An electric bulb rated for $500\, watts$ at $100\, volts$ is used in a circuit having a $200-volt$ supply. The resistance $R$ that must be put in series with the bulb, so that the bulb draws $500\, W$ is .................. $\Omega$
Medium
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Rated current through the circuit $i = \frac{{500}}{{100}} = 5\,A$
Potential difference across $R$, $100 = 5 \times R\, \Rightarrow \,R = 20\,\Omega $
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