The electromotive force of a primary cell is $2\, volts$. When it is short-circuited it gives a current of $4$ amperes. Its internal resistance in $ohms$ is
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(a) $E = V + ir$
After short-circuiting, $V = 0;$ $ \Rightarrow $ $r = \frac{E}{i} = \frac{2}{4} = 0.5\,\Omega $
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