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When a circular  coil of radius 1m and 100turns is rotated  in a horizontal  uniform  magnetic  field, the  peak value of emf. Induced is 100v. The coil is unwound  and the  rewound  into circular  coil of radius 2m. If it is  rotated  , now with the same speed, under similar conditions,  the new peak value of emf  developed  is

Ans- 200v

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A coil having N turns and area A rotates in a uniform magnetic field B with angular velocity ω,
Then the peak value of the emf (Vmax) induced in the coil is given by (Vmax) = NABω

Now the fact is that the area of the coil in the 2nd case is 4A since the radius is doubled. 

But the number of turns will be N/2 in the second case since the length of the wire required for a turn is doubled.

If V1 and V2 are the peak values of the emf in the two cases, 

We have V1/V2 = 100 /V2 = NABω / [(N/2)×(4A)Bω]
Or, 100 /V2 = ½ 
from which
V2 = 200 volt.

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