Question

The resistance of the filament of a bulb increases with rise in temperature. a bulb rated 100 W, 220 V is connected across a 220 V supply. If the voltage drops $10\%$, then the power consumed by bulb will be

• A. 100 W
• B. 81 W
• C. less than 81 W
• D. more than 81 W

Answer 1

Answer: (D)

more than 81 W

For a bulb rated 100 W, 220 V, resistance is

$R=\frac{V^2}{P}$

$R=\frac{{220}^2}{100}$

$R=484\Omega$

Now, the voltage drops 10%,

$Q=\frac{{(220-220\left(\frac{10}{100}\right))}^2}{484}$

$Q=\frac{{198}^2}{484}=81W$

Since, resistance of the filament of a bulb increases with rise in temperature, the power consumed by bulb will be more than $81W$.