A flexible wire bent in the form of a circle is place in a uniform magnetic field perpendicularly to the plane of the coil. The radius of the coil changes as shown in Figure. The graph of magnetude of induced emf in the coil is represented by

Options

A. (a)
B. (b)
C. ( c)
D. (d)

Answer

B

Explanation

(b) In the `r- t` graph, it is clear that from `a to b` there is no change in radius and hence no change in area and magnetic flux. Same is the situaion from `c to d`.
Now, `|e| = (d)/(dt)(phi)`
`|e| = B(d)/(dt)(pir^(2)) = Bpi2r(dr)/(dt)`
Since `r prop t `:.` (dr)/(dT) = constant`
``:.` |e| prop r`