The coil of an electric bulb takes 40 watts to start glowing. If more than 40 W is supplied, 60% of the extra power is converted into light and the remaining into heat. The bulb consumes 100 W at 220 V. Find the percentage drop in the light intensity at a point if the supply voltage changes from 220 V to 200 V.
Case-I
Given -
When the supply voltage is 220 V.
Power consumed by the bulb = 100 W
Excess power = 100 − 40 = 60 W
Hence, power converted to light = 60% of 60 W
= 36 W
Case-II
Given
When the supply voltage is 200 V.
Power consumed, 
Where
P is the power consumed by the bulb
V is the voltage
Putting the values in the above formula, we get
Excess power
= 82.64 − 40 = 42.64 W
Power converted to light = 60% of 42.64 W = 25.584 W
Percentage drop in the light intensity is-
