A uniform but time-varying magnetic field B(t) exists in a circular region of radius a and is directed into the plane of the paper, as shown in the figure. 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

Magnetic field B(t) is directed into the plane of the paper P lies outside the field.
`oint vecE.vec(dl) = (dphi)/(dt)`
or `E(2pir) =d/(dt) (vecB.vecA)`
`2pirE=pia^2 ((dB)/(dt))cos 0^@` or `E=a^2/(2r) (dB)/(dt)` or `E prop 1/r`