A long solenoid having 200 turns per cm carries a current of 1.5 amp. At the centre of it is placed a coil of 100 turns of cross-sectional area 3.14 × 10^–4 m² having its axis parallel to the field produced by the solenoid. When the direction of current in the solenoid is reversed within 0.05 sec, the in ducede.m.f. in the coil is

0.48 V
0.048 V
0.0048 V
48 V

Solution

The two rails of a railway track, insulated from each other and the ground, are connected to millivoltmeter. What is the reading of the millivoltmeter when a train passes at a speed of 180 km/hr along the track, given that the vertical component of earth’s magnetic field is 0.2 × 10wb/m² and rails are separated by 1 metre

10^–2 volt
10 mV
1 volt
1 mV

Solution

ε = Blv = (0.2 × 10^-4) (1) (180 × 5 /18) = 10^-3V = 1mV

A conducting square loop of side L and resistance R moves in its plane with a uniform velocity vperpendicular to one of its side. A magnetic in duction B constant in time and space, pointing perpendicular and into the plane of the loop exists everywhere.

The current induced in the loop is

\(\frac{B\iota v}{R}\) clockwise
\(\frac{B\iota v}{R}\) anticlockwise
\(\frac{2B\iota v}{R}\) anticlockwise
zero

Solution

Since the magnetic field is uniform the flux f through the square loop at any time t is constant, because

f = B × A = B × L² = constant

∴ ε = - ^dφ⁄_dt = zero

The flux linked with a coil at any instant ‘t’ is given by φ= 10t²– 50t + 250. The induced emf at t = 3s is

–190 V
–10 V
10 V
190 V

Solution

The self inductance of the motor of an electric fan is 10 H.In order to impart maximum power at 50 Hz, it should be connected to a capacitance of

8μF
4μF
2μF
1μF

Solution

The current in a coil of L =40 mH is to be increased uniformly from 1A to 11A in 4 milli sec. The induced e.m.f. will be

100 V
0.4 V
440 V
40 V

Solution

Magnetic flux fin weber in a closed circuit of resistance 10 Ω varies with time φ(sec) as f = 6t²– 5t + 1. The magnitude of induced current at t = 0.25 s is

0.2 A
0.6 A
1.2 A
0.8 A

Solution

A circular coil and a bar magnet placed nearby are made to move in the same direction. If the coil covers a distance of 1 m in 0.5. sec and the magnet a distance of 2 m in 1 sec, the induced e.m.f. produced in the coil is

zero
0.5 V
1 V
2 V

Solution

Vel.of coil = \(\frac{1}{0.5}\) = 2m/s

velocity of magnet = ²⁄₁ = 2m/s

As they are made to move in the same direction, their relative velocity is zero. Therefore, induced e.m.f. = 0.

A coil having 500 square loops each of side 10 cm is placed normal to a magnetic field which increases at the rate of 1 Wb/m². The induced e.m.f. is

0.1 V
5.0 V
0.5 V
1.0 V

Solution

In the figure the flux through the loop perpendicular to the plane of the coil and directed into the paper is varying according to the relation φ = 6 t² + 7t + 1 where φ is in milliweber and t is in second. The magnitude of the emf induced in the loop at t = 2 s and the direction of induce current through R are

39 mV; right to left
39 mV; left to right
31 mV; right to left
31 mV; left to right

Solution

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