Q127 / 200 · NEET 2012 (Screening) · Physics

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

  • A
    20\%
  • B
    $2.5 \%$
  • Correct answer
    C
    $5 \%$
  • D
    $10 \%$

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 $5 \%$.

Hence, the correct answer is option: 'C'.

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