The internal resistance of a $2.1\, V$ cell which gives a current of $0.2\, A$ through a resistance of $10\,\Omega$ is ............. $\Omega$
AIPMT 2013, Medium
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$I=\frac{\varepsilon}{R+r}$
or $I R+I r=\varepsilon$
Here, $R=10\, \Omega, \,\,r=?$,
$\varepsilon=2.1\, \mathrm{V},\,\, I=0.2\, \mathrm{A}$
$ \therefore 0.2 \times 10+0.2 \times r=2.1 $
$2+0.2\, r=2.1 $
$ 0.2 \,r=0.1$ or $r=\frac{1}{2}=0.5\,\Omega$
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