Page 27 - 12-phy-15 ELECTROMAGNETIC INDUCTION

15. Electromagnetic Induction eLearn.Punjab

On further increase in q current decreases, and at q = 180°=p rad the current becomes zero as

the loop is again perpendicular to B. For 180 <q < 270 current increases but reverses its direction
o
o
as is clear from the figure. Current is now directed along dcbad. At q = 270 =3p/2rad. current is
o
maximum in the reverse direction as the loop is parallol to B. At q =360 = 2p rad, one rotation is
o
completed, the loop is perpendicular to B and the current decreases to zero. After one rotation the

cycle repeats itself. The current alternates in direction once in one cycle. Therefore, such a current

is called the alternating current. It reverses its direction f times per second.
In actual practice a number of coils are wound around an iron cylinder which is rotated in the
magnetic field. This assembly is called an armature. The magnetic field is usually provided by

an electromagnet. Armature is rotated by a fuel engine or a turbine run by a waterfall. In some

commercial generators, field magnet is rotated around a stationary armature.
Example 15.6: An alternating current generator operating at 50 Hz has a coil of 200 turns. The
coil has an area of 120 cm . What should be the magnetic field in which the coil rotates in order to
2
produce an emf of maximum value of 240 volts?

Solution: For Your Information

Frequency of rotation = f = 50Hz

No. of turns of the coil N 200= =

2
2
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Area of the coil = A = 120 cm = 1.2x10 m 2
Maximum emf = e max = 240V
Magnetic flux density = B = ?

First, we shall find the angular speed .w

2 f
Using w= p

22
w= 2x x50 = 314.3 rads − 1 Faraday’s homopolar generator
7 with which he was able to produce

e a continuous induced current.
U sin g e= N ABw or B = N Aw
o
o

240V
B =
2
1
200x314.3 rads x1.2x10 m 2
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−
1
B = 0.32Vsrad m − 2 = 0.32T
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