## Wednesday, 1 October 2014

### ELECTROMAGNETIC INDUCTION MULTIPLE CHOICE QUESTIONS

1. The number of turns of primary and secondary coils of a transformer are 5 and 10 respectively and the mutual inductance of the transformer is 25 henry. Now the number of turns in the primary and secondary of the transformer are made 10 and 5 respectively. The mutual inductance of the transformer in henry will be
 6.25 25 12.5 50

2. The current passing through a choke coil of 5 henry is decreasing at the rate of 2 amperejsec. The e.m.f. developing across the coil is
 10 V 2.5 V -10 V -2.5 V

3. Fan is based on
 Electric motor Both Electric dynamo None of these

4. Work of electric motor is
 To convert ac into de Both (a) and (b) To convert de into ac To convert ac into mechanical work

5. The coil of a dynamo is rotating in a magnetic field.~ihe developed induced e.m.f. changes and the number of magnetic lines of force also changes. Which of the following conditions is correct
 Lines of force minimum but induced e.m.f. is zero Lines of force maximum but induced e.m.f. is not zero Lines of force maximum but induced e.m.f. is zero Lines of force maximum but induced e.m.f. is also maximum

6. If the current is halved in a coil then the energy stored is how much times the previous value
 1/2 2 1/4 4

7. If a coil of metal wire is kept stationary in a non-uniform magnetic field, then. [BHU 2000]
 An emf is induced in the coil Neither emf nor current is induced A current is induced in the coil Both emf and current is induced

8. Initially plane of coil is parallel to the uniform magnetic field 8. In time M it becomes perpendicular to magnetic field, then charge flows in it depend on this time as
 ∝ ∆t ∝ (∆t)° ∝ 1/∆t ∝ (∆t)²

9. A transformer is used to
 Change the alternating potential Both alternating current and alternating voltage Change the alternating current To increase the power of current source

10. A wire coil carries the current i. The potential energy of the coil does not depend upon
 The value of i Whether the coil has an iron core or not The number of turns in the coil The resistance of the coil

11. The primary winding of a transformer has 500 turns and its secondary has 5000 turns. If primary is connected to ac supply of 20 V and 50 Hz thensecondary will have an output of
 2 Vand 5Hz 200 V and 50 Hz 2 Vand 50Hz 200 Vand 500Hz

12. A solenoid of length i metre has self im:iuctance L henary if number of turns are doubled, its self inductance
 Remains same Becomes 4L henry Becomes 2L henry Becomes L/√2 henry

13. The current flowing in a coil of self inductance 0.4 mH is increased by 250 mA in 0.1 sec. The e.m.f. induced will be
 + 1 volt +1 mV - 1 volt - 1mV

14. Two identical circular loops of metal wire are lying on a table without touching each other. Loop A carries a current which increases with time. In response the loop B
 Remain stationery Is repelled by the loop A Is attracted by the loop A Rotates about its CM with CM fixed

15. In a transformer, the number of turns of primary coil and secondary coil are 5 and 4 respectively. If 220 V is applied on the primary coil, then the ratio of primary current to the secondary current is
 4: 5 5 : 9 5 : 4 9 : 5
16. The working of a dynamo is based on the principle of . [CPMT 1996; MP P]
 Heating effect of current Chemical effect of current Magnetic effect of current Electromagnetic induction

17. The number of turns in the coil of an ac generator is 5000 and the area of the coil is 0.25 m2; the coil is rotated at the rate of 100 cycle per second in a magnetic field of 0.2 weberjm2. The pack value of the e.m.f. generated is nearly. [PMT (AMU) 1995]
 786 kV 220 kV 440 kV 157.1 kV

18. large transformer when used for some time. become hot and are cooled by circuiatmg oil. The heating of transformer is due to
 Heating effect of current alone Both the hysteresis loss and heating effect of current Hysteresis loss along None of the above

19. An e.m.f. of 12 volt is produced in a coil when the current in it changes at the rate of 45 amp/minute. The inductance of the coil is
 0.25 henry 9.6 henry 1.5 henry 16.0 henry

20. A square loop PQRS is carried away from a current carrying long straight conducting wire CD (figure). The direction of induced current in the loop will be.
 Anticlockwise Some times clockwise sometimes anticlockwise Clockwise Current will not be induced

21. A coil of area 100 cm2 has 500 turns. Magnetic field of 0.1 weberjmetre² is perpendicular to the coil. The field is reduced to zero in 0.1 second. The induced emf in the coil is
 1 V 50 V 5 V Zero

22. The resistance and inductance of series circuit are 5 Ω and 20 H respectively. At the instant of closing the switch, the current is increasing at the rate 4 A-s. The supply voltage is
 20 V 120 V 80 V 100 V

23. In a step-up transformer the turn ratio is 1: 10. A resistance of 200 ohm connected across the secondary is drawing a current of 0.5 amp. What is the primary voltage and current
 50 V, 1 amp 25 V, 4 amp 10 V, 5 amp 20 V, 2 amp

24. When two inductors L1 and L2 are connec\ed in parallel, tl'}e equivalent inductance is
 L1 + L2 Less than both L1 and L2 Between L1 and L2 None of the above

25. The starter motor of a car draws a current i = 300 A from the battery of voltage 12 V. If the car starts only after 2 minutes, what is the energy drawn from the battery
 3 kj 7.2 kj 30 kj 432 kj

27. Figure shows a horizontal solenoid connected to a battery and a switch. A copper ring is placed on a frictionless track, the axis of the ring being along the axis of the solenoid. As the switch is closed, the ring will . Remain stationary Move away from the solenoid Move towards the solenoid Move towards the solenoid or away from it depending on which terminal (positive or negative) of the battery is connected to the left end of the solenoid

28. The inductance of a closed-packed coil of 400 turns is 8 mH. A current of 5 mA is passed through it. The magnetic flux through each tum of the coil is
 1/4π μ₀Wb 1/3π μ₀Wb 1/2π μ₀Wb 0.4 μ₀Wb

29. When the speed of a de motor increase the armature current
 Increases Does not change Decreases Increases and decreases continuously

30. A coil having an area of 2 m² placed in a magnetic field which changes from 1 to 4 weber/m² in 2 seconds. The e.m.f. induced in the coil will be
 4 volt 2 volt 3 volt 1 volt

31. The number of turns in the primary and secondary coils of a transformer are 1000 and 3000 respectively. If 80 volt ac is applied to the primary coil of the transformer, then the potential difference per turn of the secondary coil would be
 240 volt 24 volt 2400 volt 0.08 volt

32. Find out the e.m.f. produced when the current changes from 0 to 1A in 10 second given, L = 10 ,μH
 1 V 1 mV 1 μH 1 V

33. The current through an inductor of 1 H is given by I= 3 t sin t. The voltage across the inductor of 1 H is
 3 sin t + 3 cos t 3 sin t + 3t cost cost + tsin t 3t cos t - 3 sin t

34. Figure shows two bulbs 81 and 82, resistor Rand an inductor L When the switch Sis turned off. Both B1 and B2 die out promptly B1 dies out promptly but B2 with some delay Both B1 and B2 die out with some delay B2 dies out promptly but B1 with some delay

35. The self inductance of a straight conductor is
 zero infinity Very large very small
36. A thin circular ring of area A is held perpendicular to a uniform magnetic field of induction 8. A small cut is made in the ring and a galvanometer is connected across the ends such that the total resistance of the circuit is R. When the ring is suddenly squeezed to zero area, the charge flowing through the galvanometer is
 BR / A ABR AB / R B²A / R²

37. The figure shows three situation in whiol,f'Jdentical circular conducting loops are in uniform magnetic field that are either increasing or decreasing in magnitude at identical rates. in each,the dashed line coincides with a diameter Rank the situations according to magnitudeo the current induced in the loops greatest first . ia = ib < ic (ic != 0) ia>= ib > ic (ic != 0) ia = ib > ic (ic = 0) ia < ib < ic (ic != 0)

38. A coil of 40 Ω resistance has 100 turns and radius 6 mm is connected to ammeter of resistance of 160 ohms. Coil is placed perpendicular to the magnetic field. When coil is taken out of the field, 32 μC charge flows through it. The intensity of magnetic field will be
 6.55 T 0.655T 5.66 T 0.566T

39. The armature current in a de motor is maximum when the motor has
 Picked up maximum speed Intermediate speed Just started None of these

40. Figure shows two coils placed close to each other. When the current through one coil is increased gradually 61 by shifting the position of the rheostat . A current flows along ABC in the other coil No current flows in the other coil A current flows along CBA in the other coil An alternating current flows in the other coil

41. Output voltage of a transformer does not depend upon
 Number of turns in secondary coil Number of turns in primary coil Input voltage ac frequency

42. A coil of wire of a certain radius has 600 turns and a self-inductance of 108 mH. The self-inductance of a second similar coil of 500 turns will be
 74 mH 76 mH 75 mH 77 mH

43. Two circular coils A and Bare facing each other as shown in figure. The current i through A can be altered . There will be repulsion between A and B if i is increased There will be neither attraction nor repulsion when i is changed There will be a'ttractiori between A and B if i is increased Attraction or repulsion between A and B depends on the direction of

44. A magnet is moved in the direction indicated by an arrow between two coils AB and CD as shown in fig. What is the direction of the induced current in each coil . A to Bin coil X and C to D in coil Y B to A in coil X and C to D in coil Y A to Bin coil X and D to C in coil Y B to A in coil X and D to C in coil Y

45. Two coils A and B having turns 300 and 600 respectively are placed nea.r each other, on passing a current of 3.0 ampere in A, the flux linked with A is 1.2 x 10-4 weber and with B it is 9.0 x 10-5 weber. The mutual inductance of the system is
 2 x 10-5 henry 4 x 10-5 henry 3 x 10-5 henry 6 x 10-5 henry
46. In a transformer, the coefficient of mutual inductance between the primary and the secondary coil is 0.2 henry. When the current changes by 5 ampere/second in the primary, the induced e.m.f. in the secondary will be. [MP PMT 1989]
 5 V 25 V 1 V 10 V

47. S.I. unit of magnetic flux is
 Weber m-2 Weber / m Weber Weber / m4

48. The coefficients of self induction of two coils are L1 and L2 To induce an e.m.f. of 25 volt in the coils change of current of 1A has to be produced in 5 second and 50ms respectively. The ratio of their self inductances L1 : L2will be
 1:5 100:1 200:1 50:1

49. 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
 5 A 110 A 105 A 215 A

50. A varying current at the rate of 3 A/s in coil generates an e.m.f. of 8 mV in a near by coil. The mutual inductance of the two coils is
 2.66 mH 2.66 H 2.66 mH x 10 -3 0.266 H