Question

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
a. 90 W
b. 81 W
c. 81 to90w
D. 90-100w​

Answer 1

Answer:

Option b. 81W

Explanation:

$P = {V}^{2}/ R \\ = > R= {V}^{2}/P$

= 220*220/100

=484ohm

Now when the voltage drops by 10℅, the actual voltage is

= 90/100 * 220

= 198 V

So Power = 198*198/484 = 81W