Find the energy stored at t = 5 s. Assume zero initial current (SSC-2014)
312.5 kJ
0.625 kJ
3.125 kJ
156 .25 kJ
Answer.4. 156 .25 kJ
Explanation:-
Energy stored in inductor “E” = 1/2(LI2)
Voltage across inductor E(l) = L di(t)/dt
or i(t) = 1/L di(t)/dt
= 30/5 x 125/3 = 250A
Energy stored in an inductor = (5 x 2502)/2 = 156.25 kJ
Ques 82. The energy stored in the magnetic field of a solenoid 30 cm long and 3 cm diameter with 1,000 turns of wire carrying current of 10 A is (SSC-2014)
1.15 J
0.015 J
0.15 J
0.5 J
Answer.3. 0.15 J
Explanation:-
Inductance in solenoid is given as
L =μoN2A/l
Where μο =vacuum permeability = 4π x 10-7 wb/m
N = Number of turns
A = area of cross-section
l = length of the solenoid
L =(μoN2πr2)/l
Energy stored = LI2/2
E = (29.43 x 10-4 x 102) /2
=0.14715 J ≅ 0.15 J
Ques 82. Point out the WRONG statement (SSC-2014)
The magnetizing force at the center of a circular coil varies
Inversely as its radius
Directly as the number of its turns
Directly as the current
Directly as its radius
Answer.4. Directly as its radius
Explanation:-
At the center of a current-carrying coil, the magnetic field intensity is directly proportional to the current and inversely proportional to the radius of the coil.
Ques 83. A 2 cm long coil has 10 turns and carries a current of 750 mA. the magnetising force of the coil is (SSC-2014)
375 AT/m
225 AT/m
675 AT/m
450 AT/m
Answer.1. 375 AT/m
Explanation:-
For a coil of wire Magnetizing force (H)
= NI/l = 10 x 750 x 10-3/2 x 10-2
= 375 AT/m
Ques 84. The mutual inductance between two unity coupled coils of 9 H and 4 H will be (SSC-2014)
36 H
2.2 H
6 H
13 H
Answer.3. 6 H
Explanation:-
Mutual inductance
M = K√L1L2
For mutual inductance between two unity coupled coils, K =1
M = √L1L2 = √9 x 4 = 6H
Ques 85. A magnet is kept in the medium of air surrounded by an iron ring. The magnetic lines of force from the magnet will be (SSC-2014)
Very small in the ring
Crowded in the ring
Passing out of the ring
Evenly distributed within the ring
Answer.2. Crowded in the ring
Explanation:-
If soft iron surrounds a magnet, being in contact with its poles, that magnet may receive a much higher charge because Magnetic fields are distorted by ferromagnetic materials in their vicinity. If a piece of Iron is placed near a magnet, most of the flux lines will be crowded in the iron ring. It is because iron is a good conductor of magnetic flux.
Ques 86. The magnetic field energy in an inductor change from the maximum value to the minimum value in 5 msec when connected to an A.C. source. The frequency of the source is (SSC-2014)
500 Hz
20 Hz
50 Hz
200 Hz
Answer.3. 50 Hz
Explanation:-
Magnetic energy stored in an inductor = LI2/2.
The energy stored is maximum when the current is maximum and energy stored is minimum when the current is minimum (i.e. zero). ln an a.c. circuit, current goes from the maximum value to zero in T/4 sec. where T is the time period of a.c.
T/4 = 5
T = 20 ms = 20 x 10-3
Frequency = 1/T = 1/20 x 10-3 = 50 Hz
Ques 87. The magnetic potential difference in a magnetic circuit is given by (SSC-2014)
BIH
HJL
BL
Hl
Answer.4. Hl
Explanation:-
The flux producing the ability of the coil is proportional to the number of turns N and the current I. The product NI is called the magnetomotive force (MMF) and determines the amount of flux developed in the magnetic circuit.
MMF = NI ampere-turns
The reluctance of the magnetic circuit is given as
S = 1/μA
Ques 88. Consider a constant uniform magnetic field. A conductor moves across this field at a constant velocity. The emf induced in the conductor is termed as (SSC-2014)
Self-Induced EMF
Induced EMF
Statically Induced EMF
Dynamically Induced EMF
Answer.4. Dynamically Induced EMF
Explanation:-
In dynamically induced emf the magnetic field system is kept stationary, and the conductor is moving, or the magnetic field system is moving, and the conductor is stationary thus by following either of the two processes the conductor cuts across the magnetic field and the emf is induced in the coil. This phenomenon takes place in electric generators and back emf of motors and also in transformers.
Ques 89. A magnetic circuit carries a flux Φi, in the iron part and a flux Φg in the air gap. Then leakage coefficient is (SSC-2014)
Φi − Φg
Φi/Φg
Φg/Φi
Φi x Φg
Answer.2. Φi/Φg
Explanation:-
The ratio of total flux to the useful flux is called as the leakage coefficient in the magnetic circuit i.e Φi/Φg
Ques 90. The flux through each turn of a 100-turns coil is (t3 – 2t)mWh, where ‘t’ is in seconds. Find the magnitude of the induced emf at t = 2s (SSC-2014)
