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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)?

14000 ohm
400 ohm
40 ohm
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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