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EMF, internal resistance, cells, and electric power sheet
3-mark answer frame

Distinguish between the EMF and the terminal voltage of a cell. Derive the relation between them, and explain why they differ.

How to lay out the answer. Follow the frame, then write it in your own words.

  1. 1 · Define EMF. The potential difference between the terminals of a cell when no current is drawn from it.
  2. 2 · Define terminal voltage. The potential difference between the terminals when the cell gives a current I.
  3. 3 · Draw the circuit. [Cell E with its internal resistance r drawn beside it; a key K and a resistor R in the loop; a voltmeter across the cell]
  4. 4 · Derive the relation. E = IR + Ir and V = IR, so E = V + Ir, or V = E − Ir.
  5. 5 · Explain the difference. Part of the EMF is used up in driving the current through the cell's internal resistance, so V is less than E whenever current flows.
  6. 6 · Close in one line. On open circuit I = 0 and V = E; the more current is drawn, the lower V becomes.

Scoring tip: Use a high-resistance voltmeter in your explanation: it draws almost no current, so on open circuit it reads the EMF.