Required Practical: Resistance

This lesson covers the Required Practical on investigating how the length of a wire affects its resistance, as specified by AQA GCSE Physics (4.2.1). This is Required Practical Activity 15 and is one of the most frequently examined practicals in the electricity topic.

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Aim

To investigate how the length of a wire affects its resistance.

The hypothesis is that as the length of a wire increases, the resistance increases proportionally. This is because a longer wire provides more opportunities for the charge carriers (electrons) to collide with the metal ions in the lattice.

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Equipment List

Equipment	Purpose
Constantan wire (e.g., SWG 26 or 28)	The wire being investigated
Metre ruler	To measure the length of the wire
Crocodile clips (x2)	To make electrical contact at each end of the measured length
Ammeter	To measure the current through the wire
Voltmeter	To measure the p.d. across the wire
Power supply (low voltage, e.g., 1–2 V)	To provide the potential difference
Switch	To control the circuit and prevent overheating
Connecting leads	To connect the circuit
Tape or clamps	To hold the wire straight and secure against the ruler

Exam Tip: Constantan wire is used rather than copper wire because constantan has a high resistivity (giving measurable resistance even at short lengths) and its resistance does NOT change significantly with temperature. This is critical for obtaining valid results.

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Variables

Variable Type	Variable	Details
Independent	Length of the wire	Varied from 10 cm to 100 cm in 10 cm intervals
Dependent	Resistance of the wire	Calculated from V and I readings using R = V/I
Control variables	Type and thickness of wire	Use the same piece of constantan wire throughout
	Temperature of wire	Use a low current and switch off between readings
	Material of wire	Same material (constantan) for all measurements

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Method

1. Set up the circuit with the power supply, switch, ammeter, and the constantan wire connected in series. Connect the voltmeter in parallel across the section of wire being tested.

2. Attach the constantan wire along the metre ruler using tape, keeping it straight and taut.

3. Position one crocodile clip at the 0 cm mark on the ruler.

4. Position the second crocodile clip at the 10 cm mark, so the length of wire in the circuit is 10 cm.

5. Close the switch and record the ammeter reading (current, I) and the voltmeter reading (potential difference, V).

6. Open the switch immediately after taking readings to prevent the wire heating up (which would change its resistance).

7. Calculate the resistance using R = V / I.

8. Repeat the experiment for lengths of 20 cm, 30 cm, 40 cm, 50 cm, 60 cm, 70 cm, 80 cm, 90 cm and 100 cm.

9. Repeat each measurement at least three times and calculate a mean resistance for each length.

10. Plot a graph of resistance (y-axis) against length (x-axis).

graph LR
    A[Power Supply] --> B[Switch]
    B --> C[Ammeter]
    C --> D["Constantan Wire (variable length)"]
    D --> A
    E[Voltmeter] -.-> |parallel across wire| D

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Safety Precautions

Hazard	Risk	Precaution
Hot wire	The wire can become hot if high currents flow, causing burns	Use a low voltage (1–2 V); switch off between readings
Sharp wire ends	Cuts from wire edges	Handle wire carefully; use crocodile clips to avoid touching bare wire
Electrical connections	Loose connections can cause sparking	Ensure all connections are secure before switching on

Exam Tip: In any required practical question, you MUST be able to describe at least one safety precaution relevant to the experiment. For this practical, always mention keeping the current low and switching off between readings to prevent overheating.

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Expected Results

If the wire obeys Ohm's Law (which constantan wire does at constant temperature), you would expect:

Length (cm)	Resistance (approximate, in ohms)
10	0.5
20	1.0
30	1.5
40	2.0
50	2.5
60	3.0
70	3.5
80	4.0
90	4.5
100	5.0

These values are approximate and depend on the thickness and type of wire used, but the pattern should show a directly proportional relationship.

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Analysing the Results

Graph

Plot resistance (y-axis) against length (x-axis).

For a wire of uniform cross-section at constant temperature:

• The graph should be a straight line through the origin.
• This confirms that resistance is directly proportional to length.
• The gradient of the line relates to the resistivity of the material and the cross-sectional area.

Calculating Resistance from Readings

For each length, use:

R = V / I

Then calculate the mean of the repeated readings:

Mean R = (R1 + R2 + R3) / 3

Identifying Anomalous Results

• Look for results that do not fit the pattern (anomalies).
• Common causes of anomalies: poor contact between crocodile clips and wire, wire not straight, temperature increase during measurement.
• Anomalous results should be identified and excluded from the mean calculation (but NOT deleted from the table — circle them and note they are anomalous).

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Sources of Error and Improvements

Source of Error	How It Affects Results	Improvement
Wire heating up	Resistance increases with temperature, giving readings that are too high	Use low voltage; switch off between readings; allow cooling time
Poor crocodile clip contact	May add extra resistance or give inconsistent readings	Clean the wire with emery paper; ensure firm clip contact
Measuring length inaccurately	Incorrect length values lead to inaccurate resistance calculations	Use a metre ruler; align wire carefully; measure from clip centre to clip centre
Parallax error when reading ruler	Incorrect length readings	Read the ruler at eye level, perpendicular to the scale
Wire not straight	Effective length differs from measured length	Tape wire to ruler; keep it taut

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How This Links to Theory

The resistance of a wire is given by: