• # question_answer 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,                                                      [MP PET 1993] 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

$\frac{{{R}_{1}}}{{{R}_{2}}}=\frac{{{P}_{2}}}{{{P}_{1}}}=\frac{100}{40}=\frac{5}{2}$. 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.