MAGNETISM ELECTROMAGNETIC INDUCTION & ALTERNATING CURRENT

2. A bar magnet having dipole moment 80 units is cut into two halves of equal length. The moment of each half will be:   a. 80 units   b. 40 units   c. 60 units   d. 20 units   e. 160 units
3. If the distance between two dissimilar magnetic poles held ?d? apart is doubled, the force of attraction between them:   a. doubles   b. quadruples   c. becomes half   d. becomes one fourth   e. remains the same
4. The distance between two magnetic poles and also their pole strength are doubled. The force between them:   a. doubles   b. becomes half   c. triples   d. quadruples  

5. All magnetic materials lose their magnetic properties, when:   a. dipped in water   b. dipped in oil     c. brought near a piece of iron   d. brought in contact with copper   e. strongly heated
6. Magnetic lines of force:   a. always intersect   b. are closed curves   c. tend to crowd together far away from the poles of a magnet   d. do not pass through vacuum   e. have none of the above properties
7. A magnet is placed in earth?s magnetic field with its south pole facing north. At the neutral point:

8. If a bar magnet is place with its north pole pointing north, the neutral points of magnetic field are located:   a. on the axial line of the magnet   b. on the equatorial line of the magnet   c. near the north pole of the magnet   d. near the south pole of the magnet   e. neutral points do not exist
9. A sensitive magnetic instrument can be shielded effectively from an outside magnetic field by placing it inside a box of:   a. plastic   b. wood   c. copper   d. soft iron   e. none of the above
10. Magnetic field can be measured using:   a. pyrometer   b. hydrometer   c. thermometer   d. altimeter            e. flux meter
11. Unit of magnetic susceptibility is:   a. weber   b. henry   c. weber/meter   d. ampere   e. none of these
12. Three identical bar magnets, each of moment M are arranged as in figure.

13. The magnetic induction at point distant ?r? from a short dipole, on its axial line varies with ?r? as:   a. r   b. r²   c. r³   d. 1/r²   e.1/r³ 
14. Two points A and B are situated along the extended axis of a 2 cm long bar magnet, at distances ?r? and ?2r? respectively, from the pole nearer to the point. The ratio of the magnetic fields at A and B is:

15. A short magnetic dipole of moment 6.75 Am² produces a neutral point on its axial line. If the horizontal earth?s magnetic induction is 5 × 10^-5 T, the distance of the neutral point from the magnet is:

16. For a bar magnet placed at an angle ? with a field, the couple (torque) is proportional to:

17. Rate of change of torque ? with angle ? is maximum for a magnet suspended freely in a uniform magnetic of induction B, when ? is:   a. 0º   b. 30    c. 90   d. 45   e. 60º
18. The work done in turning a magnet of moment M by an angle 90º from the magnetic meridian is ?n? times the work required to turn through an angle 60º. Then, ?n? is:  a. ½  b. 2  c. ¼  d. 4   e. 1
19. A thin rectangular bar magnet suspended freely in a magnetic field has period of oscillation 4 s. If it is broken into two halves, each half having half the original length, and one of the pieces is suspended in the same field, the period of oscillation will be:  a. 4 s   b. 2 s   c. 0.5 s   d. 8 s   e. 0.25 s
20. The magnetic south pole of earth is:   a. near the geographic north pole   b. near the geographic south pole  c. vertically above the equator   d. none of the above
21. The angle between the earth?s magnetic axis and geographic axis is about:

22. Earth?s magnetic field always has a horizontal component except at:   a. the magnetic poles   b. the magnetic equator   c. the geographic poles   d. a latitude of 45º
23. A magnetic needle pivoted about a horizontal axis makes an angle with the horizontal because:   a. the gravitational force of earth attracts one pole and repels the other   b. one pole is heavier than the other   c. magnetic needle cannot be in equilibrium   d. due to earth?s magnetism one pole dips   e. none of these
24. The angle of dip at the magnetic equator is:   a. zero   b. 90   c. 45   d. 60   e. 180º
25. At a certain place the angle of dip is 30º and the horizontal component of earth?s magnetic induction is 0.5 oersted. Then, earth?s total magnetic induction in oersted is:   a. 0.5   b. 1   c. ?3   d. 1/?3   e. none
26. An imaginary line passing through all the places having zero dip is called:   a. isoclinic line   b. isogonic line   c. aclinic line   d. agonic line   e. none of these
27. A small piece of an unmagnetised substance gets repelled when brought near a powerful magnet. Then the substance is:   a. diamagnetic   b. paramagnetic   c. ferromagnetic   d. non magnetic
28. The temperature above which a ferromagnetic substance becomes paramagnetic is called:   a. critical temperature   b. curie temperature   c. Boyle temperature   d. Debye temperature  e. none of these
29. Soft iron is used in many parts of electrical machines because it has:   a. low hysteresis loss and low permeability   b. low hysteresis loss and high permeability   c. high hysteresis loss   d. high susceptibility
30. The area enclosed by a hysteresis loop is a measure of:   a. retentivity of the material   b. coercivity of the material   c. permeability   d. susceptibility   e. energy loss.
31. Lenz?s law gives the direction of:  a. magnetic field  b. motion of conductor  c. induced current  d. none
32. When a magnet is moved towards a coil with its north pole facing the coil, the nearer end of the coil acts as a:  a. north pole  b. south pole  c. positive charge  d. negative charge
33. Two pure inductors of self-inductance L each are joined in series, the net inductance is:   a. L   b. L/2   c.2L   d. L/4  
34. When a wire loop is rotated in a magnetic field, the direction of induced e. m. f. changes once in every: 

35. If N is the number of turns in a coil, its self-inductance varies as:  a. Nº  b. N  c. N²  d. 1/N²
36. Among the following the combination having dimensions of frequency is:  a. 1/RC b. RL/C c. R/LC d. C/L      e. RC
37. A copper ring having a cut so as not to form a loop is held horizontally and a bar magnet is allowed to fall through it with its length along the axis of the ring. The acceleration of the magnet is:  a. g  b. g/2  c. 2g  d. 0        
38. The time constant of inductance is:  a. LR  b. L/R  c. ?(L/R)  d. R/L
39. A circuit has capacitance 10 mF and resistance 5 mega ohm. It is connected to a d. c. of 100 V. The value of steady current is:  a. 20 ? A  b. 10 A  c. 20 mA  d. 0.
40. A circuit has inductance 10 mH and resistance 10 ohm. It is connected to a source of e. m. f. (D. C.) 100 V. The value of steady state current is:  a. 10 A  b. 10000 A  c. 0.1 A  d. none of these.
41. A circuit with L and C has natural frequency f. If the capacitance and inductance are doubled, the frequency becomes:  a. f/4  b. 2f  c. 4f  d. f/2.
42. In an a. c. circuit maximum value of voltage is 423 V. Its effective value is:  a. 400 V b. 300 V  c. 323 V  d. 340 V.
43. An inductive circuit has zero resistance. When an a. c. voltage is applied across this circuit, the current lags behind the voltage by:  a. 30º  b. 45º  c. 90º  d. 0º
44. The reactance of a capacitance is 5 ohm at 50 Hz. If the frequency is increased to 100 Hz, the new reactance is:  a. 5 ohm  c. 10 ohm  c. 2.5 ohm  d. 125 ohm.
45. An a. c. source produces 120 V at 60 Hz. The value of voltage after 1/720 s from start is:  a. 84.8 V  b. 42.4 V  c. 106.8 V  d. 20.2 V.
46. An a. c. circuit has an inductance of 0.5 H and capacitance of 0.8 mF in series. The current in the circuit is maximum when the angular frequency of the a. c. is:  a. 500 Hz 

47. A 50 W/100 V lamp is to be connected to an ac mains of 200 V/50 Hz. The capacitance of the capacitor to be connected is:  a. 4.6  b. 2.9  c. 1.5  d. 19.2 mF
48. For a resistance R and a capacitor C in series, the impedence is twice that of a parallel combination of the same elements. The frequency of the applied ac is:  a. 2?/RC 

49. In a series L-C circuit, the voltage across L and C are:  a. in phase  b. out of phase by 90º  c. out of phase by 180º  d. none of these
50. An alternating voltage of angular frequency ? is applied to an L-C circuit. If for the circuit, ?² = 1/LC, then:  a. the current is maximum  b. the current is minimum  c. the voltage is minimum  d. the impedence is maximum.
51. A coil, a capacitor and an ac of 24 V r. m. s. are connected in series. By varying the frequency of the source, a maximum current of 6 A is observed. If this coil is connected to a battery of e. m. f. 12 V and internal resistance 4 ohm, the current through it will be:  a. 2.4 A  b. 1.8 A  c. 1.5 A  d. 1.2 A.
52. In a series combination of R, L and C, in resonance with an ac, if R = 20 ohm, the impedence of the combination is:  a. 20 ohm  b. zero   c. 10 ohm  d. 400 ohm.
53. In a series L-C-R circuit, the voltage across the resistance, capacitance and inductance is each 10 V. If the resistance is short circuited, the current in the circuit will be:  a. zero  b. infinite  c. 10 A  d. 5 A.
54. Reciprocal of impedence is called:  a. reactance  b. inductance  c. conductance  d. admittance                                                                     A                       B
55. In the circuit, when ? is resonant frequency,

60. A thin magnet suspended freely in a magnetic field has period of oscillation 4 s. If the magnet is broken into two halves, with each half having the original length, and one of the pieces is oscillated in the same field, the period of oscillation will be:

61. Two magnetic dipoles are placed co-axially, separated by a distance ?r?. The force between them depends on ?r? as:

62. If A and B are the angles of dip in two vertical planes at right angles to each other, and C is the true angle of dip, then:

63. The S. I (or Sommerfeld) unit of magnetic permeability is:

64. The true dip at a place is 45º. If the plane of the dip circle is turned through 60º from the magnetic meridian, the apparent dip will be:

65. If a bar magnet is placed with its north pole facing north, the neutral point will be on:

66. Three identical bar magnets, each of moment M are placed to form an equilateral triangle, with the north pole of one touching the south pole of the other. The resultant moment of the system is:

67. The dip needle in a plane perpendicular to the magnetic meridian will be:

68. If the net dipole moment of the atom of a substance is zero, the substance exhibits:

69. A metal rod of length 15 cm rotates about an axis passing through one end of it, with uniform angular velocity 60 radian/second, in a magnetic field 0.1 T, directed parallel to the axis of rotation of the rod. The e. m. f. induced between the ends of the rod is:

70. Two identical, co-axial circular loops carry equal current in the same direction. When the coils are moved towards each other:

71. Two concentric co-planar circular metal loops each having resistance 0.1 m?/m have diameters 0.2 m and 2 m. A voltage that varies with time, V = 4 + 2.5 t volt is applied to the larger loop. Then, the current in the smaller loop is:

72. An airplane with wingspan 20 m is flying horizontally at a speed 250 m/s, towards south. Earth?s horizontal component of magnetic field at the place is 0.02 m T and the angle of dip is 60º. The e. m .f. induced across the tip of the wings is:

73. A horizontal metal rod of length 1 m and mass 0.2 kg falls down in a uniform magnetic field 0.6 T, directed perpendicular to the length of the rod. The current in it when it attains terminal velocity is:

74. A metal ring is kept suspended by a thread. If one pole of a magnet is moved towards it:

75. Inductive time constant of a circuit is the time taken by the current in the circuit:

76. When the number of turns and the length of a solenoid are both doubled, its self-inductance becomes:

77. When the current in a coil changes from 8 A to 2 A in 0.03 s, the self-induced back e. m. f. in it is 2 V. The self inductance of the coil is:

78. Two coils have self-inductances 8 m H and 2 m H. At a certain instant, the current in both of them is increasing at the same rate. Then, the ratio of the induced e. m. f. in them at that instant is:

79. Which of the following statements is true?

80. When the current in an inductor is doubled, by what factor does the energy stored in it change?

81. When the current through a solenoid increases at a constant rate, the induced current:

82. A coil of area 500 cm² having 1000 turns is placed in a magnetic field of 0.04 m T, with its plane perpendicular to the field. If it is turned through 180º in 0.1 s, the induced e. m. f. in it is:

83. Two rails of a railway track are insulated from each other and from the ground. A millivoltmeter is connected across them. If a train travels over them at a speed of 20 m/s, the reading in the voltmeter is (given: vertical component of earth?s field is 0.02 m Wb/m² and distance between the rails is 1 m):

84. The unit of inductance is:

85. A bulb ?A?, with a resistance in series with it and another one, ?B?, with an inductance in series, are connected in parallel to a power supply. When the supply is switched off:

86. A fully charged capacitor is allowed to discharge through an inductance of 0.5 m H. The initial current through the inductance rises from 0 to 1.5 A in 2.5 microsecond. The instantaneous voltage across it is:

87. The core in the transformer is laminated so that:

88. A horseshoe magnet is placed with its north pole on the upper side. If a copper plate is pushed into the space between the poles, on viewing from above, the induced eddy current in the sheet will flow in:

89. The ac produced in an alternator is represented by V = 170 Sin 377 t. The frequency of the voltage is:

90. A direct current of 5 A is superimposed on an alternating current I = 10 Sin (314 t) ampere. The resultant current is:

91. The reactance of a coil connected to our domestic power supply is 100 ?. The self-inductance of the coil is:

92. A ideal coil of inductance 5 m H is connected to an alternating voltage V = 10 Sin(100t) volt. The peak current in the coil is:

93. A solenoid having resistance 10 ? and inductance 5 m H is connected to a battery of e. m. f. 10 V. The current through it in the steady state is:

94. Hot wire ammeters are used to measure:

95. A direct current of 2 A and an alternating current with peak value 2 A are allowed to pass through two identical resistances. The ratio of heat produced in them is:

96. The armature of a D. C. motor has resistance 20 ohm. It draws 1.5 A when run by a 200 V D. C. supply. The back e. m. f. induced in the motor is:

97. In a series L-C-R circuit, L = 10 m H, C = 5 m F and R = 10 W. For a particular ac frequency, the circuit is found to resonate. Now, if the resistance in the circuit is doubled, the resonant frequency will be:

98. A conducting wire is kept stretched between the pole pieces of a powerful magnet, so that it is perpendicular to the lines of force. If an alternating current is passed through the wire, it will:

99. An electric heater of 400 W is connected to a 200 V, 50 Hz mains. The peak value of current through the heater is:

100. A circular coil of 5 turns and radius 30 cm is rotated about its diameter in a uniform magnetic field of 0.01 T, perpendicular to the direction of the magnetic lines of force. If the frequency of rotation is 200 rpm and resistance of the coil is 9.87 W, the amplitude of the current induced in the coil is: