A circuit consists of a resistance R connected to n similar cells. If the current in the circuit is the same whether the cells are connected in series or in parallel then the internal resistance r of each cell is given by
Answer & explanation
Correct answer: option 3
$ \text{ If n cells are connected in series then } I = \frac{nE}{R + nr}$
$ \text{If n cells are connected in parallel } I'= \frac{E}{R + r/n}$
$ I = I' $
$ \frac{nE}{R + nr} = \frac{E}{R + r/n}$
$ nR + r = R + nr $
$ R = r$