State Ohm's law. Describe an experiment to verify it with a circuit diagram and a V-I graph.

1. 1 · State the law. At constant temperature, the steady current through a conductor is directly proportional to the potential difference across its ends: V ∝ I, so V = IR.
2. 2 · List the apparatus. Battery, plug key, ammeter, rheostat, a resistor (wire), a voltmeter and connecting wires.
3. 3 · Draw the circuit. [Circuit: battery, key, ammeter, rheostat and resistor in series; voltmeter across the resistor]
4. 4 · Write the method.
   • Close the key. Note the ammeter (I) and voltmeter (V) readings.
   • Move the rheostat to change the current. Take 4 or 5 sets of readings.
   • Find V/I for each set: it stays the same. This constant is R.
5. 5 · Draw the graph. [V-I graph: I on the x-axis, V on the y-axis, a straight line through the origin]
6. 6 · Real-life link. Copper house wiring has low resistance; the nichrome coil of an iron box has high resistance.
7. 7 · Close. Since V/I is constant and the graph is a straight line, Ohm's law is verified.

Scoring tip: Keep the current small and open the key between readings, so the wire does not heat up and change its resistance.