Question

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

• 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

Answer 1

Answer: (A)

40 W in series and 100 W in parallel

Answer 2

$\frac{R_{1}}{R_{2}} = \frac{P_{2}}{P_{1}} = \frac{100}{40} = \frac{5}{2}$. Resistance of 40 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.