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