The resistance of the filament of a lamp increases with the increase in temperature. A lamp rated `100 W` and `220 V` is connected across `220 V` power supply. If the voltage drops by `10%`, then the power of the lamp will be

Options

A. 90 W
B. 81 W
C. Between 90W and 100 W
D. Between 81 W and 90 W

Answer

D

Explanation

Initially `P = V^(2)/R rArr 100 = (220)^(2)/R`
When voltage drops,
`P = (220 xx 0.9)^(2)/R^(.) = 81 W` Here `R. lt R`
`therefore P gt (220 xx 0.9)^(2)//R = 81 W`