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If voltage across a bulb rated 220 volt-100 Watt drops by 2.5% of its rated value

If voltage across a bulb rated 220 volt-100 Watt drops by 2.5% of its rated value, the percentage of the rated value by which the power would decrease is:

Options

(a) 20%
(b) 2.50%
(c) 5%
(d) 10%

Correct Answer:

5%

Explanation:

Resistance of bulb is constant
P = V² / R
∆p / p = 2∆V / V + ∆R / R
∆p / P = 2 x 2.5 + 0 = 5%

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