PHYSICSHIGHER SECONDARY

SECOND YEAR

VOLUME - II

TAMILNADU TEXTBOOKCORPORATIONCOLLEGE ROAD, CHENNAI - 600 006

Untouchability is a sin

Untouchability is a crime

Untouchability is inhuman

Revised based on the recommendation of theTextbook Development Committee

REVIEWERS

P. SARVAJANA RAJANSelection Grade Lecturer in PhysicsGovt.Arts CollegeNandanam, Chennai - 600 035

S. KEMASARISelection Grade Lecturer in PhysicsQueen Marys College (Autonomous)Chennai - 600 004

Dr. K. MANIMEGALAIReader in PhysicsThe Ethiraj College for WomenChennai - 600 008

G. SANKARISelection Grade Lecturer in PhysicsMeenakshi College for WomenKodambakkam, Chennai - 600 024

G. ANBALAGANLecturer in PhysicsAringnar Anna Govt. Arts CollegeVillupuram.

AUTHORS

S. PONNUSAMYAsst. Professor of PhysicsS.R.M. Engineering CollegeS.R.M. Institute of Science and Technology(Deemed University)Kattankulathur - 603 203

S. RASARASANP.G. Assistant in PhysicsGovt. Hr. Sec. SchoolKodambakkam, Chennai - 600 024

GIRIJA RAMANUJAMP.G. Assistant in PhysicsGovt. Girls Hr. Sec. SchoolAshok Nagar, Chennai - 600 083

P. LOGANATHANP.G. Assistant in PhysicsGovt. Girls Hr. Sec. SchoolTiruchengode - 637 211Namakkal District

Dr. N. VIJAYANPrincipalZion Matric Hr. Sec. SchoolSelaiyurChennai - 600 073

Dr. HEMAMALINI RAJAGOPALSenior Scale Lecturer in PhysicsQueen Marys College (Autonomous)Chennai - 600 004

This book has been printed on 60 G.S.M paper

Price : Rs.

This book has been prepared by the Directorate of School Education on behalf ofthe Government of Tamilnadu

Printed by offset at :

CHAIRPERSON

Dr. S. GUNASEKARANReader

Post Graduate and Research Department of PhysicsPachaiyappas College, Chennai - 600 030

c Government of TamilnaduFirst edition - 2005Revised edition - 2007

III

PrefaceThe most important and crucial stage of school education is the

higher secondary level. This is the transition level from a generalisedcurriculum to a discipline-based curriculum.

In order to pursue their career in basic sciences and professionalcourses, students take up Physics as one of the subjects. To providethem sufficient background to meet the challenges of academic andprofessional streams, the Physics textbook for Std. XII has been reformed,updated and designed to include basic information on all topics.

Each chapter starts with an introduction, followed by subject matter.All the topics are presented with clear and concise treatments. Thechapters end with solved problems and self evaluation questions.

Understanding the concepts is more important than memorising.Hence it is intended to make the students understand the subjectthoroughly so that they can put forth their ideas clearly. In order tomake the learning of Physics more interesting, application of concepts inreal life situations are presented in this book.

Due importance has been given to develop in the students,experimental and observation skills. Their learning experience wouldmake them to appreciate the role of Physics towards the improvementof our society.

The following are the salient features of the text book.

The data has been systematically updated.

Figures are neatly presented.

Self-evaluation questions (only samples) are included to sharpenthe reasoning ability of the student.

While preparing for the examination, students should notrestrict themselves, only to the questions/problems given in the selfevaluation. They must be prepared to answer the questions andproblems from the text/syllabus.

Dr. S. GunasekaranChairperson

IV

CONTENTS

Page No.

6. Atomic Physics 1

7. Dual Nature of Radiation and

Matter and Relativity 56

8. Nuclear Physics 83

9. Semiconductor Devices andtheir Applications 131

10. Communication Systems 198

Appendix 238

Logarithmic and other tables 242

1

At atmospheric pressure, air and other gases are poor conductorsof electricity. This is because, they do not have free charged particles.However, electric current may be passed through a gas if by somemechanism, charged particles are produced in the gas. This can be donein many ways, such as (i) by applying a large potential difference acrossa gas column at very low pressure and (ii) by allowing X-rays to passthrough the gases.

The study of electric discharge through gases gives valuableinformation regarding the structure of atoms. This has led to thediscovery of electrons by J. J. Thomson and later on, to the discoveryof X-rays by Roentgen.

6.1 Discharge of electricity through gases at low pressure -Discovery of electrons

A discharge tube is anarrangement to study theconduction of electricity throughgases. It is a closed, strong glasstube of length 50 cm and diameter4 cm, filled with a gas. Two metalelectrodes C and A are fitted insidethe tube at the ends as shown in Fig 6.1. The side tube P is connectedto a high vacuum pump and a low pressure gauge. The electrodes C andA are connected to the secondary of a powerful induction coil, whichmaintains a potential difference of 50,000 V. The electrode C connectedto the negative terminal of the induction coil is called the cathode andthe electrode A connected to the positive terminal is called the anode.

When the pressure of the gas inside the discharge tube is reducedby working the vacuum pump, to about 110 mm of Hg, no dischargeoccurs through the tube. At a pressure of about 100 mm of Hg, thedischarge of electricity through the gas begins and irregular streaks of

6. Atomic Physics

Fig 6.1 Discharge tube

- +Induction coil

C A

P

2

light appear, accompanied by a crackling sound. As the pressure isreduced to the order of 10 mm of Hg, the irregular streaks broaden outinto a luminous column extending from the anode, almost upto thecathode. This column is known as the positive column. With furtherreduction in pressure to around 0.01 mm of Hg, the positive columndisappears and Crookes dark space fills the whole tube. At this stage,the walls of the glass tube fluoresce with green colour. This greenishglow in the final stage of the gaseous discharge is found to be afluorescence of the glass produced by some invisible rays emanatingfrom the cathode (shown in the wrapper). These rays are called cathoderays and are found to be electrons.

6.1.1 Properties of Cathode rays

Cathode rays have the following properties:

(i) They travel in straight lines.

(ii) Cathode rays possess momentum and kinetic energy.

(iii) Cathode rays produce heat, when allowed to fall on matter.

(iv) Cathode rays produce fluorescence when they strike a numberof crystals, minerals and salts.

(v) When cathode rays strike a solid substance of large atomicweight, X-rays are produced.

(vi) Cathode rays ionize the gas through which they pass.

(vii) Cathode rays affect the photographic plates.

(viii) The cathode rays are deflected from their straight line pathby both electric and magnetic fields. The direction of deflection showsthat they are negatively charged particles.

(ix) Cathode rays travel with a velocity upto (1/10)th of the velocityof light.

(x) Cathode rays comprises of electrons which are fundamentalconstituents of all atoms.

3

6.1.2 Positive rays (or) Canal rays

While conductingexperiments on the gas dis-charge, in 1886, GermanPhysicist, E.Goldstein,discovered that if thecathode used is perforated,luminous streams appe-ared in the tube behind the cathode. These streams were called ascanal rays. The discharge tube designed by Goldstein is shown inFig 6.2. The tube contains an anode (A), a perforated cathode (K) anda fluorescent screen (S). At a pressure of about 1mm of mercury, aluminous stream of particles were observed behind the cathodeproceeding in a direction opposite to that of the cathode rays. Goldstein,called them as canal rays, since they pass through and emerge from theholes, in the cathode in straight lines, opposite to the direction of thecathode rays. From the nature of the deflection produced, by amagnetic field or electric field, these rays were found to be positivelycharged particles. Hence, canal rays are most commonly known aspositive rays.

Properties of Canal rays

(i) They are the streams of positive ions of the gas enclosed in thedischarge tube. The mass of each ion is nearly equal to the mass of theatom.

(ii) They are deflected by electric and magnetic fields. Theirdeflection is opposite to that of cathode rays.

(iii) They travel in straight lines.

(iv) The velocity of canal rays is much smaller than the velocity ofcathode rays.

(v) They affect photographic plates.

(vi) These rays can produce fluorescence.

(vii) They ionize the gas through which they pass.

Fig 6.2 Production of canal rays.

+A SK

4

6.1.3 Determination of specific charge (e/m) of an electron Thomsons method.

In 1887, J.J. Thomson, measured the specific charge (e/m) ofthe cathode ray particles. The specific charge is defined as the chargeper unit mass of the particle. Thomson discovered that the value of(e/m) was independent of the gas used and also independent of thenature of the electrodes.

Principle

The fact that the cathode rays (electrons) are deflected byelectric and magnetic fields is made use of in this method.

Experimental arrangement

A highly evacuated discharge tube used in this experiment is asshown in Fig. 6.3. Cathode rays are produced by the discharge between

the cathode and the anodes D1and D2. A thin pencil of cathoderay comes out through fine pinholes in the anode discs. Thecathode rays then passbetween