Discussion
The current in a coil decreases from 10 A to 4 A in 0.1 second. If the inductance of the coil is 4 H, find the emf induced in the coil.
Answer: C
The induced emf is 240 V.
The emf of self-induction is the inductance times the rate of change of current, \(e = L\,\frac{\Delta I}{\Delta t}\).
\[ \begin{aligned} \Delta I &= 10 - 4 = 6\ \text{A} \\ e &= 4 \times \frac{6}{0.1} = 240\ \text{V} \end{aligned} \]
Using the final current of 4 A instead of the change gives 160 V, and using the initial 10 A gives 400 V. Only the change in current induces an emf.
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