Question

An electric bell has a rating of 500 W 100 V. It is used in a circuit having a 200 V supply. What resistance must be connected in series with the bulb so that it delivers 500 W?

• A. 10 ohm
• B. 20 ohm
• C. 30 ohm
• D. 40 ohm

Answer 1

The correct option is B 20 ohm

The current flowing in the bulb of 500 W operating at 100 V is
$I=\frac{500}{100}=5A$
resistance of bulb $=\frac{100}{5}=20$ ohm say $R_1$ to deliver 500 W the current in the bulb must remain 5 A when it is operated with 200 V supply the resistance R to be connected in series for this purpose is given by
$\frac{200}{R_1+R_2}=5$
$R_1+R_2=40$
$R_1+20=40$
R=20 ohm