Question

An electric bulb rated for $500 \, W$ at $100 \, V$ is used in a circuit having a $200 \, V$ supply. The resistance $R$ that must be put in series with the bulb, so that the bulb is drawn $500 \, W$ is

• A. $18 \, $ $\Omega$
• B. $20 \, $ $\Omega$
• C. $40 \, $ $\Omega$
• D. $700$ $\Omega$

Answer 1

Answer: (B)

$P=VI, \, I=\frac{P}{V}$
or $ \, I=\frac{500 \, W}{100 \, V}=5 \, A$
Now, $ \, 5R=100$
or $ \, \, R=20 \, \Omega$ .