A uniform but time varying magnetic field B(t) exists in a circular region of radius a and is directed into the plane of paper as shown. The magnitude of the induced electric field at point P at a distance r from the centre of the circular region.

Options

A. is zero
B. decreases as `(1)/(r )`
C. increases as r
D. decreases as `(1)/(r^(2))`

Answer

B

Explanation

`oint vec(E ).vec(dl)=|(-d phi)/(dt)|=(d)/(dt)(pi a^(2)B)`
or `E2 pi r=pi a^(2)|(dB)/(dt)|therefore E=(a^(2))/(2r)|(dB)/(dt)| therefore E prop (1)/(r )`.