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Capacitors: Explain result from experiment

  1. May 15, 2008 #1
    [SOLVED] Capacitors: Explain result from experiment

    1. The problem statement, all variables and given/known data
    I made an experiment today at school with a parallel plate capacitor. I induced 100 V and increased the distance between the plates. I measured the charge and calculated the capacitance of the capacitor, which of course got smaller when the distance got bigger.

    Now the thing is: When I calculate the capacitance theoretically (using C = K*A/d), where A is the area of the plate, I get values for the capacitance, which are a lot smaller than the measured values when the distance is very "large". Why is that?

    I would think it would have to be something like the plates are getting discharged the further they are apart.
     
  2. jcsd
  3. May 15, 2008 #2

    Hootenanny

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    Erm, how did you measure the charge?

    Edit: Also, how did you increase the distance of the plates?
     
    Last edited: May 15, 2008
  4. May 15, 2008 #3
    I measured the charge using a Coulomb-metre. I increased the plates using plastic-"feet". My teacher gave them to me.
     
  5. May 15, 2008 #4

    Hootenanny

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    One possible explanation for the discrepancy between the theoretical and experimental values is edge effects. Your expression for the capacitance ignores edge effects and assume a completely uniform electric displacement field, which is fine when the separation is small. However, once d becomes large the edge effects become more significant and would reduce the magnitude of the electric field between the plates and hence increase the capacitance.
     
    Last edited: May 15, 2008
  6. May 15, 2008 #5
    What are edge-effects in this case?
     
  7. May 15, 2008 #6

    Hootenanny

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    Your expression for capacitance assumes that the electric displacement field is completely uniform. However, in reality at the edges of the capacitor the field is non-uniform, the field lines arc as in the diagram below.

    [​IMG]
     
  8. May 15, 2008 #7
    Ahh, I see. Yes that actually does make a lot of sense.. Of course my first assumptions (the plates getting discharged) is wrong, since nothing would discharge them.

    I understand it. Thanks!

    Edit: But how would that result in the experimental value being lower than the theoretical? Would in be the other way around?
     
    Last edited: May 15, 2008
  9. May 15, 2008 #8

    Hootenanny

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    Hence the theoretical result (ignoring edge effects) would predict a lower capacitance than is observed experimentally.
     
  10. May 15, 2008 #9
    Sorry, I really have a bad habit of just skating through a post, and asking all sorts of questions which are already answered in the post. I will stop that.

    Thanks again!
     
  11. May 15, 2008 #10

    Hootenanny

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    Don't worry about it :smile:
     
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