Question

What is the resistance, under normal working conditions of a 240 V electric lamp rated at 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 = `V^2/R` 

∴ R = `(240)^2/60`  = 960 Ω 

 I = `P/V` = 0.25  A

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

`(240V)/((960 + 960))Ω`  = 0.125 A

(i.e., current is halved), hence heating (= 2 I Rt ) in each bulb becomes one-fourth, so each bulb appears less bright.