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Capacitance of a system

  1. May 1, 2008 #1
    I'm trying to figure out the capacitance of the attached system.

    may I look at this problem as N capacitors attached in a cascade connenction?

    I'm also trying to figure out the force that will be felt by the point charge.
    I thought about using the image method... could you thing of a better solution? I'm worried it will be very long and frustrating...

    thanks!
     

    Attached Files:

  2. jcsd
  3. May 1, 2008 #2
    Yes, all those are in series. You can replace them by a single capacitance.

    I don't understand the statement "..and can be seen as a point charge Q". Do you mean to say that the charge on the capacitor plate is Q?
     
  4. May 1, 2008 #3
    Thanks very much for the help!

    sorry for the bad explanation.
    what i was told is that the electrode of the capacitor is a point charge Q.
    I guess it means two things:
    1. the charge on the capacitor's upper plate is Q.
    2. the capacitor's upper plate is very small on the horizontal axises - which sounds very strange...

    This leads me to my next question:

    could there be a capacitor without two metal plates on its two ends?
    It is not drawn on the picture I got (only N dielectric plates), but maybe I was supposed to assume that they are there...

    Again - thanks very much for the help!
     
  5. May 1, 2008 #4
    Thought about it a little more and I guess what they've ment in the question is that the upper electrode could be considered a point charge when calculating the force it feels...
     
  6. May 3, 2008 #5
    Hey guys!

    I think I've solved it, attached is my solution.

    I have some last little points I would love to understand better:

    1) why is the field D contant though the capacitor? (I've read it in a notebook but could not understand why this is true - they've claimed it is because of the symmetry of the problem...)
    2) In the solution i've calculated E_n using Gauss envelopes, and then saw that D is constant, how can I calculate D directly?
    3) sometimes we write D=eps*E+P and sometimes we ommit P... why is that? how could I explain this?

    thank you very much for the help!
     

    Attached Files:

  7. May 3, 2008 #6

    reilly

    User Avatar
    Science Advisor

    Re 1&2. You have, for all practical purposes done a direct computation.Otherwise, look at the integral for the field, and you'll be direct, and 2, you'll see the important symmetry in action

    Regards,
    Reilly Atkinson
     
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