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Homework Help: Parallel Plate Capacitor and distance

  1. Mar 29, 2006 #1
    I'm going through my book and one of the Example exercises has the following problem:
    Two plates, each of area 3X10^-4 m^2 are used to construc a parallel-plate capacitor with capacitance 1 pF. (a) Find the necessary seperation distance.

    Seems pretty obvious right? C = Eo(A/d)
    So, I figure I'm finding d = C/(Eo)(A)
    but it doesn't come out right, so I must be missing something either in my math (not surprising) or the area. Do I need to do a calculation with the area? Like 2 X A? That doesn't work either, and it doesn't seem right to do it that way. Help, please?? :yuck:
  2. jcsd
  3. Mar 29, 2006 #2


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    Its just your manipulation which is letting you down. It should be;

    [tex]C = \frac{E_{0}A}{d}[/tex]
    [tex]C \cdot d = E_{0}\cdot {A}[/tex]
    [tex]d = \frac{E_{0}A}{C}[/tex]

  4. Mar 29, 2006 #3

    You are awesome!! Thank you!!

  5. Mar 29, 2006 #4


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    If you are assume the dielectric is air (k ~ 1) and [itex] \epsilon_o = 8.854 x 10^{-12}[/itex] F/m, you may get d = 2.656 mm. A more general formula for this type question may be found here.
    In agreement with Hoot, i noticed the same fault in rearranging your equation. It is a good idea to do a couple of things to double check maths. One is to do the algebra step by step, the other is to do a quick dimensional analysis of the final equation. In your case, you have A left in the denominator. The only way that would work and have answer come out in meters is to have [itex]m^3[/itex] in the numerator, which you don't have.
  6. Mar 29, 2006 #5
    Wow. Thank you, that is most helpful. The link is terrific. Again, thank you!
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