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Three identical bulbs are connected so that bulb B1 is in series with the parallel combination of bulbs B2 and B3, and a switch S is connected in series with B3 (so closing S puts B3 into the circuit). With switch S closed, the power consumed by bulb B2 is P. What power does bulb B2 consume when the switch S is opened?

1. 9P/4
2. 4P/9
3. 9P/16
4. 16P/9

Correct answer: 9P/4

Solution

Opening S removes B3, raising the voltage and current through B2; the current ratio is 3/2, so the power scales by (3/2)² = 9/4.

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