Quiznetik

Electromagnetic Theory | Set 3

1. What will be the current passing through the ring shaped air cored coil when number of turns is 800 and ampere turns are 3200 ?

Correct : B. 4

2. What will be the magnetic potential difference across the air gap of 2 cm length in magnetic field of 200 AT/m ?

Correct : B. 4 AT

3. A certain amount of current flows through a ring-shaped coil with fixed number of turns. How does the magnetic induction B varies inside the coil if an iron core is threaded into coil without dimensional change of coil ?

Correct : B. Increases

4. The magnetic reluctance of a material

Correct : A. decreases with increasing cross sectional area of material

5. The initial permeability of an iron rod is

Correct : D. the permeability almost in non-magnetised state

6. How does the magnetic compass needle behave in a magnetic field ?

Correct : C. It assures a position which follows a line of magnetic flux

7. In a simple magnetic field the strength of magnet flux

Correct : D. first increases and then decreases till it becomes zero

8. The stray line of magnetic flux is defined as

Correct : D. a line of magnetic flux which does not follow the designed path

9. The bar magnet has

Correct : A. the dipole moment

10. Which of the following materials are dia-magnetic ?

Correct : C. Silver and copper

11. Which of the following type of materials are not very important for engineering applications ?

Correct : C. Diamagnetic

12. The susceptibility of paramagnetic materials generally lies between

Correct : A. KT3 and 1CT6

13. For which of the following materials the saturation value is the highest ?

Correct : D. Ferrites

14. The magnetic materials exhibit the property of magnetisation because of

Correct : C. spin of nucleus

15. For which of the following materials the net magnetic moment should be zero ?

Correct : C. Antiferromagnetic materials

16. The attraction capacity of electromagnet will increase if the

Correct : D. flux density increases

17. Which of the following statements is correct ?

Correct : A. The conductivity of ferrites is better than ferromagnetic materials

18. Temporary magnets are used in

Correct : D. all of the above

19. Main causes of noisy solenoid are

Correct : C. both of above

20. Core of an electromagnet should have

Correct : C. both of the above

21. Magnetism of a magnet can be destroyed by

Correct : D. by all above methods

22. An air gap is usually inserted in magnetic circuits to

Correct : C. prevent saturation

23. The relative permeability of a ferromagnetic material is

Correct : D. more than 100 or 1000

24. The unit of magnetic flux is

Correct : B. weber

25. Permeability in a magnetic circuit corresponds to______ in an electric circuit.

Correct : C. conductivity

26. Point out the wrong statement. Magnetic leakage is undesirable in electric machines because it

Correct : A. lowers their power efficiency

27. Relative permeability of vacuum is

Correct : A. 1

28. Permanent magnets are normally made of

Correct : A. alnico alloys

29. Energy stored by a coil is doubled when its current is increased by percent.

Correct : C. 41.4

30. Those magnetic materials are best suited for making armature and transformer cores which have____permeability and_______hystersis loss.

Correct : C. high, low

31. The rate of rise of current through an inductive coil is maximum

Correct : B. at the start of the current flow

32. When both the inductance and resistance of a coil are doubled the value of

Correct : A. time constant remains unchanged

33. The initial rate of rise of current through a coil of inductance 10 H when suddenly connected to a D.C. supply of 200 V is_______Vs

Correct : B. 20

34. A material for good magnetic memory should have

Correct : D. high retentivity

35. Conductivity is analogous to

Correct : C. permeability

36. In a magnetic material hysteresis loss takes place primarily due to

Correct : D. it high retentivity

37. Those materials are well suited for making permanent magnets which have______retentivity and _______coercivity.

Correct : B. high, high

38. If the area of hysteresis loop of a material is large, the hysteresis loss in this material will be

Correct : C. large

39. Hard steel is suitable for making permanent magnets because

Correct : A. it has good residual magnetism

40. Silicon steel is used in electrical machines because it has

Correct : C. low hysteresis loss

41. Conductance is analogous to

Correct : A. permeance

42. The property of a material which opposes the creation of magnetic flux in it is known as

Correct : D. reluctance

43. The unit of retentivity is

Correct : B. weber/sq. m

44. Reciprocal of reluctance is

Correct : B. permeance

45. While comparing magnetic and electric circuits, the flux of magnetic circuit is compared with which parameter of electrical circuit ?

Correct : B. Current

46. The unit of reluctance is

Correct : D. 1/henry

47. A ferrite core has less eddy current loss than an iron core because

Correct : D. ferrites have high hysteresis

48. Hysteresis loss least depends on

Correct : D. ambient temperature

49. Laminated cores, in electrical machines, are used to reduce

Correct : B. eddy current loss

50. The property of coil by which a counter e.m.f. is induced in it when the current through the coil changes is known as

Correct : A. self-inductance

51. As per Faraday's laws of electromagnetic induction, an e.m.f. is induced in a conductor whenever it

Correct : C. cuts magnetic flux

52. Which of the following circuit element stores energy in the electromagnetic field ?

Correct : A. Inductance

53. The inductance of a coil will increase under all the following conditions except

Correct : A. when more length for the same number of turns is provided

54. Higher the self-inductance of a coil,

Correct : D. longer the delay in establishing steady current through it

55. In an iron cored coil the iron core is removed so that the coil becomes an air cored coil. The inductance of the coil will

Correct : B. decrease

56. An open coil has

Correct : B. infinite resistance and zero inductance

57. Both the number of turns and the core length of an inductive coil are doubled. Its self-inductance will be

Correct : B. doubled

58. If current in a conductor increases then according to Lenz's law self-induced voltage will

Correct : C. produce current opposite to the in-creasing current

59. The direction of incViced e.m.f. can be found by

Correct : B. Lenz's law

60. Air-core coils are practically free from

Correct : C. both (a) and (b)

61. The magnitude of the induced e.m.f. in a conductor depends on the

Correct : D. rate of change of flux-linkages

62. Mutually inductance between two magnetically-coupled coils depends on

Correct : D. all of the above

63. A laminated iron core has reduced eddy-current losses because

Correct : B. the laminations are insulated from each other

64. The law that the induced e.m.f. and current always oppose the cause producing them is due to

Correct : B. Lenz

65. Which of the following is not a unit of inductance ?

Correct : B. Coulomb/volt ampere

66. In case of an inductance, current is proportional to

Correct : B. magnetic field

67. Which of the following circuit elements will oppose the change in circuit current ?

Correct : B. Inductance

68. For a purely inductive circuit which of the following is true ?

Correct : C. Actual power of the circuit is zero

69. Which of the following is unit of inductance ?

Correct : B. Henry

70. An e.m.f. of 16 volts is induced in a coil of inductance 4H. The rate of change of current must be

Correct : D. 4 A/s

71. The core of a coil has a length of 200 mm. The inductance of coil is 6 mH. If the core length is doubled, all other quantities, remaining the same, the in ductance will be

Correct : A. 3 mH

72. The self inductances of two coils are 8 mH and 18 mH. If the co-efficients of coupling is 0.5, the mutual inductance of the coils is

Correct : C. 6 mH

73. Two coils have inductances of 8 mH and 18 mH and a co-efficient of coupling of 0.5. If the two coils are connected in series aiding, the total inductance will be

Correct : B. 38 mH

74. A 200 turn coil has an inductance of 12 mH. If the number of turns is increased to 400 turns, all other quantities (area, length etc.) remaining the same, the inductance will be

Correct : D. 48 mH

75. Two coils have self-inductances of 10 H and 2 H, the mutual inductance being zero. If the two coils are connected in series, the total inductance will be

Correct : C. 12 H

76. In case all the flux from the current in coil 1 links with coil 2, the co-efficient of coupling will be

Correct : B. 1.0

77. A coil with negligible resistance has 50V across it with 10 mA. The inductive reactance is

Correct : D. 5000 ohms

78. A conductor 2 metres long moves at right angles to a magnetic field of flux density 1 tesla with a velocity of 12.5 m/s. The induced e.m.f. in the conductor will be

Correct : C. 25V

79. Lenz's law is a consequence of the law of conservation of

Correct : C. energy

80. A conductor carries 125 amperes of current under 60° to a magnetic field of 1.1 tesla. The force on the conductor will be nearly

Correct : B. 120 N

81. Find the force acting on a conductor 3m long carrying a current of 50 amperes at right angles to a magnetic field having a flux density of 0.67 tesla.

Correct : A. 100 N

82. The co-efficient of coupling between two air core coils depends on

Correct : C. mutual inductance and self inductance of two coils

83. An average voltage of 10 V is induced in a 250 turns solenoid as a result of a change in flux which occurs in 0.5 second. The total flux change is

Correct : D. 0.02 Wb

84. A 500 turns solenoid develops an average induced voltage of 60 V. Over what time interval must a flux change of 0.06 Wb occur to produce such a voltage ?

Correct : C. 0.5 s

85. Which of the fpllowing inductor will have the least eddy current losses ?

Correct : A. Air core

86. A coil induces 350 mV when the current changes at the rate of 1 A/s. The value of inductance is

Correct : B. 350 mH

87. Two 300 uH coils in series without mutual coupling have a total inductance of

Correct : B. 600 uH

88. Current changing from 8 A to 12 A in one second induced 20 volts in a coil. The value of inductance is

Correct : C. 5 H

89. Which circuit element(s) will oppose the change in circuit current ?

Correct : B. Inductance only

90. A crack in the magnetic path of an inductor will result in

Correct : D. reduced inductance

91. A coil is wound on iron core which carries current I. The self-induced voltage in the coil is not affected by

Correct : B. variation in voltage to the coil

92. The polarity of the induced voltage can be determined by using the left-hand generator rule.

Correct : A. Yes

93. Increasing the field or increasing the current will decrease the force on the conductor.

Correct : B. No

94. Reversing the field or the current will reverse the force on the conductor.

Correct : A. Yes

95. When a conductor moves in the field, so that it makes an angle 8 with the lines of flux, the force F is given as : F = Bl sin2 0.

Correct : B. No

96. The self-inductance of the coil may be defined as equal to the e.m.f. induced in volts when the current in the circuit changes at the rate of unit weber turns.

Correct : B. No