When the resistance of $9 \,\Omega$ is connected at the ends of a battery, its potential difference decreases from $40\, volt$ to $30\, volt$. The internal resistance of the battery is ............... $\Omega$
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The internal resistance of battery is given by
$r = \left( {\frac{E}{V} - 1} \right)R = \left( {\frac{{40}}{{30}} - 1} \right) \times 9$$ = \frac{{9 \times 10}}{{30}} = 3\,\Omega $
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