The back e.m.f. induced in a coil, when current changes from 1 ampere to zero in one milli second, is 4 volts, the self inductance of the coil is
1 H
4 H
10-3H
4×10-3H
e=−Ldidt but e=4V and didt=0−110−3=−1/10−3
∴−110−3(−L)=4⇒L=4×10−3 henry