Question

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

• A. 40 W in series and 100 watt in parallel
• B. 100 W in series and 40 watt 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

The correct option is A 40 W in series and 100 watt in parallel

Resistance, $R=V^2/P$
We assume rating voltage of both bulb is same, so $R\propto \frac{1}{P}$
Thus, $40W$ bulb has more resistance than the $100W$ i.e $R_{40}>R_{100}$
When they are connected in series, same current flow through them.

Thus, heat produced,$H=I^{2}Rt$ or $H\propto R$

So, $40W$ bulb will more brighter than $100W$.
When they are connected in parallel, same potential across them.

Thus, heat produced, $H=\left(V^2/R\right)t$ or $H\propto 1/R$

So, $100W$ bulb will more brighter than $40W$.