$n$ identical cells are joined in series with two cells $A$ and $B$ with reversed polarities. $emf$ of each cell is $E$ and internal resistance is $r$. Potential difference across cell $A$ and $B$ is : $(n > 4)$
Medium
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Net $\mathrm{emf}$ of the circuit $= (n -2) E -2E = (n -4)E$ total resistance of the circuit $= nr$
$\therefore $ Current in the circuit will be,
$\mathrm{i}=\frac{(\mathrm{n}-4) \mathrm{E}}{\mathrm{nr}}$
Hence, potential difference across $\mathrm{A}$ or $\mathrm{B}$ is,
${\mathrm{V}=\mathrm{E}+\mathrm{Ir}=\mathrm{E}+\frac{(\mathrm{n}-4) \mathrm{E}}{\mathrm{nr}} \cdot \mathrm{r}} $
${=2 \mathrm{E}\left(1-\frac{2}{\mathrm{n}}\right)}$
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