• In a closed circuit a current flows through a conductor when a potential difference is applied across it, and the conductor’s resistance opposes the flow.
• How are the three quantities related?
• Activity verifying Ohm’s law.

• Apparatus battery, fixed resistor, rheostat, switch, ammeter, voltmeter, wires.
• Connect the ammeter in series with the resistor and rheostat, and the voltmeter across the fixed resistor.
• Steps:

1. Set the rheostat at its maximum value so that the current is least (this protects the components).
2. Close the circuit and record the voltmeter reading V and the ammeter reading I.
3. Reduce the rheostat resistance; record the new V and I.
4. Repeat several times and tabulate the readings.
5. Plot a graph of V (horizontal axis) against I (vertical axis).

I = V / R, or V = I R, or R = V / I

• The resistance can be found from the graph R = ΔV / ΔI, which is the reciprocal of the slope of an I–V graph (where I is plotted on the vertical axis, slope = I/V = 1/R, so R = 1/slope).
• Scientist profile.
• Georg Simon Ohm (1789–1854), a German physicist who found the law named after him and the relationship between current, voltage and resistance; the unit of resistance is named in his honour.

Worked example 1

• Find the current in a metal wire of resistance 2 Ω when the potential difference across it is 6 V.
• I = V/R = 6 / 2 = 3 A.

Worked example 2: from a graph

• An I–V graph for a fixed resistor is a straight line through the origin passing through the point (V = 2.0 V, I = 0.40 A).
• Find R.
• R = V/I = 2.0 / 0.40 = 5 Ω. Check with another point: at 1.0 V the current should be 0.20 A.

Worked example 3

• What potential difference is needed to drive 0.25 A through a 48 Ω resistor?
• V = I R = 0.25 × 48 = 12 V.
• What happens if the potential difference doubles?
• The resistance of the conductor does not change (it depends on length, area, material and temperature); the current doubles.