundamental force governs the large scale Marie Curie ? motion in the universe? 33. Who first discovered radioactivity ? 57. What is meant by charge independent character of 34. Who first gave theory of expanding universe ? strong nuclear force ? 35. Mention some important contributions of Albert 58. Which class of elementary particles experiences the Einstein to physics. weak nuclear forceand not the strong nuclear force? 36. Name the Indian physicist, who was first awarded 59. Among which type of elementary particles does the Nobel Prize. the electromagnetic force act ? 37. What was the impo~t discovery of C.V. Raman? 60. Name the class of elementary particldes on which IT 38. What was the major contribution of Indian strong nuclear force acts. physicist S.N. Bose? 61. Which particle initiates the electromagnetic force 39. With which field work was the famous Indian between two electrons? , physicist H.J. Bhabha associated? 62. What are the exchange particles for the operation of 40. Name the Indian physicist associated with the (I) strong nuclear force and (il) weak nuclearlorce? triple helical structure of proteins. 63. How much is the range of gravitational force ? 41. Mention the major areas of modem scientificresearch What are the messenger particles for this force? H in chemical and biologicalsciencesthat are bound to 64. Name the physicists who first unified the electric affectthe human society of the twenty-first century. and magnetic phenomena. 42. On which scientificprincipledoes an aeroplanework? 65. Name the unified domain of weak nuclear force 43. On which scientific principle calculators and and electromagnetic force. computers are based ? 66. Name the physicists who first predicted the O 44. Which technology of physics has triggered the existence of electro-weak force. computer revolution in the last three decades of 67. What are the conserved quantities in nature? Give the twentieth century ? two examples of such quantities. 45. What does the word LASER stand for? On what 68. State the law of conservation of momentum. From basic principle does it work? which symmetry principle is this law obtained? 46. Name the phenomenon used in the production of 69. Which symmetry laws lead to (i) law of conser- M ultra high magnetic fields. vation of energy and (ii) law of conservation of 47. Name the philosopher who said about science "we angular momentum ? know very little yet it is astonishing that we know 70. Some conservation laws are true for one funda- so much and still astonishing that so little knowledge mental force but not for other. Give two such (of science)can give us so much power". examples. Answers 1..Science is a systematised knowledge about the 2. The word science originated from the Latin verb various natural phenomena gained by the man scientia which means to know. through his careful experimentation, keen obser- 3. Bothof these words stand for organised knowledge. vation and accurate reasoning. 1.18 PHYSICS-XI 4. The basic aim of science is to search for truth. It 33. A.H. Becquerrel in 1896. tends to analyse the natural phenomena occurring 34. Edwin Hubble of U.S.A. around us'. 35.. (I) Mass-energy equivalence (il) Photoectric effect 5. Physical sciences deal with the properties and and (iii) Theory of relativity. behaviour of non-living matter while biological 36. C.V. Raman. sciences deal with living things. 37. Inelastic scattery of light by molecules. 6. Scientific attitude requires a flexible open-minded 38. Quantum statistics (Bose-Einstein statistics). approach towards solving problems in which 39. Cascade process in cosmic radiation. other important points of view are not neglected. 40. G.N. Ramchandran. 7. Theory is the name given to a minium number of 41. Genetic engineering, biotechnology, new chemical R laws in terms of which the behaviour of a physical materials, etc. system can be explained. 42. Bernoulli's principle in fluid dynamics. 8. Physics is the study of the basic laws of nature and 43. Digital logic of electronic circuits. their manifestations in various natural phenomena. 44. Technology of silicon chip. 9. The word physics originates from a Greek word SI meaning nature. 45. The word LASERstands for light amplification by stimulated emission of radiation. It works on the 10. (i) Unification and (ii) Reductionism. principle of population inversion. 11. Meso-scopic physics which deals with a few tens 46. Superconductivity. or hundreds of atoms. 47. Bertrand Russel.' 12. No, physics has a very wide scope. We find application of physics in every walk of life. 48. (i) Gravitational force (il) Electromagnetic force 13. One such law is the Newton's law of gravitation. It (iil) Strong nuclear force and states that every body in the universe attracts (iv) Weak nuclear fOIDi. every other body with a force which is directly 49. Gravitational force has the longest range and IT proportional to the product of their masses and nuclear force has the shortest r~ge. inversely proportional to the square of the distance 50. Electromagnetic force> Weak nuclear force> between them. Gravitational force. 14. (i) The fall of an apple from a tree and motion of 51. Fe: Fw : FE: Fl'J = 1: 1