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CLASS - XSolutions

CLASS - X

FOUNDATION OLYMPIAD&

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hysic

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IntegratedSyllabus

PH

YS

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S

UNIQUE ATTRACTIONS

�

� Cross word Puzzles

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Basic Practice�

Further Practice�

Brain Nurtures�

� Numerical Problems

Conceptual Questions

� Multiple Answer Questions

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FOUNDATION & OLYMPIAD

E: X P L D R E: R

PHYSICS

CLASS - 10

BRAIN MAPPING ACADEMY

MapptngYour Future

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Published by:

Brain Mapping Academy#16–11–16/1/B, First Floor,Farhat Hospital Road,Saleem Nagar, Malakpet,Hyderabad–500 036Andhra Pradesh, India.✆ 040–65165169, 66135169E–mail: [email protected]: www.bmatalent.com

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ALL RIGHTS RESERVEDNo part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher.

Publication Team

Editor: E.V.S.S. Lakshmi

Design & Typing: M. Manjula, M. Nagender

ISBN: 978-93-80299-01-3

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Every care has been taken by the compilers andpublishers to give correct, complete and updated information. In case there is any omission, printing mistake or anyother error which might have crept in inadvertently,neither the compiler / publisher nor any of thedistributors take any legal responsibility.

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PrefaceSpeed and accuracy play an important role in climbing the competitive ladder. Students have to integrate

the habit of being able to calculate and function quickly as well as efficiently in order to excel in the learning

culture. They need to think on their feet, understand basic requirements, identify appropriate information

sources and use that to their best advantage.

The preparation required for the tough competitive examinations is fundamentally different from that of

qualifying ones like the board examinations. A student can emerge successful in a qualifying examination by

merely scoring the minimum percentage of marks, whereas in a competitive examination, he has to score

high and perform better than the others taking the examination.

This book provides all types of questions that a student would be required to tackle at the foundation level.

The questions in the exercises are sequenced as Basic Practice, Further Practice, Multiple Answer Questions,

Paragraph Questions, Numerical Problems, Conceptual Questions and Brain Nurtures. Simple questions involving

a direct application of the concepts are given in Basic Practice. More challenging questions on direct application

are given in Further Practice. Questions involving higher order thinking or an open-ended approach to

problems are given in Brain Nurtures. These questions encourage students to think analytically , to be

creative and to come up with solutions of their own. Constant practice and familiarity with these questions

will not only make him/her conceptually sound, but will also give the student the confidence to face any

entrance examination with ease.

Valuable suggestions as well as criticism from the teacher and student community are most welcome and

will be incorporated in the ensuing edition.

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1. Ligtht–I ................................................. 01

2. Light–II .................................................. 40

3. Electrostatics ........................................ 69

4. Current Electricity ................................ 96

5. Heating & Chemical Effectsof Electric Current................................ 129

6. Magnetic Effects of ElectricCurrent ................................................. 162

7. Heat ................................................. 186

8. Modern Physics ................................... 213

Answers ................................................ 240

Solution to Crossword Puzzles ........... 255www.bmata

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IIT Foundation and Olympiad Explorer Physics / Class - X

© Brain Mapping Academy6. Magnetic Effects of Electric Current 162

1

Chapter

6Magnetic Effects of

Electric Current

Magnetic Effects of

Electric Current

Chapter

Common Misconception

1. The source of magnetic field is a magnetic charge.

2. When a current flows through a conductor, it has electric field around it.

1. No. The source of magnetic field is not a magnetic charge since an isolated magnetic poles do not exist, whereas the source of electric field is electric charge.

2. When a current flows through a conductor it has magnetic field around it, but not electric field.

Fact

SYNOPSIS

INTRODUCTION

An electric current can also produce magnetic effect. The term �magnetic effect of current�means that �a current flowing in a wire produces a magnetic field around it�. In otherwords, electric current can produce magnetism. The magnetic effect of current wasdiscovered by Oersted in 1820. Oersted found that a wire carrying a current was able todeflect a compass needle. Now, the compass needle is a tiny magnet which can be deflectedonly by a magnetic field. Since a current carrying wire was able to deflect a compassneedle, it was concluded that a current flowing in a wire always gives rise to a magneticfield around it. The importance of magnetic effects of current lies in the fact that it givesrise to mechanical forces. The electric motor, electric generator, telephone and radio,all utilize the magnetic effects of current. The magnetic effects of current is also calledelectromagnetism which means electricity produces magnetism.

Magnetic field pattern due to straight current - carrying conductor (straight current - carryingWire)

The magnetic field lines around a straight conductor (straight wire) carrying current areconcentric circles whose centre lie on the wire. When current in the wire flows in theupward direction, then the lines of magnetic field are in the anticlockwise direction. Ifthe direction of current in the wire is reversed, the direction of magnetic field lines alsogets reversed.

It has been shown by experiments that the magnitude of magnetic field produced bya straight current - carrying wire at a given point is : (i) directly proportional to thecurrent passing in the wire, and (ii) inversely proportional to the distance of that pointfrom the wire. So greater the current in the wire, stronger will be the magnetic field

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IIT Foundation and Olympiad Explorer Physics / Class -X

© Brain Mapping Academy6. Magnetic Effects of Electric Current163

produced. And greater the distance of a point from the current-carrying wire, weakerwill be the magnetic field produced at that point. In fact, as we move away from acurrent-carrying straight wire, the concentric circles around it representing magneticfield lines, becomes larger and larger indicating the decreasing strength of the magneticfield.

(•)

MC

B

A

Magnetic

field lines(anticlockwise)

Current

(Upwards)

Magnetic field pattern due to a straight current - carrying wire

Direction of magnetic Field produced by current - carrying conductor

If the direction of current is known, then the direction of magnetic field produced by astraight wire carrying current can be obtained by using Maxwell�s right - hand thumbrule. According to Maxwell�s right - hand thumb rule : Imagine that you are grasping thecurrent carrying wire in your right hand so that your thumb points in the direction ofcurrent, then the direction in which your fingers encircle the wire will give the directionof magnetic field lines around the wire. Maxwell�s right - hand thumb rule is also knownas Maxwell�s corkscrew rule.

B

Direction of

magnetic

field

Direction of

current

Straight wire

carrying currentRight

hand

A

Right - hand thumb rule to find the direction of magnetic field

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If a conductor of infinitely long wire at a perpendicular distance r from it, at that point,

Magnetic induction, 0 iB

4 rµ=

π

Magnetic Field Pattern due to a Circular Loop Carrying Current

When a current is passed through the circular loop of wire, a magnetic field is producedaround it. The pattern of magnetic field due to a current carrying circular loop is shownin figure. The magnetic field lines are circular near the current-carrying loop. As wemove away, the concentric circles representing magnetic field lines become bigger andbigger. At the centre of the circular loop, the magnetic field lines are straight. By applyingright - hand thumb rule, it can be seen that each segment of circular loop carryingcurrent produces magnetic field lines in the same direction within the loop. At the centreof the circular loop, all the magnetic field lines are in the same direction and aid eachother, due to which the strength of magnetic field increases.

The magnitude of magnetic field produced by a current - carrying circular loop at itscentre is :

i) directly proportional to the radius of circular loop (or circular wire), and

ii) inversely proportional to the radius of circular loop (or circular wire).

M

TMagnetic

field lines

Circular loop of wire

carrying currentCircular

currentC

Magnetic field lines due to circular loop(or circular wire) carrying current.

A circular coil of radius �r� with n turns carries current magnetic field at the axis of theloop,

( )2

03

2 2 2

ni rB

2 r x

µ=+

At the centre of coil, x = 0

0 niB

2rµ∴ =

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Magnetic Field due to a Solenoid

The solenoid is a long coil containing a large number of close turns of insulated copperwire. The magnetic field produced by a current carrying solenoid is similar to the magneticfield produced by a bar magnet. The magnetic field lines inside the solenoid are in theform of parallel straight lines. This indicates that the strength of magnetic field is thesame in a region , it is said to be uniform magnetic field. Thus, the magnetic field isuniform inside a current - carrying solenoid. The uniform magnetic field inside thecurrent - carrying solenoid has been represented by drawing parallel straight field lines.Even the earth�s magnetic field at a given place is uniform which consists of parallelstraight field lines.

The strength of magnetic field produced by a current carrying solenoid depends on :

i) The number of turns in the solenoid. Larger the number of turns in the solenoid,greater will be the magnetism produced.

ii) The strength of current in the solenoid. Larger the current passed through solenoid,stronger will be the magnetic field produced.

iii) The nature of �core material� used in making solenoid. The use of soft iron rod ascore in solenoid produces the strongest magnetism.

X

NS

Magnetic field lines

Solenoid

B

Magnetic field due to a current carrying solenoid is similar to that of a bar magnet

.

Electro magnet

An electric current can be used for making temporary magnets known as electromagnets.An electromagnet works on the magnetic effect of current. An electromagnet consists ofa long coil of insulated copper wire wound on a soft iron core. The core of an electromagnetmust be of soft iron because soft iron loses all of its magnetism when current in the coilis switched off.

Factors affecting the strength of an electromagnet.

The strength of an electromagnet depends on :

i) The number of turns in the coil. If we increase the number of turns in the coil, thestrength of electromagnet increases.

ii) The current flowing in the coil. If the current in the coil is increased, the strength ofelectromagnet increases.

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iii) The length of air gap between its poles. If we reduce the length of air gap between thepoles of an electromagnet, then its strength increases.

N S

C

Soft iron core Coil of insulated copper wire

Electromagnet

Force on Current Carrying Conductor placed in a Magnetic FieldOersted�s experiment which shows that a current carrying wire exerts a force on acompass needle and deflects it from its usual north - south position. Since a compassneedle is actually a small freely pivoted magnet, it can also say that a current carryingwire exerts a mechanical force on a magnet, and if the magnet is free to move, this forcecan produce a motion in the magnet. When a current carrying conductor is placed in amagnetic field, a mechanical force is exerted on the conductor which can make theconductor move. This is known as the motor principle and forms galvanometer.

When a current carrying conductor is placed in a magnetic field, a mechanicalforce is exerted on the conductor which makes it move. The direction of force acting ona current carrying wire placed in a magnetic field is (i) perpendicular to the direction ofcurrent, and (ii) perpendicular to the direction of magnetic field. The maximum force isexerted on a current carrying conductor only when it is perpendicular to the direction ofmagnetic field. No force acts on a current carrying conductor when it is parallel to themagnetic field. The direction of force on a current carrying conductor placed in a magneticfield can be reversed by reversing the direction of current flowing in the conductor. Thedirection of force on a current carrying conductor placed in a magnetic field can also bereversed by reversing the direction of magnetic field.

Fleming’s Left - Hand Rule for the Direction of ForceAccording to Fleming�s left - hand rule : Hold the forefinger, the centre finger and thethumb of your left hand at right angles to one another. Adjust your hand in such a waythat the forefinger points in the direction of magnetic field and the centre finger pointsin the direction of current, then the direction in which thumb points, gives the directionof force acting on the conductor.

Left hand

Magnetic field

Cu

rren

t

Force or

Motion (a)

Cu

rren

t

Magnetic field

Force or Motion

A B East

D

South

C

Vertically downwards (b)

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Electric Motor

A motor is a device which converts electrical energy into mechanical energy.

Principle of a Motor

A motor works on the principle that when a rectangular coil is placed in a magnetic fieldand current is passed through it, a force acts on the coil which rotates it continuously.

ELECTRO MAGNETIC INDUCTION

Michael Faraday showed that a changing magnetic field can induce an electromotiveforce (e.m.f) which drives an induced current through a closed circuit. This phenomenonis known as Electromagnetic Induction.

Laws of Electromagnetic Induction

The electromotive force generated in a conductor is proportional to the rate of changeof magnetic lines of force linking the circuit.

The negative rate of change of magnetic flux associated with a coil of (1 loop) is equal tothe induced emf (e) in that coil.

∴ e= ddt

φ

Where e = induced emf, φ = magnetic flux

For N loops of the coil,

e = Nddt

− φ;

e = ( )dN BA

dt− B

Aφ ∴ =

Magnetic flux density = M.Flux

Area

e = ( )dNBA

dt−

The negative sign indicates that induced emf (e) has a nature to oppose the change inmagnetic flux. .

Lenz’s Law

The direction of the induced e.m.f. and hence the induced current in a closed circuit isalways such that its magnetic effect opposes the motion or change producing it. Lenz�slaw is analogous to Law of conservation of energy.

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Flemming’s Right Hand rule

When the fingers of right hand palm are stretched in such a way that the Fore - finger,Central finger and Thumb are mutually perpendicular to each other. Then The Fore -finger indicates the direction of magnetic field, Thumb represents the direction of motionof the conductor, then the Middle finger indicates the direction of induced current.

SELF - INDUCTANCE

If a current is passed through a conducting coil of one loop, there develops magnetic fluxφ around it.

∴ The magnetic flux φ is directly proportional to i.

Where L = coefficient of self inductance

When the main current i in the loop is not constant and it is changing, it causes acontinuous change in magnetic flux associated with the coil. This change in flux inducesan emf in the loop.

∴ Induced emf e = ddt

− φ

e = ( ) [ ]dLi) Li

dt− ∴ φ =

e = �L didt

The self inductance of the coil is numerically equal to induced emf in the circuit, if rateof change of current is 1 ampere per second.

L = sec

eampere

1−

In general L =

edidt

−

S. I unit = volt

sA

×

= 2

J 1J. JC As

A A A× = =

If i ≠ constant ⇒ current is changing due to change in current i

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P SSoft Iron

Core

E1

E2

N1 N

2

N > > N2 1

E > > E2 1

(Step-up transformer)

P SSoft Iron

Core

E1

E2N > > N

2 1

E > > E2 1

(Step-down transformer)

Electric Generator

The electric generator converts mechanical energy into electrical energy.

Principle of Electric Generator

The electric generator is an application of electromagnetic induction. The electricgenerator works on the principle that when a straight conductor is moved in a magneticfield, then current is induced in the conductor.

SOLVED EXAMPLES

Example 1

What is the basic difference between magnetic and electric field ?In what way is anelectric field different from a magnetic field ?

Solution :

A charged particle always experiences force due to an electric field. It does not matter ;whether the charged particle is at rest or is in motion. How ever, due to magnetic field,force on a charged particle acts only, when it moves inside the magnetic field and itsmotion is not along the direction of the magnetic field. Further, when electric field acts,the kinetic energy of the charged particle is a affected. However, there is no effect onthe kinetic energy of a charged particle due to its motion inside the magnetic field.

Example 2

An electron is projected in the direction of magnetic field. How will its motion be affectedby the action of magnetic field ?

Solution :

Force acts on the electron due to the magnetic field, when it is projected in the directionof magnetic field. Hence its motion will not be affected.

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Example 9

The induced e.m.f. is also called back e.m.f. why ?

Solution :

The induced e.m.f. is also called back e.m.f as it opposes the applied voltage.

Example 10

What is the magnitude of the induced current in the circular loop KLMN of radius r, ifthe straight wire PQ carries a steady current of magnitude I ampere?

IP Q

M

LN

K

Solution :

No induced emf will be produced in the circular loop. It is because, due to steady currentflowing.

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Mag

net

ic e

ffec

ts o

f ele

ctri

c cu

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Oers

ted

exp

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Ele

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Summative Exercise

BASIC PRACTICE

SECTION A

FILL IN THE BLANKS

1. Oersted�s experiment gives dependence of the direction of magnetic field with___________.

2. The source of electric field is ___________.

3. The direction of magnetic field lines due to a straight current carrying conductor may befound by applying ___________.

4. The lines magnetic induction due to a straight current carrying conductor are ___________.

5. If a copper wire carries a direct current, the magnetic field associated with the currentwill be only ___________ the rod.

6. ___________ law gives the quantitative relationship between current and magnetic fielddue to the current carrying conductor.

7. The direction of magnetic field on the conductor in Fleming�s left hand rule is repre-sented by ___________ finger.

8. The strength of magnetic field produced by a current carrying circular coil can be__________ by increasing the number of turns of wire in the coil.

9. An electromagnet is a __________ magnet.

10. An electric generator converts mechanical energy into __________ energy.

11. The SI unit of inductance is __________.

12. __________ is increased in a step down transformer.

13. The property of a conductor which enables to induce an emf due to change of current inthe same coil is __________.

14. The magnitude of induced emf is directly proportional to the rate of change of __________linked with the circuit.

15. According to Ampere�s swimming rule, if a man swims along a direction opposite to thedirection of the current, south pole of the needle deflects towards his __________.

TRUE OR FALSE

16. A current flowing in a wire always give rise to a magnetic field.

17. Magnetic lines of force around any current carrying conductor are circular in nature.

18. According to right hand thumb rule ; the direction in which our fingers are folded givesthe direction of current.

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19. The magnetic produced by a current carrying solenoid is similar to the magnetic fieldproduced by a bar magnet.

20. When a current carrying conductor is placed in a magnetic field, a mechanical force isexerted on the conductor which can make the conductor move.

21. Fleming�s right hand rule is used to determine the direction of force acting on a currentcarrying wire placed in a magnetic field.

22. Maximum force acts on a current carrying conductor when it is parallel to the magneticfield.

23. Magnetic field is not associated with a decelerated charge.

24. A transformer is a device which gives only ac voltage.

25. Electro magnetic induction is a phenomenon of production of electric current in a coil,when the magnetic flux linked with the coil is changed.

MATCH THE FOLLOWING

26. Column I Column II

A. Fleming�s Left hand Rule p. direction of magnetic flux

B. Fleming�s Right hand Rule q. direction of magnetic force on a current carrying conductor in a uniform magnetic field

C. Ampere�s Swimming Rule r. direction of induced current and its nature

D. Lenz�s law s. direction of induced current

27. Column I Column II

A. Magnetic induction at a point near p. e = ( )BN d / dt− φ

a straight current carrying conductor

B. Force on a current carrying conductor q. Vp > Vs

C. Faraday�s law of electro magnetic induction r. B = i

2 r°µπ

D. Step up transformer s. Vp < Vs

E. Step down transformer t. F = BIL

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FURTHER PRACTICE

MULTIPLE CHOICE QUESTIONS

1. Which statement about the action of a transformer is correct ?

(A) Alternating current always flows in the secondary coil when an alternating voltage is applied to the primary coil.

(B) The current in the secondary coil is always larger than the current in the primary coil.

(C) An e.m.f. is induced in the secondary coil when an alternating voltage is applied to the primary coil.

(D) An e.m.f. is induced in the secondary coil when a steady direct current flows in the primary coil.

2. A current flows in a conductor from east to west. The direction of the magnetic field ata point above the conductor is

(A) towards north (B) towards south (C) towards east (D) towards west

3. The magnetic field at a distance r from a long wire carrying current I is 0-4 tesla. Themagnetic field at a distance 2r is

(A) 0-1 tesla (B) 0-2 tesla (C) 0-8 tesla (D) 1-6 tesla

4. A positively charged particle moving due east enters a region of uniform magnetic fielddirected vertically upwards. The particle will

(A) get deflected in vertically upward direction.

(B) move in circular path with an increased speed.

(C) move in circular path with a decreased speed.

(D) move in a circular path with a uniform speed.

5. A charged particle is moving through uniform magnetic field, then magnetic field :(A) always exerts a force on the particle

(B) never exerts a force on the particle.

(C) exerts a force, if the particle is moving along the field.

(D) exerts a force, if the particle is moving perpendicular to the direction of the field.

6. In electromagnetic induction, the induced e.m.f. is independent of

(A) change of flux (B) time (C) tesla (D) weber

7. Magnitude of e.m.f. produced in a coil, when a magnet is inserted into it ; does notdepend upon :

(A) number of turns in the coil. (B) resistance of the coil.

(C) magnetic moment of magnet. (D) speed of the magnet.

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8. A magnet is moved towards a coil, first quickly and then slowly. The induced e.m.f.produced is:

(A) larger in first case

(B) smaller in first case

(C) equal in both cases

(D) larger or smaller, depending upon resistance of the coil

9. Refer to the figure. Deflection in the galvanometer (G) occurs, when :

G

S N

COIL

(A) the magnet is pushed into the coil

(B) the magnet is rotated into the coil

(C) the magnet is stationary at the centre of the coil.

(D) the number of turns in the coil is reduced.

10. A field due to a long straight wire carrying a current I is proportional to

(A) I (B) I2 (C) I3 (D) I

11. In the right -hand thumb rule, the thumb points towards the :

(A) direction of current (B) direction of magnetic field

(C) direction of earth�s magnetic field (D) all of these

12. When a charged particle moves through a magnetic field it suffers a change in its :

(A) energy (B) mass (C) speed (D) direction of motion

13. The split - ring type commutator is used in generators to :

(A) convert AC to DC (B) convert DC to AC

(C) induce emf (D) induce magnetism

14. In an electrical motor, the energy transformation is from :

(A) electrical to light (B) electrical to mechanical

(C) light to electrical (D) mechanical to mechanical

15. A straight wire of diameter 0.5 mm carrying a current of 1 A is replaced by another wireof 1 mm diameter carrying the same current. The strength of magnetic field far away is:

(A) twice the earlier value (B) half of the earlier value

(C) quarter of its earlier value (D) unchanged

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25. If in a transformer the number of turns of primary coil and secondary coil are 5 and 4respectively and 240 V is applied to primary coil, then the ratio of current in primary andsecondary coil is :

(A) 4 : 5 (B) 5 : 4 (C) 5 : 10 (D) 8 : 12

26. The SI unit of inductance, the henry, can be written as :

(A) weber ampere�1 (B) volt - s ampere�1

(C) joule ampere�1 (D) ohm s�1

27. The number of turns in the primary and the secondary coils of a transformer are 1000and 3000 respectively. If the primary of the coil is connected to 80 volt ac, then potentialdifference per turn of the secondary coil is :

(A) 240 V (B) 24 V (C) 0.24 V (D) 0.08 V

28. An ideal transformer is used on 220 V line to deliver 2 A at 110 V. The current throughthe primary is :

(A) 10 A (B) 5 A (C) 1 A (D) 0.1 A

29. In a transformer, number of turns in primary and secondary are 500 and 2000 respectively.If current in primary is 48 A, current in the secondary is :

(A) 144 A (B) 24 A (C) 48 A (D) 12 A

30. Power is transmitted from a power house on high voltage ac because :

(A) the rate of transmission is faster at high voltage

(B) it is more economical due to less power wastage

(C) the life of the current carrying wire is prolonged

(D) a precaution against the theft of transmission line

MULTIPLE ANSWER QUESTIONS

1. The direction of magnetic field around a current carrying conductor is determined bywhich of the following principles ?

(A) Fleming�s right hand rule (B) Ampere�s right hand rule

(C) Maxwell�s cork screw rule (D) Ampere�s swimming rule

2. The magnetic field at a point due to a current carrying conductor is directly proportionalto:

(A) resistance of the conductor

(B) thickness of the conductor

(C) current flowing through the conductor

(D) distance from the conductor

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3. If a long cylindrical conductor carries a steady current parallel to its length :

(A) the electric field along the axis is zero

(B) the magnetic field along the axis is zero

(C) the magnetic field outside the conductor is zero

(D) the electric field outside the conductor is zero

4. The path of a charged particle moving in a magnetic field can be a :

(A) straight line (B) circle (C) parabola (D) helix

5. In case of motion of a charged particle in a steady magnetic field :

(A) speed remains constant (B) momentum remains constant

(C) kinetic energy remains constant (D) work done is a always zero

6. A portion moving with constant velocity passes through a region of space without anychange in its velocity. If E and B represent electric and magnetic fields respectively, thisregion of space may have :

(A) E = 0 and B = 0 (B) E = 0 and B ≠ 0 (C) E ≠ 0 and B = 0 (D) E ≠ 0 and B ≠ 0

7. A long straight wire of radius r carries a current distributed uniformly over its cross-section. The magnitude of the magnetic field is :

(A) maximum at the axis of wire

(B) minimum at the axis of the wire

(C) maximum at the surface of the wire

(D) minimum at the surface of the wire

ASSERTION AND REASON TYPE QUESTIONS

The questions given below consists of statements of an Assertion and a Reason. Use thefollowing key to choose the appropriate answer.

(A) If both assertion and reason are CORRECT and reason is the CORRECT explanation

of the assertion.

(B) If both assertion and reason are CORRECT, but reason is NOT THE CORRECT

explanation of the assertion.

(C) If assertion is CORRECT, but reason is INCORRECT.

(D)If assertion is INCORRECT, but reason is CORRECT.

(E) If both assertion and reason are INCORRECT.

1. Assertion : Magnetic field interacts with a moving charge and not with a stationarycharge.

Reason : A moving charge produces a magnetic field.

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2. Assertion : Force experienced by moving charge will be maximum if direction of velocityof charge is parallel to applied magnetic field.

Reason : Force on moving charge is independent of direction of applied magneticfield.

3. Assertion : A stationary charged particle in a magnetic field does not experience a force.

Reason : The force acting on a charged particle does not depend on velocity of theparticle.

4. Assertion : A magnetic field exert a force on a moving charge in the same direction asthe direction of field itself.

Reason : The direction of force is given by Fleming�s left hand rule.

5. Assertion : A current carrying conductor experiences a force in a magnetic field.

Reason : The net charge on a current carrying conductor is zero.

6. Assertion : When current is represented by a straight line, the magnetic field will becircular.

Reason : According to Fleming�s left hand rule, direction of force is parallel to themagnetic field.

PARAGRAPH QUESTIONS

Passage - I

Three specimens of magnetic material were tested using the apparatus shown in thediagram.

A

Iron nails

Specimen under

test

When the switch is closed, the specimen picks up some of the iron nails but when theswitch is opened, many or most of the nails fall off. The number of nails picked up andleft on were found for three specimens. The table shows the results.

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Specimen Number of nails picked up Number of nails left on

X 35 4

Y 20 10

Z 40 3

i) Which material is the best electromagnet among the three ?

(A) X (B) Y (C) Z (D) all the above

ii) Which material among the following is suitable for making use in compasses ?

(A) X (B) Y (C) Z (D) all the above

iii) What does the number of nails left on the material infer to ?

(A) Ability to retain magnetism when current is removed

(B) Ability to induce e.m.f

(C) To undergo magnetostriction on removal of current

(D) Ability to change strength of magnetic field

iv) Which among the following pair of elements are suitable in the construction ofelectric bell ?

(A) X, Y (B) Y, Z (C) X, Z (D) X, Y, Z

SECTION – B

NUMERICAL PROBLEMS

1. A horizontal wire 0.1 m long carries a current of 5 A. Find the magnitude and directionof the magnetic field, which can support the weight of the wire. Assume wire to be ofmass 3 × 10�3 kg m�1.

2. A transformer consists of a coil of 1200 turns and another coil, with a total of 120 turns,which can be tapped at various places. Primary voltage is 240 V. At which pair of terminalswould you connect to a 12 V, 24 V lamp for it to be lit normally ?

3. A long straight conductor carries a current of 5 amps. Find the magnetic field at a

distance of 0.2 metre away from the wire. Given 7 1 10 4 10 wb A m− − −µ = π× .

4. A current of 1 A flows in a length 0.1 m placed in a magnetic field of 0.5 T. Calculate theacting on the wire when the wire makes an angle of (i) 90° and (ii) 0°, with respect to themagnetic field.

5. A current of 0.24 A flows through a circular coil of 72 turns, the average diameter of thecoil being 20 cm. What is the strength of field produced at the centre of the coil ?

6. When current changes from 4 A to 0 A in 0.1 s in an inductor the induced emf is found tobe 100 V, then find the value of self � inductance.

7. In a transformer the number of turns of primary coil and secondary coil are 5 and 4respectively and 240 V is applied on primary coil, then find the ratio of current in primaryand secondary coils.

8. A transformer steps up an ac supply from 220 to 2200 V. If the secondary coil of thetransformer has 2000 turns, Then find the number of turns in its primary coil.

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9. The primary winding of a transformer has 500 turns whereas its secondary has 5000turns. The primary is connected to an ac supply of 20 V, 50 Hz. What will be the outputof the secondary ?

10. A solenoid is 2.0 m long and 3.0 cm in diameter. It has 5 layers of winding of 1000 turnseach and carries a current of 5.0 A. What is the magnetic field at its centre ?

CONCEPTUAL QUESTIONS

1. Can a current carrying straight electric wires attract the nearby iron objects towardsthem ?

2. In what respect does a wire carrying a current differ from a wire, which carries nocurrent ?

3. Explain the rule, with illustration, related to the direction of current through a conductorand magnetic field.

4. An electric charge enters in electric field at right angles to the direction of electric field.What is the nature of the path followed ?

5. An electron is not deflected, while moving through a certain region of space. Can we besure that there is no magnetic field in the region ?

6. In a field, the force experienced by a charge depends upon its velocity and becomes zero,when it is at rest. What is the nature of the field ?

7. Why the core used in a transformer and other electromagnetic devices is laminated?

SECTION – C

PREVIOUS CONTEST QUESTIONS

1. Magnetic field at the centre of a coil carrying current in clock wise direction is(A) towards the centre of the coil (B) away from the centre of the coil(C) towards you (D) away from you

2. An electron is moving towards east in a magnetic field acting vertically downwards. Sothe electron is deflected towards.(A) south (B) north (C) east (D) west

3. A conductor of length l carrying current i is placed perpendicular to magnetic field ofindication B. The force experienced by it is(A) i lB (B) iB / l (C) il / B (D) lB / i

4. An electric current passes through a long straight wire. At a distance 5 cm from wire,the magnetic field is B. The field at 20 cm from the wire would be(A) 2B (B) B / 4 (C) B / 2 (D) B

5. Two circular coils are made of two identical wires of the same length. If the number ofturns of the two coils are 4 and 2, then the ratio of magnetic inductions at the centreswill be(A) 4 : 1 (B) 2 : 1 (C) 1 : 2 (D) 1 : 1

6. An electric charge in uniform motion produces(A) an electric field only (B) a magnetic field only(C) both electric and magnetic fields (D) no such field at all

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7. If a copper rod carries a direct current, the magnetic field associated with the currentwill be

(A) only inside the rod (B) only outside the rod

(C) both inside and outside the rod (D) neither inside nor outside the rod

8. The magnetic field at a distance �r� from a long wire carrying current �i� is 0.4 Tesla. Themagnetic field at a distance �2r� is

(A) 0.2 tesla (B) 0.8 tesla (C) 0.1 tesla (D) 1.6 tesla

9. The current in a choke changes from 0 to 10 A in 5 seconds. If the inductance of thechoke is 2H the induced emf is

(A) 2 V (B) 8 V (C) 4 V (D) 12 V

10. The current passing through a choke coil of 5 henry is decreasing at the rate of 2 amp/sec. The e.m.f. developed across the coil is

(A)10 volts (B) �10 volts (C) 2.5 volts (D) �2.5 volts

BRAIN NURTURES

1. A long wire is first bent into a circular coil of one turn and then into a circular coil ofsmaller radius having n turns. If the same current passes in both the cases, find theratio of the magnetic fields produced at the centres in the two cases.

2. Under which condition, an electron moving through a magnetic field experiencesmaximum force ?

3. Under what condition is the force acting on a charge moving through a uniform magneticfield minimum ?

4. What will be the path of a charged particle moving along the direction of a uniformmagnetic field ?

5. An electron and a proton moving with the same speed enter the same magnetic fieldregion at right angles to the direction of the field. For which of the two particles will theradius of circular path be smaller.

6. Which of the graphs shows the variation of magnetic induction B with distance �r� froma long wire carrying a current ?

(A)

B

r

(B)

B

r

(C)

B

r

(D)

B

r

7. Two flat circular coils are made of two identical wires each of length 20 cm, one coil has4 turns while the second has 2. If the same current flows through the two, then findratio of the field at their centre.

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Crossword Puzzle

1

2 3 4

5

6

7 8

9

10

11

ACROSS

5 Temporary magnet

7 Electric motor converts electrical energy to thisform of energy

9 Static device that works on principle ofelectromagnetic induction

10 In Fleming’s left hand rule the direction of force isrepresented using

11 In a setup transformer, voltage of this coil isgreater

DOWN

1 Electricity produces magnetism is called by thisname

2 SI unit of inductance

3 A long coil containing a large number of closeturns of insulated copper wire.

4 Discovered that a wire carrying current is capableto deflect a compass needle

6 This device converts mechanical energy toelectrical energy

8 The direction of induced emf and its nature canbe found by this law.

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` 75Detailed solutionsfor all problems

of IIT Foundation &Olympiad Explorer

are available in this book

CLASS - XSolutions

CLASS - X

FOUNDATION OLYMPIAD&

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ss - X

CH

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IS

TR

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IntegratedSyllabus

PH

YS

IC

S

UNIQUE ATTRACTIONS

�

� Cross word Puzzles

� Graded Exercise

Basic Practice�

Further Practice�

Brain Nurtures�

� Numerical Problems

Conceptual Questions

� Multiple Answer Questions

� Paragraph Questions

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