Question

What is the resistance, under normal working conditions, of an electric lamp rated at '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 = `"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

`(240  "V")/((960 + 960)  Omega) = 0.125  "A"`

(i.e., current is halved), hence heating (= I²Rt) in each bulb becomes one-fourth, so each bulb appears less bright.