How to determine Young Modulus of A wire by Experiment? [HELP]

Hey,

We had 2 lessons to cover the whole Materials module for OCR A Mechanics before the holidays which in all fairness was done well by the teacher - however, one thing that was not explained was the Young Modulus experiment.

I can't seem to find anything in my revision guide either.

But there are some 7 mark questions all about the experiment, in terms of what do you measure, how do you carry it out, how do you determine youngs modulus etc in the past papers and I haven't got much of a clue.

I know you measure the diameter of the wire, find its area, calculate stress and strain, plot a graph and find the gradient (Youngs modulus) but thats really vague and not enough for 7/8 marks surely?

Please can someone help?

Reply 1

lucas13

15

measure the unstretched length of wire, stretch it using a weight, measure the extension, measure the cross sectional area using a micrometer.

then put the values into the formula E=Fl/eA

Reply 2

PhysicsGog

The wire being extended needs to be compared to a reference wire to account for variations due to temperature change. The extension should be measured using a vernier scale as it will be small.

Reply 3

subzero0137

12

Measure the diameter of the wire using a micrometer. Half the diameter to get radius, from there you can calculate the cross-sectional area

A

. Now measure the length of the wire using a rule, this will be your original length

l

. Now stretch the wire by hanging weights (force)

F

and measure the new length of the wire. Substract this value from your original length

l

to get extension

\delta l

. Young Modulus

E = \frac{F \times l}{A \times \delta l}

Reply 4

Stonebridge

13

The method most loved by A-Level specifications is called "Searle's Method" and is similar to that described by post #3
1. There needs to be a reference wire to compensate for length changes due to temperature changes.
2. Contrary to posts #4 and #2, it is NOT done using a single measurement and plugging numbers into the formula.
3. Experimentally the modulus is calculated by drawing a graph of extension against load and finding the gradient.

Further information here
http://en.wikibooks.org/wiki/A-level_Applied_Science/Choosing_and_Using_Materials/Properties#Searle.E2.80.99s_method_for_finding_the_Young.E2.80.99s_modulus_of_a_wire
and here
http://books.google.co.uk/books?id=WN6TuD8gTYEC&pg=PA318&lpg=PA318&dq=searle's+method+for+determination+of+young's+modulus&source=bl&ots=HWKOEl-SpK&sig=RPzkrc5REmkT10cTupMkNzrcz8s&hl=en&sa=X&ei=4_T6Tr-_EoTT8gPGh8HEAQ&ved=0CH8Q6AEwCQ#v=onepage&q=searle's%20method%20for%20determination%20of%20young's%20modulus&f=false
Here's some background. Point 6 and diagram 2.7b reference Searle's Method, although other less accurate methods exist, as shown.
I doubt many schools have the equipment for this (Searle's Method) nowadays.

(edited 12 years ago)

Reply 5

subzero0137

12

Original post by Stonebridge

The method most loved by A-Level specifications is called "Searle's Method" and is similar to that described by post #3
1. There needs to be a reference wire to compensate for length changes due to temperature changes.
2. Contrary to posts #4 and #2, it is NOT done using a single measurement and plugging numbers into the formula.
3. Experimentally the modulus is calculated by drawing a graph of extension against load and finding the gradient.

Why not? Isn't Young Modulus simply Stress multiplied by (original length/change in length)?

Reply 6

Stonebridge

13

The question asks about an experimental method.
You are talking about a calculation plugging numbers into a formula. Read the links. What do they say?
Standard experimental procedure would be to plot a straight line graph of the data and find the gradient. This is the classic way to reduce experimental errors by combining multiple readings.