Current Electricity - Result Question 66

71. 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 :

[2012]

(a) $20 %$

(b) $2.5 %$

(c) $5 %$

(d) $10 %$

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Answer:

Correct Answer: 71. (c)

Solution:

  1. (c) Resistance of bulb is constant

$P=\frac{V^{2}}{R} \Rightarrow \frac{D p}{p}=\frac{2 D V}{V}+\frac{D R}{R}$

$\frac{D p}{p}=2 \times 2.5+0=5 %$