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,
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(a) $\frac{{{R_1}}}{{{R_2}}} = \frac{{{P_2}}}{{{P_1}}} = \frac{{100}}{{40}} = \frac{5}{2}$. 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.
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