Question
Radius of a circular loop placed in a perpendicular uniform magnetic field is increasing at a constant rate of $r_0 ms ^{-1}$. If at any instant radius of the loop is $r$, then emf induced in the loop at that instant will be
✓
Answer
(d)
$\frac{d r}{d t}=r_0 \Rightarrow \phi=B \pi r^2$
$\varepsilon=\frac{-d \phi}{d t}=-B \pi(2 r) \frac{d r}{d t}$
$\varepsilon=-2 \pi B r r_0$
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