12.3 Resistance
Method sheet
AQA Physics
Investigating resistivity
Specification references







PS 1.1
PS 2.2, 2.3
PS 3.1, 3.2, 3.3
PS 4.2
3.5.1.1 Basics of electricity
3.5.1.3 Resistivity
This practical includes techniques and skills relevant to Required Practical 5
Learning objectives
After completing this practical, you should be able to:




measure the current in and potential difference across a length of wire to
determine its resistance
use a micrometer to measure the diameter of a wire and hence determine the
cross-sectional area of the wire
determine the resistivity of a material
compare the measured resistivity with the expected resistivity, and account for
any differences between these values.
Background
The resistance, R, of a wire (or any object with a uniform cross-sectional area) is:



proportional to the length of the wire, L
inversely proportional to the cross-sectional area, A
proportional to the resistivity of the material, ρ.
R
L
A
The resistivity is a measure of the difficulty for a current to pass through the
material. The equation above can be rearranged to give resistivity as the subject.

RA
L
   m2 
 .
This demonstrates that the unit for resistivity is Ω m 
 m 
In this experiment you will be testing the resistivity equation. You will be required to
design and use a simple circuit to measure the resistance of a constantan wire and
plot a graph of your results. After carrying out your readings you will plot a graph of
resistance against length for a piece of constantan wire. This will enable you to use
and interpret the graph to find the resistivity of constantan.
© Oxford University Press 2015
www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original
1
12.3 Resistance
Method sheet
AQA Physics
Safety



Check electrical equipment for loose wires and connections.
Do not allow the current to go higher than 1 A as this will cause the constantan
wire to overheat.
Do not use a wire shorter than 10 cm as this will also cause the wire to overheat.
Equipment and materials




a length of constantan wire
masking tape
a metre rule (attach the wire to the
metre rule with a short piece of
masking tape near each end)
a micrometer





a power supply or two 1.5 V cells
a voltmeter
a milliammeter
two 4 mm plugs (or crocodile clips) to
be used as contacts
electrical wires
Method
1
2
3
4
5
Draw a circuit diagram suitable for measuring the resistance of a wire,
incorporating the wire mounted on a ruler as shown in Figure 1. You will need
to measure the potential difference across the wire and the current through it.
Choose a sensible range of lengths of wire to carry out measurements on.
Do not use lengths shorter than 10 cm.
Record the length of wire and touch the wire with the edges of the 4 mm plugs
(or crocodile clips), take the readings of potential difference and current, and
then remove the plugs. Repeat the measurements twice more for the same
length of wire. Unplug the circuit and allow it to cool between readings.
Repeat step 3 for each of your chosen lengths of wire.
Use a micrometer to measure the diameter of the wire you have used. In several
places along the wire take two measurements at right angles to each other, and
then calculate the average cross-sectional area of the wire.
Figure 1 The wire is mounted along a ruler to allow for measurement of length
Results
Tabulate your results. Use the table below as a guide for the values you need to
record.
Length
/m
Potential
difference
/V
© Oxford University Press 2015
Current
/A
Resistance
/
Average resistance
(for each length) / 
www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original
2
12.3 Resistance
Method sheet
AQA Physics
Questions
1
2
3
4
5
6
Plot a graph of resistance against length, using the average value of resistance
for each length. Carefully draw a line of best fit through your points.
Calculate the gradient of your graph.
Use the gradient to calculate the resistivity of the wire.
When you measure the gradient from your line of best fit, you draw a triangle to
show the values you are reading from the graph. Suggest why it is important that
this triangle is as large as possible.
Pure constantan has a resistivity of 5.10  10–7 Ω m at room temperature.
Compare your value of resistivity with this value by calculating the percentage
difference between the two. By taking 10% as an acceptable error in your
experimental technique, comment on the accuracy of your result.
Discuss which of your measurements had the greatest effect on the accuracy of
your experiment.
© Oxford University Press 2015
(4 marks)
(2 marks)
(1 mark)
(1 mark)
(2 marks)
(2 marks)
www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original
3