Question

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

• A. 100W
• B. 40W
• C. 20W
• D. 10W

Answer 1

Answer: (D)

10W

Answer 2

$P = \frac{V^{2}}{R}$ $\Rightarrow \frac{P_{2}}{P_{1}} = \frac{V_{2}^{2}}{V_{1}^{2}}$ ($\because R$ is constant)

$\Rightarrow \frac{P_{2}}{P_{1}} = \left( \frac{100}{200} \right)^{2} = \frac{1}{4} \Rightarrow P_{2} = \frac{P_{1}}{4} = \frac{40}{4} = 10W$