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Homework Help: Electric field in parallel plate capacitor

  1. Mar 17, 2013 #1
    1. The problem statement, all variables and given/known data

    The parameters of an air-filled parallel plate capacitor are: d = 5mm and A = 500cm2. The
    charge of the electrodes are Q = 10nC and −Q. Inside of the capacitor in the middle point
    there is a point charge of 0, 02Q. Find the magnitude of the force acting to the point charge!

    2. Relevant equations

    I'm guessing these formulas

    C=Q/V = ε0 * A/d

    V = E*d

    3. The attempt at a solution

    The answer is supposed to be 4.52 * 10-6 N

    However I plug in the values and get 9.02 N.. Am I using the wrong formulas, or am I just calculating it wrong?
  2. jcsd
  3. Mar 17, 2013 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    You don't want to compute capacitance.

    What did you get for the E field between the plates? You can use for example σ = D where σ = surface charge density and D = εE. Or use Gauss' theorem.

    Then F = EQ' where Q' = 0.02Q.

    The given answer is correct BTW.
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