An electric bell has a rating of 500 W 100 V. It is used in a circuit having a

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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?

Options:

A . 10 ohm

B . 20 ohm

C . 30 ohm

D . 40 ohm

Answer: Option B
:
B

The current flowing in the bulb of 500 W operating at 100 V is
$I = \frac{500}{100} = 5 A$
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

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