Question

An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be –

• A. 100 W
• B. 75 W
• C. 50 W
• D. 25 W

Answer 1

Answer: (D)

25 W

Answer 2

Energy consumed by bulb $𝑃=\frac {𝑉^2}{R}$
⟹$𝑅 = \frac {𝑉^2}{P}$
Here, $V = 220 \ V$ and $P = 100\ W$
$𝑅 = \frac {(220)^2}{100}$
$R = 484\ Ω$
The resistance of the bulb remains constant if the supply voltage is reduced to $110 V$. If the bulb is operated on $110 V$, then the energy consumed by it is given by the expression for power
$𝑃 = \frac {𝑉^2}{R}$

$P= \frac {(110)^2}{484}$

$P= \frac {12100}{484}$

$P= 25 \ 𝑊$

Hence, the correct option is (D): $25\ W$