Define emf and internal resistance of a cell. Obtain the relation V = ε − Ir for its terminal voltage, and draw the graph of V against I.

1. 1 · Define. EMF ε: the PD between the terminals when no current flows. Internal resistance r: the resistance of the electrolyte inside the cell.
2. 2 · Set up the circuit. Cell (ε, r) joined to an external R; current I = ε/(R + r).
3. 3 · Find the drop. Inside the cell the potential falls by Ir, so V = IR = ε − Ir.
4. 4 · Draw. [Circuit: cell with r drawn as a small resistor beside it, external R, voltmeter across the terminals; and a falling straight-line V-I graph with ε and ε/r marked]
5. 5 · Real life. Headlights dim when a vehicle starts: a big current means a big Ir drop.
6. 6 · Close in one line. So, V is less than ε while the cell gives current, and equals ε only on open circuit.

Scoring tip: Draw r as a small resistor next to the cell symbol, inside a dashed box, so the examiner sees where the Ir drop happens.