State Ohm's law. With a circuit diagram, describe an experiment to show that the current through a resistor is proportional to the potential difference across it.

1. 1 · State the law. At constant temperature, the current through a conductor is directly proportional to the potential difference across it: V = Ri.
2. 2 · Draw the circuit. [Sketch: cells, plug key, ammeter and resistor in series; voltmeter across the resistor]
3. 3 · Take readings. Use one, two and then three cells. Record the current i and the p.d. V each time.
4. 4 · Work out V/i. V/i comes out nearly the same every time. This constant is the resistance R.
5. 5 · Plot the graph. [Sketch: straight line through the origin, V on one axis and i on the other]
6. 6 · Conclude. The graph is a straight line through the origin, so i ∝ V and the resistor obeys Ohm's law.

Scoring tip: Write 'provided the temperature stays the same' in the law. Without it the statement is incomplete.