Question

Three 60 W, 120 V light bulbs are connected across a 120 V power line as shown in Fig. Find the voltage across each bulb.

Answer 1

1. As bulbs B and C are in parallel, voltage across B and C will be the same, i.e., $V_B=V_C$.
Further, if R is the resistance of each bulb (as bulbs are identical), the resistance of bulbs B and C together $(=R/2)$ is in series with resistance R of bulb A and as in series, potential divides in proportion to resistance.

$V_A=\frac{R}{R-0.5R}=\frac{2}{3}{V}_s=\frac{2}{3}\times 120=80V$

$V_B=V_C=\frac{0.5R}{R+0.5R},V=\frac{1}{3}{V}_S=\frac{1}{3}\times 120=40V$

$[or{V}_B=V_C=V-V_A=120-80=40V]$