Current, potential difference or resistance can be calculated using the…

The idea

The defining equation of this topic is potential difference = current × resistance, V = I R, with V in volts, I in amperes and R in ohms, Ω; it rearranges to R = V ÷ I and I = V ÷ R. The required practical puts it to work. Build a circuit with the test wire, a battery and an ammeter in series and a voltmeter across the wire, include a variable resistor to change the readings, and calculate R = V ÷ I for each length of wire. Resistance is proportional to length, provided the wire does not warm up, so keep the current low and switch off between readings. Then repeat for resistors in series, where resistance adds, and in parallel, where the total is less than the smallest.

What earns the marks

• The battery in series with the component and the ammeter.
• The voltmeter connected in parallel with the component.
• Including a variable resistor, variable power supply or potentiometer.
• Calculating resistance as potential difference divided by current with the unit ohms.
• Keeping the temperature of the wire constant, for example by switching off between readings.

Common mistakes and how to put them right

• Leaving the circuit switched on so the wire heats up, which raises the resistance as the length increases.

• Measuring the length of wire from the end rather than between the crocodile clips.

• Adding resistances in parallel as though they combine like series resistors.

• Recording the ammeter reading in milliamperes but using it as though it were amperes.

Exam tips

• In a six-mark method, state the circuit, the range of lengths, the readings taken and the equation used.
• Say that repeat readings are taken at each length and a mean resistance calculated.
• Check any parallel answer against the rule that the total must be smaller than the smallest single resistance.