1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Parallel plate capacitor electric field problem

  1. Sep 12, 2009 #1
    1. The problem statement, all variables and given/known data

    Two circular disks spaced 0.50 mm apart form a parallel-plate capacitor. Transferring 1.8*10^9 electrons from one disk to the other causes the electric field strength to be 1.3* 10^5 N/C. What are the diameters of the disks?

    2. Relevant equations

    E_capacitor= Q/(epsilon_0)*A

    3. The attempt at a solution
  2. jcsd
  3. Sep 12, 2009 #2


    User Avatar
    Homework Helper
    Gold Member

    Look at your second equation. What can you find from it?
  4. Sep 13, 2009 #3
    I'm doing the same problem.

    All I can find from the second equation is that Q = Epsilon * A * E

    Which doesn't really help because even if I break down A into pi * r^2 I still don't know r.. because if I knew r I would know d, which is what we're looking for.

    I don't get how that helps.
  5. Sep 13, 2009 #4
    Well I have another equation I was solving for earlier.

    r = sqrt(Q/pi*eta)

    I suppose I could substitute that into the equation set equal to Q and solve for Q.

    Then maybe I could plug that Q from Q=Epsilon*A*E into the r = sqrt(Q/pi*eta) equation to find r.

    Hmm. I can't stand this because of the precision required for these stupid online assignments, I get zero credit or full credit, none in between. Rewarded none for the effort I've put forth the same as someone who did nothing.

    Oh well, computers give grades.
  6. Sep 13, 2009 #5
    And now I end up with

    Q=(Epsilon*E*Q) / eta

    that makes no sense because if I go any further the Q's will cancel out *sigh*.. so I know I did something wrong.

    Back to the drawing board.
  7. Sep 13, 2009 #6


    User Avatar
    Homework Helper
    Gold Member

    Look at your second equation.

    You know Q, you know ε0 and you know E. Can you find A? Once you know A can you find the diameter?
    Last edited: Sep 13, 2009
  8. Sep 13, 2009 #7
    What is Q?

    N electrons * Charge of electron?

    I'm going back and re-reading the entire chapter leading up to this one in cased I missed something.

    I'm completely lost when it comes to electricity and thats not like me.
  9. Sep 13, 2009 #8
    thanks for the help. now if only webassign would stop lagging all the time and I could see if my answer is correct or not...
  10. Sep 13, 2009 #9


    User Avatar
    Homework Helper
    Gold Member

    Yes, Q is the total charge of the N electrons.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook