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Homework Help: Weird plate capacitor

  1. May 6, 2008 #1
    1. The problem statement, all variables and given/known data

    I have a capacitor which i just like a plate capacitor, but instead of the upper electrode being a plate it is a point charge Q. the lower plate is held at a fixed potential V=0.

    I am supposed to calculate the capacitance of the capacitor.

    3. The attempt at a solution

    I've used the image method with a negative charge (-Q) at the same distance from the lower plate (forcing V=0 on this plate).
    now I wanted to calculate the potential difference between the "plates", but it is obvisously inifinite since as we get closer to the point charge the potential grows like 1/x^2.
    and thus C = Q/delta_V = 0
    what am I missing?

    thanks :)
  2. jcsd
  3. May 7, 2008 #2
    someone? even if you are not sure I will be glad to hear what you think...
  4. May 7, 2008 #3


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    I looks to me like you are right. If one of the plates is an ideal point, you are going to have a really hard time shoving any charge into that capacitor. I'll vote for C=0.
  5. May 7, 2008 #4
    I've wrote in email to my lecturer asking about it.
    I'll let you know how it goes...
  6. May 7, 2008 #5
    Okey so my lecturer had answered to use a small radius spherical electrode instead of a point charge.
    actually I'm pretty confused, so I'll try to work on it for a little while and let you know if I made some progress and/or have any questions.

    thanks for the help!
  7. May 8, 2008 #6
    I solved the question, it's not that difficult after all - just a lot of calculations.
    solution is attached if you are interested :) if you see an error i would love to know about it..

    now - supposed I know the capacitance and the force felt by the upper electrode, and I know all dielectric coefficients eps_k, could I derive the dielectric layers' lengths??

    I can see only two equations, thus missing (N-2) to solve the problem, but I was told it is possible...

    Attached Files:

    Last edited: May 8, 2008
  8. May 8, 2008 #7
    I've made progress, I'm stuck with 6/8 equations, could you think about anything I might have missed??? thanks!

    Attached Files:

  9. May 9, 2008 #8
    I have the feeling I got the first two sections wrong...
    something about my dealing with the dielectrics.
    could you have a look and tell me what you think?
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