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21.

The inductance of a coil in which a current of 0.1 A increasing at the rate of 0.5 As^{-1} represents a power flow of $\frac{1}{2}$ W, is

2 H

8 H

20 H

10 H

22.

A point source of electromagnetic radiation has an average power output of 1500 W. The maximum value of electric field at a distance of 3 m from this source in Vm^{-1} is

500

100

$\frac{500}{3}$

$\frac{250}{3}$

23.

Electromagnets are made of soft iron because soft iron has

low susceptibility and low retentivity

low susceptibility and high retentivity

high permeability and low retentivity

high permeability and high coercivity

24.

The flux linked with a circuit is given by $\mathrm{\varphi}={\mathrm{t}}^{3}+3\mathrm{t}-7$.The graph between time (x-axis) and induced emf (y-axis) will be a

straight line through the origin

straight line with positive intercept

parabola not through the origin

parabola through the origin

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25.

If the self-inductance of 500 tum coil is 125 mH, then the self inductance of similar coil of 800 turns is

48.8 mH

200 mH

187.5 mH

320 mH

26.

A copper disc of radius 0.1 m is rotated about its centre with 20 rev/s in a uniform magnetic field of 0.1 T with its plane perpendicular to the field. The emf induced across the radius of the disc is

$\frac{\mathrm{\pi}}{20}\mathrm{V}$

$\frac{\mathrm{\pi}}{10}\mathrm{V}$

$20\mathrm{\pi}\mathrm{mV}$

$10\mathrm{\pi}\mathrm{mV}$

27.

A varying magnetic flux linking a coil is given by $\mathrm{\varphi}-{\mathrm{Xt}}^{2}$. If at time t = 3 s, the emf induced is 9 V, then the value of X is

0.66 Wb s

^{-2}1.5 Wb s

^{-2}− 0.66 Wb s

^{-2}− 1.5 Wb s

^{-2}

28.

A short solenoid of length 4 cm, radius 2 cm and 100 turns is placed inside and on the axis of a long solenoid of length 80 cm and 1500 turns. A current of 3 A flows through the short solenoid. The mutual inductance of two solenoids is

2.96 × 10

^{-4 }H5.3 × 10

^{-5}H3.52 × 10

^{-3 }H8.3 × 10

^{-5}H

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29.

A coil having an inductance of 0.5 H carries a current which is uniformly varying from 0 to 10 A in 2 s. The emf (in volts) generated in the coil is

10

5

2.5

1.25

30.

If a coil of 40 turns and area 4.0 cm^{2} is suddenly removed from a magnetic field, it is observed that a charge of 2.0 x 10^{-4} C flows into the coil. If the resistance of the coil is 80 Ω, the magnetic flux density in Wb/m^{2} is

0.5

1.0

1.5

2.0

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