A primary cell has an $e.m.f.$ of $1.5\,volts$, when short-circuited it gives a current of $3$ amperes. The internal resistance of the cell is .............. $ohm$
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(c) Short circuit current ${i_{SC}} = \frac{E}{r}$ $ \Rightarrow $ $3 = \frac{{1.5}}{r}$ $ \Rightarrow $ $r = 0.5\,\Omega $
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