Two identical cells, when connected either in parallel or in series gives same current in an external resistance $5\,\Omega$. The internal resistance of each cell will be $.............\,\Omega$.
JEE MAIN 2023, Medium
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$i=\frac{2 \varepsilon}{5+2 r}........(1)$
$i=\frac{\varepsilon}{\frac{r}{2}+5}.........(2)$
Equating $(1)$ and $(2)$
$\frac{2 \varepsilon}{5+2 r }=\frac{\varepsilon}{\frac{ r }{2}+5} \Rightarrow r +10=5+2 r$
$r=5$
Ans.$5$
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