The average emf induced in a coil is 2 V when current is changed in 0.4 s
(A) from 5 A to 2 A and the self-inductance of the coil is 0.266 mH
(B) from 4 A to 4 A in the opposite direction, the self-inductance of the coil is 0.10 mH
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Both (A) and (B) are incorrect
Average induced emf is given by
$E = L \frac{\Delta I}{\Delta t}$
(A) Given: $E = 2\text{ V}$, $\Delta t = 0.4\text{ s}$, $\Delta I = 5 - 2 = 3\text{ A}$
$L = \frac{E \Delta t}{\Delta I} = \frac{2 \times 0.4}{3} = 0.266\text{ H} = 266\text{ mH}$
(B) When current changes from $+4\text{ A}$ to $-4\text{ A}$, $\Delta I = 8\text{ A}$
$L = \frac{E \Delta t}{\Delta I} = \frac{2 \times 0.4}{8} = 0.10\text{ H} = 100\text{ mH}$
Correct Answer: Both (A) and (B) are incorrect