Question

The magnetic flux in a closed circuit of resistance $10\Omega$ varies with time as $\varphi =(2t-4t^2+1)$. The current in the loop will change its direction after a time of

• A. $0.25\sec$
• B. $0.5\sec$
• C. $1\sec$
• D. none

Answer 1

The correct option is A $0.25\sec$

$e=-\frac{d\varphi }{dt}=[-2+8t+0]$

Since $e=0$ when flux is passing through a closed surface.
$\Rightarrow 8t=2\Rightarrow t=\frac{1}{4}\sec =0.25\sec$