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An electric lamp is marked 220 V, 100 W. If it is connected to a 110 V supply, the power it will consume is

1. 75 watt
2. 40 watt
3. 25 watt
4. 50 watt

Correct answer: 75 watt

Solution

Lamp resistance R = V^2/P = 220^2/100 = 484 ohm. At 110 V, P = 110^2/484 = 25 W.

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