Youngs modulus isnt coming out as constant

[smolly]

Homework Statement

hey
my youngs modulus for all my results for c.w has come out "blehege"
Y=youngs modulus
F=force
A= area 5.255x10-8
X=extension
L= original length 1m make things quicker
my Y for when F=1.5 and X=0.05x10-3 i get a stress of 26198585 my stress is 2000 which makes my Y=5.2x10to the 10
and later on when my F=11 and X=10x10-3 my stress is 192122958 and stress 100 which makes Y= 1.9x10 to the 10
ive done something really wrong here but i can't figure it out...can someone take these results and make them so there right and give me there method ><

Homework Equations

i used
Stress = force divided by area
strain = extension divided by original length
Youngs modulus= stress divided by strain

on my results page of my c/w my teacher wrote that it should be a constant which makes sense so i think i worked something out wrong? or is it just my equipment was so bad its going to be miles off?

 

[Dadface]

Haven't checked your maths but your equation is correct. It seems that you applied two forces only and calculated Y for each one.Y is a constant and you should have used your two values to calculate the average value.It would be better to use a range of forces and plot a graph of stress against strain.Y can be found from the gradient of the graph.

 

[smolly]

i did do results for every 0.5N so 1, 1.5, 2 , 2.5 etc till my copper wire broke at 14N i just used those 2 results cause if its constant surely 1.5 should be roughly equal to 6.5?
i just thought of this? is youngs modulus a constant decrease? or should it be constantly the same?

 

[Dadface]

Since your wire broke you stretched it beyond its elastic limit and before breaking it started to display plastic behavior.Try the graph of stress against strain(or it may be easier to plot force and extension)and calculate Y from the linear part of your graph..It is only over this linear elastic region that Y is a constant.

 

[smolly]

here is my set of results

http://i234.photobucket.com/albums/ee228/mantaray159/P210209_1546.jpg

my graph didnt work too well so i found it hard to find a linear part of it?
should my youngs modulus answers be all roughly the same? or something else?

 

[Dadface]

To measure youngs modulus accurately you need some quite sophisticated equipment and if you didnt have this your results will have large experimental errors.I think the best you can do is plot the line of best fit close to the origin.

 

[smolly]

ok thanks! so can i basically say that my youngs modulus is abit skewif cause my equipment was really inaccurate?
was a 1M ruler and my eye site^^

 

[cjl]

It's possible that it's experimental error, but try graphing it. You shouldn't get a simple linear relationship.

 

[smolly]

ill post my graph so you can see what i got
http://i234.photobucket.com/albums/ee228/mantaray159/P210209_154601.jpg

i checked wiki and its like 100-140GPA and I am only getting 52GPA as my highest and it goes down to 6GPA
i really appreciate all the help here thanks