The coil of an electric bulb takes 40watts to start glowing. If more than 40W is supplied,
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.
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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%
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