Resistors: explain the design and use of a circuit to measure the resistance of a component by measuring the current through, and potential difference across, the component

The idea

To find a component's resistance you need its current and its potential difference at the same time, then apply R = V ÷ I. Connect the component in series with a cell, switch and ammeter so the same current flows through all of them; connect a voltmeter in parallel across the component only. The ammeter measures current in amperes, the voltmeter measures potential difference in volts. Close the switch, record both readings, then vary the supply or add a variable resistor and repeat to obtain several pairs. For each pair calculate R = V ÷ I; a roughly constant value indicates ohmic behaviour at constant temperature. This method suits a fixed resistor, filament lamp or diode, and the design must ensure the meters do not alter the very quantities being measured.

What earns the marks

• Component connected in series with the cell, switch and ammeter so the ammeter measures the current through the component.
• Voltmeter connected in parallel across the component so it measures the potential difference across that component alone.
• Correct use of R = V ÷ I with the measured current and potential difference to calculate resistance in ohms.
• Repeat readings taken, for example by adjusting a variable resistor or changing the number of cells, to check consistency and reduce the effect of random error.
• Switch included so the circuit can be broken between readings, limiting heating of the component and unnecessary cell discharge.
• Recognition that the ammeter must be in series and the voltmeter in parallel, not the reverse, for valid measurements.

Common mistakes and how to put them right

• Placing the voltmeter in series with the component.

  Put it right by connecting the voltmeter in parallel across the component.

• Placing the ammeter in parallel with the component.

  Put it right by connecting the ammeter in series in the main circuit.

• Using R = I ÷ V instead of R = V ÷ I.

  Put it right by dividing the potential difference in volts by the current in amperes.

Exam tips

• Sketch the circuit first, labelling the ammeter in the main series loop and the voltmeter across the component, then describe each reading taken.
• State the equation R = V ÷ I and show one substitution with units before quoting the final resistance in ohms.
• Explain why readings are repeated, linking repeats to spotting anomalous values and improving confidence in the result rather than claiming they remove all error.