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Electricity is supplied at 220 V in India and at 110 V in the USA. If the resistance of a 60 W bulb made for use in India is R, what is the resistance of a 60 W bulb made for use in the USA?

1. R/4
2. R/2
3. R
4. 2R

Correct answer: R/4

Solution

Since R = V²/P for the same P, the USA bulb at half the voltage has one-quarter the resistance: (110/220)² = 1/4, so R_USA = R/4.

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