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6.ELECTROMAGNETIC INDUCTION Single Correct Answer Type 1. A wire of length 1 m is moving at a speed of 2ms perpendicular to its length and in a homogenous −1 magnetic field of 0. 5 T. The ends of the wire are joined to a circuit of resistance 6 Ω. The rate at which work is being done to keep the wire moving at constant speed is a) 1 12 W b) 1 6 W c) 1 3 W d) 1W 2. A horizontal loop abcd is moved across the pole pieces of a magnet as shown in fig. with a constant speed v . When the edge ab of the loop enters the pole pieces at time t = 0 sec, which one of the following graphs represents correctly the induced emf in the coil a) b) c) d) 3. An ideal transformer has 100 turns in the primary and 250 turns in the secondary. The peak value of the ac is 28 V. The r. m. s. secondary voltage is nearest to a) 50 V b) 70 V c) 100 V d) 40 V 4. Which of the following is not an application of eddy currents a) Induction furnace b) Galvanometer damping c) Speedometer of automobiles d) X-ray crystallography 5. Induced emf in the coil depends upon a) Conductivity of coil b) Amount of flux c) Rate of change of linked flux d) Resistance of coil 6. A square metallic wire loop of side 0. 1 m and resistance of 1Ω is moved with a constant velocity in a magnetic field of 2 wb/m as shown in figure. The magnetic field is perpendicular to the plane of the loop, 2 loop is connected to a network of resistances. What should be the velocity of loop so as to have steady current of 1mA in loop a) 1 cm/sec b) 2 cm/sec c) 3 cm/sec d) 4 cm/sec 7. When a certain circuit consisting of a constante.m.f. E,an inductanceLand a resistanceRis closed, the current in it increases with time according to curve 1. Page| 1
After one parameter(E, L or R)is changed, theincrease in current follows curve 2 when the circuit is closed second time. Which parameter was changed and in what direction a) Lis increased b) Lis decreased c) Ris increased d) Ris decreased 8. The average power dissipation in pure inductance is a) 1 2 LI 2 b) 2LI 2 c) 1 4 LI 2 d) Zero 9. The current from A to B is increasing in magnitude. What is the direction of induced current, if any, in the loop shown in figure. a) No current is induced b) Clock-wise current c) Anti-clock-wise current d) Alternating current 10. The current i in an induction coil varies with time t according to the graph shown in figure. Which of the following graphs shows the induced emf (e) in the coil with time a) b) c) d) 11. A step-down transformer reduces the voltage of a transmission line from 2200 V to 220 V. The power delivered by it is 880 W and its efficiency is 88%. The input current is a) 4.65 mA b) 0.045 A c) 0.45 A d) 4.65 A 12. A conducting wire is dropped along east-west direction, then a) No emf is induced b) No induced current flows c) Induced current flows from west to east d) Induced current flows from east to west 13. Lenz’s law is expressed by the following formula (here e =induced e.m.f., φ =magnetic flux in one turn and N = number of turns) a) e =− φ dN dt b) e =− N dφ dt c) e =− d dt φ ( N ) d) e = N dφ dt 14. In the circuit shown below, the ac source has voltage V = 20 cos cos (ωt) volts with ω = 2000 rad/sec. The amplitude of the current will be nearest to Page| 2
a) First clockwise then anticlockwise b) In clockwise direction c) In anticlockwise direction d) First anticlockwise then clockwise 15. A motor having an armature of resistance 2Ω is designed to operate at 220 V mains. At full speed, it develops a back e.m.f. of 210 V. When the motor is running at full speed, the current in the armature is a) 5A b) 105A c) 110A d) 215A 16. Two circuits have coefficient of mutual induction of 0. 09 henry. Average e.m.f. induced in the secondary by a change of current from 0 to 20 ampere in 0. 006 second in the primary will be a) 120 V b) 80 V c) 200 V d) 300 V 17. The time constant of an LR circuit represents the time in which the current in the circuit a) Reaches a value equal to about 37% of its final value b) Reaches a value equal to about 63% of its final value c) Attains a constant value d) Attains 50% of the constant value 18. A conducting circular loop is placed in a uniform magnetic field of induction Btesla with its plane normal to the field. Now, the radius of the loop starts shrinking at the rate . Then, the induced emf at the instant dr ( dt ) when the radius is r,is a) πrB dr ( dt ) b) 2πrB dr ( dt ) c) πr 2 dB ( dt ) d) πr 2 ( 2 )B dr ( dt ) 19. The current (I) in the inductance is varying with time according to the plot shown in figure Which one of the following is the correct variation of voltage with time in the coil a) b) c) d) 20. A transformer has 100 turns in the primary coil and carries 8 A current. If input power is one kilowatt, the number of turns required in the secondary coil to have 500V output will be Page| 3
a) 100 b) 200 c) 400 d) 300 21. A circular coil of mean radius of 7 cm and having 400 turns is rotated at the rate of 1800 revolutions per minute in the earth’s magnetic field (B = 0. 5 gauss), the maximum e.m.f. induced in coil will be a) 1. 158 V b) 0. 58 V c) 0. 29 V d) 5. 8 V 22. Consider the statements: (I)If magnetic field, B =0, then magnetic flux is also zero. (II)If magnetic flux, φ = 0,then magnetic field is also zero. a) (I)is true , (II) may be true b) Both (I) and (II) are true c) (I) may be true, (II) is true d) (I) and (II) both are false 23. A circular coil and a bar magnet placed near by are made to move in the same direction. The coil covers a distance of 1 m in 0. 5 sec and the magnet a distance of 2min 1 sec. The induced emf produced in the coil a) Zero b) 1 V c) 0. 5 V d) Cannot be determined from the given information 24. A cylindrical bar magnet is kept along the axis of a circular coil. If the magnet is rotated about its axis, then a) A current will be induced in a coil b) No current will be induced in a coil c) Only an e.m.f. will be induced in the coil d) An e.m.f and a current both will be induced in the coil 25. When power is drawn from the secondary coil of the transformer, the dynamic resistance a) Increases b) Decreases c) Remains unchanged d) Changes erratically 26. The mutual inductance of an induction coil is 5H. In the primary coil, the current reduces from 5A to zero in 10 . What is the induced emf in the secondary coil −3 s a) 2500V b) 25000V c) 2510V d) Zero 27. An alternating current of frequency 200 rad/sec peak value 1A as shown in the figure, is applied to the primary of a transformer. If the coefficient of mutual induction between the primary and the secondary is 1. 5H, the voltage induced in the secondary will be a) 300 V b) 191 V c) 220 V d) 471 V 28. An L-R circuit has a cell of e.m.f. E, which is switched on at time t = 0. The current in the circuit after a long time will be a) Zero b) E R c) E L d) E L 2+R 2 29. Current in a coil changes from 5 A to 10 A in 0.2 s. If the coefficient of self-induction is 10 H, then the induced emf is a) 112 V b) 250 V c) 125 V d) 230 V 30. Choke coil works on the principle of a) Transient current b) Self induction c) Mutual induction d) Wattless current Page| 4