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YOUNG’S MODULUS PRACTICAL

OBJECTIVE

• To determine the Young’s Modulus of a steel wire using a graphical method

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VIDEOS

• https://youtu.be/YdqvGGFIbfc• https://www.youtube.com/watch?v=Hywk0NHlCoc• https://www.youtube.com/watch?v=5v8K5r61Yuo

GETTING ORGANIZED

What needs to be measured?

Cross sectional area (diameter)Force appliedExtensionOriginal Length

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GETTING ORGANIZED

• Original length – what does this correspond to for a particular experimental set up?

• Cross-sectional area: Is the sample uniform? If sample gets longer, won’t it get thinner? Won’t it be quite small?

• Are we using engineering stress or true stress?• Should the sample be arranged vertically or

horizontally?

VERNIER CALLIPER

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• Above, we see the zero on the vernier is just beyond the 22 mm mark, so the reading must be `twenty two point something'. The line of the vernier that exactly matches a graduation on the main scale is the 6, so the vernier must be 0.6 mm beyond the 22 mm mark. Hence the reading is 22.6 mm.

• Initially you should close the jaws right up in order to note any zero error that needs to be accounted for. If it reads greater than 0 at this stage, then the reading needs to be subtracted from the final one and vice versa

MICROMETER

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• Initially you should close micrometer in order to note any zero error that needs to be accounted for. If it reads greater than 0 at this stage, then the reading needs to be subtracted from the final one and vice versa

POSSIBLE SET UPS

There are many ways one could set up this experiment, think about which of the following might be the most accurate, relevant, manageable, and eliminate the most uncertainty

Consider what we are trying to measure, and what type of stress we are applying

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YOUR SET UP

The set up you will be using will look similar to the following, but with two wires (one control and one test) connected by a vertical Vernier scale

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DETAILED METHOD1. Set up clamps, pulleys and wires as shown in the diagram.2. Ensure safety measures are in place below masses and

along wires. Put goggles on. 3. Add 100g mass to each wire to ensure tautness and no

kinks.4. Measure initial length of wires5. When both wires are taut, “zero” the Vernier scale.6. Add masses in 100g increments, making note of the

extension at each increase in mass.

UNCERTAINTY• To minimise errors the control wire is the same length, diameter and

material as the test wire. This means that errors due to expansion during the experiment are avoided as the test wire and control wire would both expand by the same amount and the scale would adjust position and eliminate the error.

• The wire must have no kinks in it otherwise there will be big extensions due to the wire straightening out rather than just stretching.

• Care must be taken that the limit of proportionality is not exceeded. This can be checked by removing the load after each addition of the weight. If the limit has not been exceeded the wire should return to the length it was before the weight was added.

• The wire is as long as possible (usually about 2m long) and it is as thin as possible so that as big an extension as possible can be recorded. (A typical extension for a 5N loading will be 1mm).

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SAFETY – HOW CAN THE FOLLOWING RISKS BE DEALT WITH?

• Heavy weights • The wire is very thin and taut. • The wire might snap. If it does it might whip into your

face and eyes.

OTHER NOTES

• Measure the wire diameter in 5 places and take the average

• Be aware of significant figures

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BEFORE THE PRACTICAL YOU NEED TO COMPLETE THE FOLLOWING

1. Objective2. Background information (what is Young’s Modulus,

how do we use a stress strain curve to determine it, etc)

3. Set up (diagram or description)4. Method5. Risk Assessment6. Table (set up for data collection)