First slide

Electrical power

Question

There are two electric bulbs of 40 W and 100 W. Which one will be brighter when connected in series and when in parallel?

Easy

A

40 W in series and 100 W in parallel
B

100 W in series and 40 W in parallel
C

40 W both in series and parallel will be uniform
D

100 W both in series and parallel will be uniform

Solution

$\frac{R_{1}}{R_{2}} = \frac{P_{2}}{P_{1}} = \frac{100}{40} = \frac{5}{2} .$ Resistance of 4o w bulb is $\frac{5}{2}$ times than 100 W. In series, $P = i^{2} R$ and in parallel, $P = \frac{V^{2}}{R}$ So 40 W in series and 100 W in parallel will glow brighter.

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