material coursework- young modulus of copper

[roserose]

1. The problem statement, all variables and given/known data
find the young modulus of copper through calculating the stress and strain, plotting on a graph and finding
the gradient of the linear part of the graph

2. Relevant equations
ym=stress/strain
stress=force/ area
strain= extension/length


3. The attempt at a solution
having spent several lessons streching a piece of wire i came to drawin my graphs.
However as the ruler has mm markings and the wire only stretched by a couple of mm's i have really really large error bars on my graph. (about 50% near the bottom).

These have streched the gradient so that it is nearly infinate. The maximum gradient is less than th average gradient and the minimum gradient is negative.

HELP!


the graph looks a bit like this:
----
-----------
--------------------
------------------------------------
-------------------(0,0)------------------

[LowlyPion]

Welcome to PF.

Well as I recall you need to identify the area of the curve that you were experiencing linear elastic deformation. As you stretch the wire then your cross sectional area shrinks and so on and so on until you are into the region of plastic deformation.

Here is a lecture that covers this in some detail with a practical demonstration. Perhaps it will help.
http://www.youtube.com/watch?v=YrRP-oGPjvk

As to your data problem it may be that the experiment and measurements were set up without sufficient precision such that your results may not be all that useful, insofar as you didn't get enough data in the region where there is elastic deformation and too many of your data points are into plastic deformation.

[roserose]

My graph looks crazy even with just the elastic values. It looked how i expected it would until i added the uncertainty bars.

Because at first the wire only stretched a couple of mm ,the uncertainty was really high- about 50%, however as the extension increased the error decreased hence the pyramid shape.
My main Problem is finding the average gradient and the range of gradients for this pyramid ,as inevitably the gradient of one side is negative.
For example on one of my graphs i found these results:
average gradient= 437MPa
steepest gradient= -1.5MPa
shallowest Gradient= 1.5 MPa

[Timiop2008]

Hi, Just to get the correct context of this coursework, this sounds to me like an AS level assignment. Please could you clarify this. Also, From the Described Experiment, to me, it sounds like you are possibly doing the 'OCR Advancing Physics B' Specification. Is this correct? (It is easier to provide help for you if you state the level you are working at).