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Cells and internal resistance
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The internal resistance of a 2.1 V cell which gives a current of 0.2 A through a resistance of 10 $\Omega$ is
$I = \frac{E}{r + R} \Rightarrow 0.2 = \frac{2.1}{r + 10}$
$r = 0.5\ \Omega$ -
A cell having an emf $\varepsilon$ and internal resistance r is connected across a variable external resistance R. As the resistance R is increased, the plot of potential difference V across R is given by
Current in the circuit: $I = \frac{\varepsilon}{R + r}$
$V = IR = \frac{\varepsilon R}{R + r} = \frac{\varepsilon}{1 + \frac{r}{R}}$
At R = 0, V = 0. As R increases V increases, but not linearly, and approaches $\varepsilon$ as R → ∞.
So the graph starts from the origin and rises as a curve that flattens towards the value $\varepsilon$.