Question

An electric bulb, marked $40W$ and $200V$, is used in a circuit of supply voltage $100V$. Now its power is

• A. 100 W
• B. 40 W
• C. 20 W
• D. 10 W

Answer 1

Answer: (D)

Power of bulb is given by, $P=\frac{V^2}{R}$
$\Rightarrow \frac{P_2}{P_1}=\frac{V_2^2}{V_1^2}(\because R$ is constant $)$
$\Rightarrow \frac{P_2}{P_1}={\left(\frac{100}{200}\right)}^2=\frac{1}{4}$
$\Rightarrow P_2=\frac{P_1}{4}=\frac{40}{4}=10W$