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Youngs modulus isnt coming out as constant

  1. Feb 21, 2009 #1
    1. The problem statement, all variables and given/known data
    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 cant figure it out......can someone take these results and make them so there right and give me there method ><


    2. Relevant 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 gonna be miles off?
     
  2. jcsd
  3. Feb 21, 2009 #2
    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.
     
  4. Feb 21, 2009 #3
    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?
     
  5. Feb 21, 2009 #4
    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.
     
  6. Feb 21, 2009 #5
    Last edited: Feb 21, 2009
  7. Feb 21, 2009 #6
    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.
     
  8. Feb 21, 2009 #7
    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^^
     
  9. Feb 21, 2009 #8

    cjl

    User Avatar

    It's possible that it's experimental error, but try graphing it. You shouldn't get a simple linear relationship.
     
  10. Feb 21, 2009 #9
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