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The resistance of hot tungsten filament is about 10 times the cold resistance. What will be the resistance of 100 W and 200 V lamp when not in use?

1. 20Ω
2. 40Ω
3. 200Ω
4. 400Ω

Correct answer: 40Ω

Solution

The resistance of the lamp when not in use is calculated using the formula for power, P = V²/R. Rearranging gives R = V²/P. Plugging in the values of 200 V and 100 W results in a resistance of 40Ω.

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