Resistance of a wire practical

Investigate length and resistance while controlling temperature, diameter and contact positions.

Practise the calculations · Print a classroom set

Apparatus and measurements

Secure a uniform resistance wire beside a metre ruler. Use one fixed contact and one movable contact to define the test length. Measure current through the wire and potential difference across that exact length.

Low-voltage supply, switch, ammeter and variable resistor in series with the test component. Voltmeter in parallel across the measured component or wire section. Schematic connections.

Larger apparatus diagram

Method and controls

Choose wire length as the independent variable. Keep the material and diameter fixed. Connect the voltmeter across the measured section and the ammeter in series.
Record potential difference and current for each length and calculate R = V/I. Use low currents and switch off between readings to limit temperature changes.
Repeat readings and plot resistance vertically against length horizontally. For an ideal uniform wire at constant temperature, resistance is proportional to length. Contact resistance can produce an intercept.

Results to analyse

Original synthetic data for practice.

Resistance of a wire practical: sample readings
Length / m	Potential difference / V	Current / A
0.20	0.30	0.50
0.40	0.60	0.50
0.60	0.90	0.50
0.80	1.20	0.50

Questions

Calculate the four resistance values and the resistance–length gradient.
Predict resistance at 1.0 m if the same relationship continues.
Explain why changing wire diameter midway through the investigation spoils the length comparison.

Show worked answers

R values are 0.60, 1.20, 1.80 and 2.40 Ω. Gradient = (2.40 − 0.60)/(0.80 − 0.20) = 3.0 Ω m⁻¹.
The line through the origin gives R = 3.0 × 1.0 = 3.0 Ω. This prediction assumes uniform wire and unchanged temperature.
Resistance also depends on cross-sectional area. A changed diameter changes area, so the resistance change cannot be attributed to length alone.

Evaluation

Use the actual contact separation, not the full ruler length. Poor contacts can add resistance; voltmeter placement determines whether their potential differences are included.

Wire can become hot. Use a current-limited low-voltage supply. The A-level resistivity resource extends this work to diameter, cross-sectional area and resistivity.

Specification links

AQA 8463: Required practical 3; 4.2.1.3. OCR Gateway J249: P3.2; PAG P6 (selected). Selected skills only; no practical-completion assessment.

AQA practical requirements · OCR Gateway specification

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