The circuit shown in figure consisting of three identical lamps and two coils is connected to a direct current source. The ohmic resistance of the coils is negligible. After some time switch S is opened. Which of the following statement(s) is/are correct for the instant immediately after opening the switch?

Question

TestHub · Accepted Answer

In steady state the coils have no resistance, so all three lamps sit directly across the battery, each carrying $i_{0} = ε / R$ .

Coil currents: $L_{1}$ feeds $B_{2} + B_{3}$ , so $I_{L_{1}} = 2 i_{0}$ ; $L_{2}$ feeds only $B_{3}$ , so $I_{L_{2}} = i_{0}$ .

Just after S is opened, inductor currents cannot change: $L_{1}$ still drives $2 i_{0}$ and $L_{2}$ still drives $i_{0}$ through the lamp loop.

KCL: $B_{2}$ gets $2 i_{0} - i_{0} = i_{0}$ and $B_{3}$ gets $i_{0}$ - brightness of both unchanged.

$B_{1}$ now carries $2 i_{0}$ instead of $i_{0}$ - its brightness suddenly increases.

Hence (B) and (C).

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Options:(select one or more)

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