Question

The coil of an electric bulb takes 40watts to start glowing. If more than 40W is supplied, 60% of the extra power is converted into light and the remaining into heat. The bulb consumes 100W at 220V. Find the percentage drop in the light intensity at a point if the supply voltage changes from 220V to 200V.

Answer 1

P = 100w V = 220v

Case I : Excess power = 100 – 40 = 60w

Power converted to light = (60 x 60)/100 = 36w

Case II : Power = (220)²/484 = 82.64w

Excess power = 82.64 – 40 = 42.64w

Power converted to light = 42.64 x (60/100) = 25.584w

ΔP = 36 – 25.584 = 10.416

Required % = (10.416)/36 x 100 = 28.93 ≈ 29%