A cell of e.m.f. $E$ is connected with an external resistance $R$, then p.d. across cell is $V$. The internal resistance of cell will be
Medium
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(b) Let the current in the circuit $ = i = \frac{V}{R}$
Across the cell, $E = V + ir$ $ \Rightarrow $ $r = \frac{{E - V}}{i} = \frac{{E - V}}{{V/R}} = \left( {\frac{{E - V}}{V}} \right)\,R$
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