12th class (Fsc) Physics unit 4 MCQs
51. The only difference between the construction of DC and AC generator is:
a. Commutator
b. Carbon brushes
c. Magnetic field
d. Coil
52. In A.C inductor behaves as:
a. Commutators
b. Capacitor
c. Transistor
d. Resistor
53. In A.C generator when plane of coil is perpendicular to the magnetic field, then output of generator is:
a. Maximum
b. NWAB
c. Zero
d. 2nf
54. Split rings are used in:
a. Transformer
b. A.C generator
c. D.C motor
d. A.C motor
55. Induced emf in A.C generator can be increased by:
a. Increasing area of coil
b. Decreasing area of coil
c. Slowing down speed of coil
d. Decreasing magnetic field
56. A.C generator based upon the:
a. Faradays Law of electromagnet induction
b. Lenz’s Law
c. Mutual induction
d. Maxwell’s relation
57. The emf produced by an alternating current generator is:
a. NꞷAB sin2θ
b. NꞷAB sinθ
c. NꞷAB cos2θ
d. NꞷAB cosθ
58. Which one is not present in A.C generator?
a. Slip rings
b. Armature
c. Commutator
d. Magnet
59. Eddy current is one cause energy loss in:
a. D.C motor
b. A.C generator
c. D.C generator
d. Transformer
60. In D.C generator, split rings act as:
a. Inductor
b. Capacitor
c. Resistor
d. Commutator
61. The devices in the circuit that consume electrical energy are known as:
a. Load
b. Dissipaters
c. Motors
d. Generator
62. The jerks in D.C motor are created by the use of:
a. Slip rings
b. Armature
c. Source of emf
d. Commutators
63. Output of D.C motor is:
a. Chemical energy
b. A.C energy
c. D.C energy
d. Mechanical energy
64. When a motor is over loaded then the magnitude of back emf:
a. Remains constant
b. Increases
c. Zero
d. Decreases
65. When the back emf is zero, its draws:
a. Maximum current
b. Zero current
c. Steady current
d. Minimum current
66. If speed of rotation of a generator is doubled the output voltage will be:
a. Four time
b. Remain same
c. One half
d. Double
67. A device which converts electrical energy into mechanical energy is called:
a. D.C motor
b. Transformer
c. D.C generator
d. A.C generator
68. With the increase in speed of motor the magnitude of back emf:
a. Decreases
b. Remain same
c. First increases than decreases
d. Increases
69. When motor is just started, back emf is almost:
a. Minimum
b. Maximum
c. Infinite
d. Zero
70. The term Φ/t has the same dimension as:
a. Time
b. Flux
c. Current
d. Potential difference
71. With the speed of motor, magnitude of back emf:
a. Decreases
b. Remain same
c. First increases then deceases
d. Increases
72. If D.C is input for step-up transformer, then output is:
a. Low
b. Zero
c. May be high or low
d. High
73. The working principle of transformer is:
a. Mutual induction
b. Self-induction
c. Electromagnetic induction
d. Faraday’s law
74. For step down transformer:
a. Ns=Np
b. Ns>Np
c. Ns>>>Np
d. Np>Ns
75. A step-up transformer is used 120v line to provide 240v. If primary coil has 100 turns the number of turns is secondary is:
a. 150
b. 50
c. 200
d. 100
76. Which one has the least resistance?
a. Voltmeter
b. Galvanometer
c. Ohm meter
d. Ammeter
77. A real transformer does not change:
a. Frequency level
b. Voltage level
c. Power level
d. Current level
78. Transformer is used to change:
a. Alternating voltage
b. Electric power
c. Phase of A.C
d. Magnetic field
79. Step up transformer is used to:
a. Step up both A.C and D.C
b. Step up D.C voltage
c. Step up A.C current
d. Step up A.C voltage
80. If a step-up transformer were 100% efficient, the primary and secondary windings would have the same:
a. Voltage
b. Current
c. Direction of winding
d. Power
81. The core of transformer is laminated so reduce:
a. Eddy current loss
b. Magnetic loss
c. Electric loss
d. Hysteress loss
82. Commentator was invented in:
a. 1935
b. 1736
c. 1885
d. 1834
83. Commentator was invented by:
a. Maxwell
b. Henry
c. William sturgeon
d. Ousted
84. Efficiency of transformer does not affect by:
a. Insulation between sheet
b. Input voltage
c. Resistance of coils
d. Core of transformer
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