Question

A 120-V house circuit has the following light bulbs turned on: 40,60 and 75 W. Find the equivalent resistance of these lights.

Answer 1

$U=U_1=U_2=U_3,$

$P=\frac{U^2}{R},$

$P=P_1+P_2+P_3,$

$R_1=\frac{U^2}{P_1}=360~\Omega,$

$R_2=\frac{U^2}{P_2}=240~\Omega,$

$R_3=\frac{U^2}{P_3}=192~\Omega.$