Interactive practical

Factors affecting resistance

Slide a crocodile clip along a resistance wire to find how resistance depends on length, then compare identical resistors in series and in parallel.

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GCSE Physics

Investigating factors affecting resistance

AQA 8463 — matched to your course.

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Available question marks4

Practical focus

Investigate how wire length and resistor arrangement affect electrical resistance.

Practical method

Aim

Investigate how the length of a wire affects its resistance (and compare wires of different thickness).

Variables

Independent variable: length of wire between the crocodile clips
Dependent variable: resistance, calculated from R = \(\dfrac{V}{I}\)
Control variables: material and diameter of the wire; temperature of the wire (low current, switch off between readings); the same meters.

Apparatus

Constantan wire taped along a metre rule
Crocodile clips
Ammeter
Voltmeter
Low-voltage power supply (or cell)
Switch
Connecting leads
Micrometer for diameter

Method

Tape the wire along a metre rule. Connect it in series with an ammeter, a switch and the power supply, with crocodile clips attached to the wire.
Connect the voltmeter in parallel across the length of wire between the clips.
Set the clips 20 cm apart, close the switch, record I and V and then open the switch.
Repeat for 40, 60, 80 and 100 cm, repeating each length three times.
Calculate R = \(\dfrac{V}{I}\) for each length and plot resistance against length.

Sample results

Constantan wire, current kept at about 0.50 A.

Length / cm	Current / A	Potential difference / V	Resistance / Ω
20	0.50	0.42	0.84
40	0.50	0.84	1.68
60	0.50	1.26	2.52
80	0.50	1.68	3.36
100	0.50	2.10	4.20

Analysis

Resistance is directly proportional to length: the graph is a straight line through the origin with gradient 0.042 Ω/cm (4.2 Ω/m). Doubling the length doubles the resistance. A thicker wire of the same material has a lower resistance for the same length.

Errors and improvements

Problem	Effect	Improvement
Wire heats up	Resistance increases during the experiment	Use a small current and switch off between readings
Contact resistance at the clips	Line does not pass through the origin	Clean the wire and press clips firmly; check the zero error
Clip position read at an angle	Length wrong	Read the clip position at eye level

Risk assessment

Hazard	Risk	Control measure
Wire becomes hot	Burns	Keep current low; do not touch the wire while current flows
Short circuit if clips touch	Overheating	Check the circuit before switching on

Common mistakes

Connecting the voltmeter in series.
Leaving the current on so the wire heats.
Measuring length from the end of the rule instead of between the clips.

Worked analysis

At 60 cm the readings are 0.50 A and 1.26 V. R = \(\dfrac{V}{I}\) = 1.26 ÷ 0.50 = 2.52 Ω.

Common misconception

A longer wire has a greater resistance, not a smaller one: electrons collide with more ions along a longer path.

Variations required by some exam boards

Resistor combinations: measure the total resistance of two known resistors first in series and then in parallel, using an ammeter and voltmeter (or an ohmmeter with the circuit switched off). In series the total is the sum; in parallel it is less than the smallest resistor. For example, 10 Ω and 10 Ω give 20 Ω in series and 5 Ω in parallel.

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2 questions available, worth 4 marks in total.

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