**Short Questions**

1. A coil of wire in the xy plane with a magnetic field directed along the Y-axis.Around which of the three co-ordinate axes should the coil be rotated in order to generate an emf and a current in the coil?

2. A glance at a dynmo will indicate whether it has sliprings or commutators.How will this information enable one to know whether the machine is an A.C. or D.C.generator?

3. A light metallic is released from above into a vertical bar magnet viewed from above does the current flow clockwise or anticlockwise in the ring?

4. A square loop of wire is moving through a uniform magnet field.The normal to the loop is oriented parallel to the magnetic field.Is an emf induced in the loop?Give a reason for your answer?

5. A suspended magnet is oscillating freely in a horizontal plane.The oscillations are strongly damped when a metal plate is placed under magnet.Explain why this occurs?

6. Can a current carrying coil be used as a compass?Explain.

7. Can a D.C motor be turned into a D.C.generator?

8. Can a step – up transformer increase the power.(b) In a transformer there is no transfer of charge from the primary to the secondary,How is then the power transferred?

9. Can a step up transformer increase the power level?

10. Can an efficient transformer step- up energy?

11. Can an electric motor be used to drive an electric generator with the output from the generator being used to operate the motor?

12. cm is moving at a speed of 0.25 m sec⁻¹ in a direction perpendicular to a point 0.50 T magnetic field. What is the emf produced in the moving rod ?

13. Define eddy current ?

14. Define efficiency of a transformer?

15. Define electromagnetic induction?

16. Define henry?

17. Define load in an electrical circuit ?

18. Define motional emf?

19. Define self – inductance and give its unit?

20. Define the phenomenon of self inductance ?

21. Define the term energy density ?

22. Derive the dimensional formula for co–efficient of self – inductance?

23. Derive the dimensional formula for the self inductance ?

24. Describe some uses of step – down transformers?

25. Differentiate between an A.C and D.C currents?

26. Discuss briefly that the Lenz’s law is exactly in accordance to the law of the law of conservation of energy?

27. Does the induced current depend on the resistance of the circuit?

28. Does the induced emf always act to decrease the magnetic flux through a circuit?

29. Does the induced emf in a circuit depend on the resistance of the circuit?Does the induced current depend upon the resistance of the circuit?

30. Does the induced emf in a circuit depends on the resistance of the circuit ?

31. Does the inductance of a coil depend on the current in the coil?What factors affect the inductance of a coil?

32. Does the voltage output of a generator change if its speed of rotation is increased?Explain.

33. Four unmarked wires emerge from a transformer.What steps would you take to determine the turns ratio?

34. Give a mathematical relation for the energy density in an inductor ?

35. How can teh power losses be minimized in a transformer?

36. How can the concept of electro – magnetic induction be used to apply breaks to fast moving automobile?

37. How is the armature rotated in commercial generators?

38. How is the efficiency of Transformer increased?

39. How power loss due to eddy currents in a transformer can be reduced?

40. How the back emf of a motor can be expressed?

41. How the magnetic field in the D.C motor is provided?

42. How would you position a flat loop of wire in a changing magnetic field so that there is no emf induced in the loop?

43. In a certain region the earth’s magnetic field point vertically down.When a plane flies due north which wingtip is possibly charged?

44. In what three ways can a voltage be induced in a wire?

45. Is it possible to change both the area and the magnetic field passing through the loop and still not have an induced emf in the loop?

46. Is the law of conservation of energy is applicable to the phenomena of induction ?

47. Name the factors on which self inductance depends ?

48. Name the factors upon which the self – inductance depends?

49. Name the main parts of an A.C generator ?

50. On which principle transformer works ?

51. rev per min. What is the back emf when the motor turns 3360 rev per min ?

52. State Faraday’s law of electromagnetic induction ?

53. State Lenz’s law ?

54. The self – inductance of an air core is independent of the value of the current in the coil.This is not true for an iron – core coil.Explain?

55. What are similarities between a motor and a generator?What are the differences?

56. What are the main causes of power loss in a transformer?

57. What happened when any motor is over loaded ?

58. What is a back emf effect in motors?

59. What is a commutator ?

60. What is a generator ?

61. What is a transformer ?

62. What is an A.C. generator ?

63. What is an inductor ?

64. What is back emf in a motor which is just started?What is induced emf in a speedy motor?

65. What is D.C generator ?

66. What is difference between an A.C generator and D.C generator?

67. What is difference between step up transformer and step down transformer?

68. What is electromagnetic induction ?

69. What is henry?

70. What is hysteresis?

71. What is induced current ?

72. What is induced emf ?

73. What is mutual induction ?

74. What is self induction ?

75. What is the induced emf produced during the phenomenon of self induction ?

76. What is the magnitude of back emf in a transformer?

77. What is the net current flowing through a coil of a motor in the presence of a back emf?

78. What is the SI unit of motional emf ?

79. What is the SI unit of mutual inductance ?

80. What is the SI unit of self inductance?

81. What is the use of self – inductance in A.C circuits?

82. What is the wave form of A.C voltage or current ?

83. What is the working principle of a D.C motor?

84. What is the working principle of a generator?

85. What is transformer?

86. When an electic motor such an electric drill is being used does it also act as a generator?If so what is the consequence of this?

87. When the primary of a transformer is connected to a.c. mains the current it. (a) is very small if the secondary circuit is open but (b) increases whether the secondary circuit is closed Explain these facts.

88. When the switch in the circuit is closed the current is established in the coil and the metal ring jumps upward. Why?

89. Whether an inductor coil may appear like a solenoid?

90. Whether energy can be stored in the magnetic field of an inductor ?

91. Whether inductors capacitor?

92. Whether transformer obeys the law of conservation of power?

93. Why self induced emf is also called as back emf?

94. Will the out put voltage of a generator change if its speed of rotation is increased?

**Long Questions**

1. Define electromagnetic induction.

2. What is induced emf and induced current? Explain with different experiments.

3. Define motional emf and derive a relation for it.

4. State and explain Faraday’s law of electromagnetic induction. Also calculate induced emf.

5. State and explain Lenz’s law and direction of induced emf.

6. Explain the phenomenon mutual induction.

7. Explain the phenomenon self induction.

8. What type of energy is stored in an inductor? Find the relations for the energy and energy density in an inductor.

9. Describe the principle, construction and working of an alternating current generator.

10. What is D.C. generator?

11. Explain back motor effect in generators.

12. What is D.C. motor?

13. Explain the back emf effect in motors.

14. Describe construction, principle and working of a transformer.

15. How the power is lost in transformer?

16. An EMF of 0.45V is induced between the ends of a metal bar moving through a magnetic field of 0.22T. What field strength would be needed on EMF of 5 V bet. The ends of the bar, assuming that all other factors remain the same?

17. The flux density B in a region between the pole faces of a horse shoe magnet 0.5 wb/m² directed vertically downwards. Find the EMF induced in a straight wire 5cm long, perpendicular to B when it is moved in a direction at an angle of 60degree with the horizontal with a speed of 100cms⁻¹.

18. A coil of wire has 10 loop. Each loop has an area of 5 x 10⁻³m². A magnetic field perpendicular to the surface of each loop at all times. If the magnetic field is charged from 0.05T to 0.06T in 0.1 sec. Find the average EMF induced in the coil during this time.

19. A circular coil has 15 turns of radius 2cm each. The plane of the coil is inclined at 40 degree to a uniform magnetic field of 0.2T. If the field is increased to 0.5T in 0.2 sec, find the magnitude of the induced EMF.

20. Two coils are placed side by side. An EMF of 0.8V is observed in one coil when the current is changing at the rate of 200 As⁻¹ in the other coil. What is the mutual inductance of the coils?

21. A pair of adjacent coils has a mutual inductance of 0.1H. If the current in the primary changes from 0 to 10 A in 0.25s, what is the average induced EMF in the secondary? What is the change in flux in it if the secondary has 500 turns?

22. A solenoid has 250 turns and its self inductance is 2.4 mH. What is the flux through each turn when the current is 2A? What is the induced EMF when the current changes at 20 As⁻¹?

23. A solenoid of length 8cm and cross sectional area 0.5cm² turns. Find the self inductance of the solenoid when the core is air. If the current in the solenoid increases through 5A in 0.2 sec, find the magnitude of induced EMF in it.

24. When current through a cell changes from 100 mA to 200 mA in 0.005 sec, an induced EMF of 40 mV is produced in the coil. (a) What is the self inductance of the coil? (b) Find the increase in the energy stored in the coil.

25. Like any field, the Earth’s magnetic field stores energy. Find the magnetic energy stored in a space where strength of earth’s field is 7 x 10⁻⁵ T, if the space occupies an area of 10 x 10⁸m² and has a height of 750m.

26. A square coil of side 16cm has 200 turns and rotates in uniform magnetic field of magnitude 0.05 T. If the peak EMF is 12 V, what is the angular velocity of the cell?

27. A generator has a rectangular coil consisting of 360 turns. The coil rotates at 420 rev per min in 0.14 T magnetic field. The peak value of EMF produced by the generator is 50 V. If the coil is 5 cm wide, find the length of the side of the coil.

28. It is desired to make an A.C generator that can produce an EMF of 5kV with 50Hz frequency. A coil of area 1m² consisting of 200 turns is used as armature. What should be the magnitude of the magnetic field in which the coil rotates?

29. The back EMF in a motor is 120 V when the motor is turning at 1680 rev/min. What is the back EMF when the motor turns at 3360 rev/min?

30. A D.C motor operates at 240V and has a resistance of 0.5 ohm. When the motor is running at normal speed, the armature is 15A. Find the back EMF in the armature.

31. A copper ring has radius of 4cm and resistance of 1 milli ohm. A magnetic field applied over the ring, perpendicular to its plane. If the magnetic field increases from 0.2 T to 0.4 T in a time interval of 5 x 10⁻³s. What is the current in the ring during?

32. A coil of 10 turns and 35 cm² area is in a perpendicular magnetic field of 0.5T. The coil is pulled out of the field in 1s. Find the induced EMF in the coil as it is pulled out of the field.

33. An ideal step down transformer is connected to main supply of 240V. It is desired to operate a 12V, 30W lamp. Find the current in the primary and the transformation ratio?