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A bulb is rated at 220 V and 100 W. If the applied voltage falls by 2.5% from its rated value, by what percentage of the rated power will the power output decrease?

1. 20%
2. 2.5%
3. 5%
4. 10%

Correct answer: 5%

Solution

For a bulb of fixed resistance, P = V^2/R so percentage change in power = 2 * percentage change in voltage = 2 * 2.5% = 5%.

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