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Homework Help: Young's modulus of a copper wire

  1. Sep 14, 2007 #1
    Hi there! Can anyone help me with a problem.

    I did an experiment at school trying to measure the Young's modulus of a copper wire. We attached one end of the copper wire to a clamp and then tied on weights of 100g at a time to the other end so that a force can be applied to the copper wire and so it stretches.

    To work out Young's modulus, we require two things: the stress and the strain of the wire.

    Stress is [tex]\sigma = \frac{F}{A}[/tex] and strain is [tex]\varepsilon = \frac {\delta \ell}{\ell_o}[/tex] [tex]\frac {change in length}{original length}[/tex].

    To work out Stress, I worked out the Force applied on the wire by the weights (using the equation Force = Mass [tex]\times[/tex] Gravitational potential energy) as 0.1kg [tex]\times[/tex] 10 and this in turn produces 1N of force everytime a weight is added. To work out the area of the copper wire, I measured the diameter using a micrometer and this gave me an measurement of d=0.35mm. From this I worked out the cross section of the wire:

    [tex]\frac{0.35}{2}[/tex] (to find the radius), then squaring this and multiplying by [tex]\pi[/tex] to obtain the area.

    For stress I measured the original length of the wire (337cm) and then measured the extension of the wire and then dividing this by the original length.

    My results table is attached.

    When I try and plot the graph, I end up with a curved graph so the Young's modulus is not a straight line and so is the not directly proportional. Can anyone help me and tell me what I have done wrong in my experiment or is a curved line correct? Are my results alright?



    Attached Files:

    Last edited: Sep 14, 2007
  2. jcsd
  3. Sep 14, 2007 #2


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    Gold Member

    Your extensions look too big. If your wire is 337 cm (?) then you've permanently stretched it if you got 27cm of stretch. Your table does not show all the units.

    If the experimental force stayed within the elastic limit of the material, you would get a straight line.
  4. Oct 1, 2008 #3


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    what are the units?
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