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In an LR circuit, C is connected to A until the current becomes constant; then at t = 0 it is suddenly disconnected from A and connected to B so the inductor discharges through the resistor. What is the ratio of the voltage across the resistor to the voltage across the inductor at time t = L/R?

1. -1
2. 1
3. (1 - e)/e
4. e/(1 - e)

Correct answer: -1

Solution

Once disconnected from the source, the inductor drives current through the resistor in a closed loop containing only L and R. By KVL the sum of the two voltages is zero at every instant, so their ratio is -1 regardless of time.

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