Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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...


    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


    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

    Reilly Atkinson
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook