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  4. If voltage across a bulb rated 220V and 100W drops by 2.5 % of its rated value, by which the power would decrease is,

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Q. If voltage across a bulb rated $220V$ and $100W$ drops by $2.5\%$ of its rated value, by which the power would decrease is,

NTA AbhyasNTA Abhyas 2020

Solution:

Given $\left(\frac{\Delta V}{V} \times 100\right)\%=2.5\%$
$P=\frac{V^{2}}{R}$ error in $R=0$
$\left(\frac{\Delta P}{P} \times 100\right)\%$ $=2\left(\frac{\Delta V}{V} \times 100\right)\%$
$=2\times 2.5\%=5\%$