JEE Main 2023 — Current Electricity Question with Solution

Let $r$ be the internal resistance of each cell.

In above shown figure, first represents the series connection of cells. Second and third figures represents the parallel connection of cells.

Now, in case of series connection, ${\epsilon }_{\mathrm{net}}=2\epsilon \mathrm{and} r_{\mathrm{net}}=2r$. So, current $i=\frac{2\epsilon }{5+2r}$ .....(1)

In case of parallel connection, ${\epsilon }_{\mathrm{net}}=\frac{\epsilon r+\epsilon r}{r+r}=\epsilon \mathrm{and} r_{\mathrm{net}}=\frac{r}{2}$. So, current $i=\frac{\epsilon }{{\displaystyle \frac{r}{2}}+5}$ .....(2)

Equating (1) and (2), we get

$\frac{2\epsilon }{5+2r}=\frac{\epsilon }{\frac{r}{2}+5}$

$\Rightarrow r+10=5+2r$

$\Rightarrow r=5 \mathrm{\Omega }$.