Question

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__ $\Omega$.

Answer 1

$i =\frac{2 \varepsilon}{5+2 r }\ldots$(1)
$i=\frac{\varepsilon}{\frac{r}{2}+5} \ldots$ (2)
Equating (1) and (2)
$\frac{2 \varepsilon}{5+2 r }=\frac{\varepsilon}{\frac{ r }{2}+5}$
$\Rightarrow r +10=5+2 r$
$r =5$