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A tungsten-filament lamp rated 100 W at 200 V has a hot filament resistance roughly 10 times its resistance when cold. What is the resistance of the filament when the lamp is switched off (cold)?

1. 14000 ohm
2. 400 ohm
3. 40 ohm
4. 4 ohm

Correct answer: 40 ohm

Solution

Operating resistance V²/P = 400 ohm; cold resistance is one-tenth, i.e. 40 ohm.

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