Question

Two lamps, one rated 100 W at 220 V, and the other 60 W at 220 V, are connected in parallel to electric mains supply. What current is drawn from the line if the supply voltage is 220 V?

Answer 1

From Joule's Law of heating,

$P=V^2/R$

For first lamp,

$R_1={220}^2/100$

For second lamp,

$R_2={220}^2/60$

When they are connected in parallel,

$\frac{1}{R_{eq}}=\frac{1}{R_1}+\frac{1}{R_2}$

$=\frac{1}{{220}^2/100}+\frac{1}{{220}^2/60}$

$⟹R_{eq}={220}^2/160$

From Ohm's Law,

$V=I{R}_{eq}$

$220=\frac{{220}^2}{160}I$

$I=0.73A$