Question

Figure shows two bulbs B${}_1$ and B${}_2$, resistor R and inductor L. When the switch S is turned off

• A. both B${}_1$ and B${}_2$ die out promptly
• B. both B${}_1$ and B${}_2$ die out with some delay
• C. B${}_2$ die out promptly but B${}_1$ with some delay
• D. B${}_1$ die out promptly but B${}_2$ with some delay

Answer 1

Answer: (D)

B${}_1$ die out promptly but B${}_2$ with some delay

In the given diagram, bulb $B_2$ and inductor $L$ constitute a series $LR$ circuit. When the switch S is turned off, current decreases in this $LR$ circuit, but does not becomes zero rapidly, because due to change in current, magnetic flux linked with inductor coil, changes and as a result an emf is induced in inductor, which opposes the decrease in current due to Lenz's law, therefore current does not become zero rapidly and bulb $B_2$ dies out with some delay.

On the other hand bulb $B_1$ and resistance R, constitute a simple R circuit , in which as the voltage becomes zero, current becomes zero and $B_1$ dies out promptly.