Question

A small, flat coil of resistance $r$ is placed at the centre of a large, closed coil of resistance $R$. The coils are coplanar and their mutual inductance is $M$. Initially, a constant current $i$ was flowing in

the inner coil. If this current is suddenly switched off, what charge will circulate in the other coil?

• A. $Mir/R^2$
• B. $MiR/r^2$
• C. $Mi/R$
• D. $Mi/r$

Answer 1

The correct option is C $Mi/R$

Initial flux $\varphi =Mi$

when current is switched off, the change in flux =$\Delta \varphi =Mi$

$Q=\frac{\Delta \varphi }{R}=\frac{Mi}{R}$