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Two bulbs, each rated 250 V, 100 W, are first connected in series and then in parallel with a supply of 250 V. The total power in each of the cases will be ______ respectively.

Asked in GUJCET 2009 · Rated bulbs and appliances

• (1) 50 W, 100 W
• (2) 50 W, 200 W
• (3) 100 W, 50 W
• (4) 200 W, 150 W

Step-by-step solution

Why the other options are wrong

• (1) Gets the series case right but counts only one bulb in parallel. Each bulb then has its full 250 V and gives its full 100 W, so the pair gives 200 W.
• (3) Imagines the series pair still taking the rated 100 W because the same current flows through both, and the parallel pair halving it because the current divides. In series each bulb gets only half the voltage; in parallel each gets the full voltage.
• (4) No single slip gives 200 W and 150 W. In series the resistance doubles, so the power must fall below one bulb's 100 W, which neither figure does.

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