ELECTROMAGNETIC INDUCTION AND ALTERNATING CURRENTS

1 )

A coil having area 2m² is placed in a magnetic field which changes from 1 wb/m² to 4 wb/m² in an interval of 2 second. The emf induced in the coil of single turn is....

4 v

3 v

2 v

1.5 v

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Answer : Free Physics Questions Online Mock Test Series By Cracktest

emf induced in coil of single turn = e = – (dɸ / dt)

= – (d / dt) (A × B) ---- ɸ = A × B

= – A(dB / dt)

∴ e = – 2 × [{4 – 1} / 2] = – 3v i.e |e| = 3v.

2 )

Two different loops are concentric & lie in the same plane. The current in outer loop is clockwise & increasing with time. The induced current in the inner loop then, is …….

clockwise

counter clockwise

direction depends on the ratio of loop radii

zero

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Answer :

Lenz' law state when emf is generated by chage in magnetic fiux

according faradays law then direction of induced emf is such that it

opposes the cause of its production. Hence induced current in the inner

loop is counter clockwise.

3 )

A ring of radius r is rotating about its diameter with angular velocity ω in a perpendicular magnetic field B^➙ It has 20 turns. The emf induced is

20BÏ€r^2 sin wt

20BÏ€r^2 cos wt

10âˆš2BÏ€r^2

20BÏ€r^2 w sin wt

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Answer :

e = – N(dɸ / dt)

= – N (d / dt) (AB cos wt)

= – NAB (– 1) sin wt × w

= NABw sin wt

Here N = 20, A = πr²

∴ e = 20Bπr²w sin wt.

4 )

A magnetic field 2 × 10^–2T acts at right angles to a coil of area 200 cm² with 25 turns. The average emf induced in the coil is 0.1 v when it removes from the field in time t. The value of t is

0.1 sec

1 sec

0.01 sec

1.5 sec

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Answer :

B = 2 × 10^–2T

A = 200 cm² = 200 × 10^–4m²

N = 25

e = 0.1 v

we know e = – N(dɸ / dt)

∴ hence e = [(NAB) / t]

∴ t = [(NAB) / e] = [{25 × 2 × 10^–2 × 200 × 10^–4} / {0.1}]

∴ t = 0.1 sec

5 )

The magnetic flux linked with a coil, in webers, is given by the equation ɸ = 4t² – t + 7. Then the magnitude of induced emf at 2 sec will be ...

17 v

21 v

15 v

13 v

View Solution

Answer :

ɸ = 4t² – t + 7

(dɸ / dt) = 8t – 1

e = – (dɸ / dt)

hence induced emf at t = 2 sec

e = |– (dɸ / dt)|

e = 8t – 1|_{at t = 2 sec}

e = (8 × 2) – 1 = 15 v

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