A cell with e.m.f. $E$ and internal resistance $r$ is connected in series with a switch $S$ and an external resistor having resistance $R$. The potential difference between $P$ and $Q$ is $V$. After $S$ is closed,
- A$V$ decreases because there is a p.d. across $R$.
- B$V$ decreases because there is a p.d. across $r$.
- C$V$ remains the same because the decrease of p.d. across $r$ is balanced by the increase of p.d. across $R$.
- D$V$ remains the same because the sum of the p.d.s across $r$ and $R$ is still equal to $E$.