An average induced e.m.f of 0.4V appears in a coil when the current in it is changed from 10 A in one direction to 10 A in opposite direction in 0.4 second.find the coefficient of self induction of the coil.

The induced emf is related to the change on current via the coefficient of self inductance as

e = -L.(dI / dt)

now, as

e = 0.4 V

dI = I = -10 A - (10A) = -20A

dt = t = 0.4 s

0.4 = -L .(-20) / 0.4

hence coefficient of self inductance will be 

L = 0.008 H = 8mH