Question

An electric bulb is rated $220V$ and $100W$. When it is operated on $110V$, what will be the power consumed?

Answer 1

$P=\frac{V^2}{R}$

If $V=220$ V we have

$100W=\frac{{220}^2}{R}$

$R=\frac{{220}^2}{100}\Omega =484\Omega$. This is the resistance of the bulb.

When $V=110$ V, power consumed $=\frac{V^2}{R}=\frac{{110}^2}{484}=25$ W.