`A 12 V` battery connected ot a `6 Omega, 10 H `coil through a switch drives a constant current in the ciructit. The switch is suddenly opened. Assuming that it took 1 ms to open the swith, calculate the average emf induced across the coil.

Explanation

The steady state current is `(12 V)/(6 Omega) = 2 A`
The final current is zero
Thus, `(di)/(dt) =- (2A)/(1 ms) =- 2 X10 ^3 As^(-1)`
The induced emf is `epsilon =- (d Phi)/(dt) =- L (di)/(dt)` `=-(10H) X (-2X10^3 A s^(-1)) = 20000V`
Such a high emf may cause sparks across the open switch