Question

What is the resistance, under normal working conditions, of an electric lamp rated '240 V, 60 W' ? If two such lamps are connected in series across a 240 V mains supply, explain why each one appears less bright.

Answer 1

Given,
Voltage,v=240 v
Power,p=60 w
As $p=\frac{v^2}{R}$
$\therefore R=\frac{(240{)}^2}{60}=960\Omega$
$I=\frac{P}{V}=0.25A$
When one lamp is connected across the mains,it draws 0.25 A current,while if two lamps are connected in series across the mains,current through each bulb becomes $\frac{240v}{(960+960)\Omega }=0.125A$
(i.e.,current is halved),hence heating $(=I^{2}Rt)$ in each bulb becomes one-fourth, so each bulb appears less bright.