Question

Two lamps one rated $60W$, $220V$ and the other $40W$, $220V$ are connected in parallel to a $220V$ electric supply mains. What is the total current drawn from the electric mains if the voltage of the electric supply is $220V$?

Answer 1

Relationship between power $\left(P\right)$, voltage$\left(V\right)$, and Resistance $\left(R\right)$
$P=\frac{V^2}{R}$
$\Rightarrow R=\frac{V^2}{P}$

Resistance of lamp of $60W,220V$ is $R_1$,

$R_1=\frac{{220}^2}{60}=806.67\Omega$

Resistance of $40W,220V$ lamp be $R_2$,

$R_2=\frac{{220}^2}{40}=1210\Omega$

These resistances are connected in parallel.

Equivalent resistance,

$R=\frac{R_1{R}_2}{R_1+R_2}$

$R=$$\frac{(806.67\times 1210)}{(806.67+1210)}=484\Omega$

Current drawn from $220V$,

$I=$$\frac{220}{484}$$=0.45A$