Topic 11.3
Deformation of solids
The Young modulus
Aims
In this experiment you will use a graph to calculate the Young modulus of copper. You will
analyse data from your experiment and make suitable calculations in order to plot a graph of
stress against strain.
Safety
 The wire should be firmly clamped between the blocks.
 The floor may need protection from falling masses and you should keep your feet clear.
 Wear eye protection because eyes must be protected from the wire when it snaps.
Equipment, materials and method
1 For this experiment you will need a length of copper wire, two G-clamps and two wooden
or plastic blocks, a metre ruler, a micrometer, masses and mass hangers, a pulley and
some tape to mark the wire.
2 Set up the apparatus as shown in the diagram below. The wire should pass between the
two blocks and be clamped tightly to the bench using the G-clamp. The pulley should be
fixed to the end of the bench.
3 Fix the marker a distance of at least 2.00 m from the clamped end. Initially add the mass
hanger to tension the wire. This can be done by folding the wire 15 cm from the end and
twisting it tightly, leaving a loop large enough for two or three mass hangers. Consider the
original length of the wire to be from the clamped end to the tape marker. The extension
will be measured using the tape marker.
4 Prepare a table with columns for length, extension, force (weight) and cross-sectional
area.
5 Measure and tabulate the diameter of the wire, taking two perpendicular measurements at
several places. Hence calculate the average cross-sectional area of the wire.
6 Add a 100g mass to the mass hanger and take measurements of the extension and
diameter. Repeat until the wire breaks. If the wire stretches (creeps), wait for it to stop
before taking measurements.
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Topic 11.3
Deformation of solids
7 Estimate the uncertainty in the measurement of the extension, the diameter of the wire and
its length.
Questions
1 What are the units of the Young modulus?
2 Write down an equation for the extension ΔL of the wire in terms of the load W, the area
of cross section of the wire A, the wire’s length L and the Young modulus E of the wire
material.
Results
Tabulate your results and then plot a graph of stress against strain. Measure the gradient of
your graph in order to determine the Young modulus of the wire.
Discussion
1 What is the greatest source of error in this experiment?
2 Estimate the percentage uncertainty in your result.
Teacher Notes
Aims
 To measure the Young modulus for copper.
 To implement relevant safety precautions when stretching a copper wire.
 To plot a stress–strain graph and use it to find the Young modulus.
Safety
 The wire should be firmly clamped between the blocks and to the bench using the Gclamp.
 The floor may need protection from falling masses, so students should keep their feet
clear.
 Eyes must be protected at all times with safety glasses for when the wire snaps.
 CLEAPSS reference: laboratory handbook/CD-ROM section 12.18.
1. This experiment should take 20–25 minutes.
2. A large number of masses may be required before the wire breaks.
3. As the wire will stretch a long way, ensure that there is sufficient space below the masses.
4. If the wire stretches (creeps), students must wait for it to stop before taking measurements.
5. Diameters should be determined by taking two perpendicular measurements at several
places along the wire.
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Topic 11.3
Deformation of solids
6. Ensure that the triangle used to measure the gradient of the graph is as large as possible in
order to minimise percentage error.
7. The cross-sectional area of the wire will decrease as the wire stretches. Ideally it should be
measured at stages during the experiment, but it is unlikely to affect the outcome.
Answers to questions
1 the Young modulus =
stress
strain
F
units N m−2
A
extension
strain =
dimensionless
length
The units of the Young modulus are therefore N m−2.
stress =
2 ΔL =
WL
AE
Discussion
3 The greatest source of error is probably determining the position of the marker tape used
to measure the length/extension. This error will be more significant when the distances are
smaller.
4 The percentage uncertainty of the length is probably significantly greater than the other
uncertainties. If so, it can be used as the percentage uncertainty in the result. If not, the
percentage uncertainties in the individual quantities measured need to be added. Note that
the percentage uncertainty for the diameter must be added in twice because the area of
cross section is proportional to the square of the diameter.
Practical assessment opportunities
5 PSA – Selecting and using equipment: 0–3 marks
(Following instructions and group work; organisation and safety assessed at end of
course.)
6 ISA preparation:
practical task – practice in recording measurements, accurate measurement of the diameter
of a wire and its extension, estimating errors and plotting a graph
written task Section A and B – opportunities to analyse data and errors and to plot a graph
and relate an equation to its gradient to find the Young modulus of the wire. Also, to draw
conclusions and to evaluate the strength of the conclusion
Equipment and materials required
Each student or pair of students will require the following equipment:
 approximately 2.5 m of copper wire (SWG 32) (One end must be firmly clamped between
two plastic or wooden blocks. It is better to make an S shape between the blocks. Then it
is harder for the wire to be pulled through. The other should be twisted into a strong loop
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Topic 11.3
Deformation of solids
from which the masses will be hung. Ensure that initially this loop is a sufficient height
above the floor.)
 some tape to mark the wire (masking tape)
 two G-clamps
 two wooden or plastic blocks
 a metre ruler
 a micrometer
 a pulley that can be fixed firmly to the edge of the bench
 approximately 20 × 100 g masses and 2 mass hangers
 graph paper
The G-clamp holds the blocks firmly together and clamps them to the bench
Safety
 The wire should be firmly clamped between the blocks.
 The floor may need protection from falling masses.
 Students should keep their feet clear.
 Eyes must be protected at all times with safety glasses in case the wire snaps.
 CLEAPSS reference: Laboratory Handbook/CD-ROM section 12.18.
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