If a coil of inductance 0.2 H and negligible resistance, carrying a steady d.c. current were suddenly short-circuited, using a resistance of 500 ohm, the current would reach zero in
A) 0.4 ms
B) 2 ms
C) 100 s
D) 500 s

Question

Quizplus · Accepted Answer

The time constant for an RL circuit is given by $\tau = \frac{L}{R}$, where $L$ is the inductance and $R$ is the resistance. Substituting the given values, $\tau = \frac{0.2}{500} = 0.0004$ seconds or 0.4 ms. However, the current does not immediately drop to zero in an RL circuit. It takes about 5 time constants ($5\tau$) for the current to effectively reach zero. Therefore, $5 \times 0.4$ ms = 2 ms.

If a Coil of Inductance 0