An electric bulb rated for 500W at 100V is used in a circuit having a 200V supply. The resistance R that must be put in series with the bulb, so that the bulb draws 500W, is
A
5Ω
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B
20Ω
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C
10Ω
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D
30Ω
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Solution
The correct option is A20Ω Here, P=VI⇒I=P/V=500/100=5A
This is the current through bulb as well as the resistor R. Voltage across R is VR=IR ⇒100=5R⇒R=20Ω