If voltage across a bulb rated $220 \mathrm{~V}-100 \mathrm{~W}$ drops by $2.5 \%$ of its rated value, the…
If voltage across a bulb rated $220 \mathrm{~V}-100 \mathrm{~W}$ drops by $2.5 \%$ of its rated value, the percentage of the rated value by which the power would decrease is
20%
$2.5 \%$
$5 \%$
$10 \%$
Solution
Power $P=\frac{V^2}{R}$
For small variation
$\begin{aligned}
\frac{\Delta P}{P} \times 100 \% & =\frac{2 \times \Delta V}{V} \times 100 \% \\
& =2 \times 2.5=5 \%
\end{aligned}$
Therefore, power would decrease by $5 \%$.