A cell of e.m.f. 1.5 V and internal resistance 0.5 Ω is connected to an external resistor. If the terminal voltage of the cell is 1.2 V, what is the value of the external resistance?

The voltage drop across the internal resistance = e.m.f. − terminal voltage = 1.5 − 1.2 = 0.3 V. Current in circuit $I = \frac{0.3}{0.5} = 0.6\,\text{A}$. External resistance $R = \frac{V_{terminal}}{I} = \frac{1.2}{0.6} = 2.0\,\Omega$. A common distractor error is using the e.m.f. directly instead of the terminal voltage to find R.