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Homework Help: Capacitance Question

  1. Nov 17, 2004 #1
    Hi Guys,

    Here is a question that I cannot get my mind on. Bad day I guess. I'm trying to figure out the answer. Here comes the question:

    How far away from each other would two metal plates, 2 square meters in area each, have to be in order to create a capacitance of 1 micro Farad? Assume that the plates are separted by air.
  2. jcsd
  3. Nov 17, 2004 #2
    Capacitance = (Permitivity of air * Area of overlap of the plates)/d between plates

    you have all the variables apart from d
    rearrange the formula with "d" on one side of the equals sign and everything else on the other.

    Permitivity of air = 1.0006 (in case you didnt know)
  4. Nov 17, 2004 #3
    The answer I get is 2*10 to the power -6. I don't think that is the right answer.
  5. Nov 17, 2004 #4
    i made an error there sorry

    1.0006 is the relative permittivity of air
    you multiply this by the permittivity of free space.

    permittivity of free space is 8.85*10^-12
    permittivity of air is (8.85*10^-12)*1.0006 = 8.85531*10^-12

    d = dist between plates = unknown
    A = overlapping area of plates = 2 metres square
    E = permittivity of air = 8.85531*10^-12
    C = capacitance of the parallel plate capacitor

    your formula was C = (E*A) / d

    you rearrange it to d = (E*A) / C

    plug in your values for E, A and C

    then see what you get.
  6. Nov 18, 2004 #5
    Thanks for helping out SpeedBird. I forgot completely about the permittivity of free space. Answer correct after I got the full menu!!

    Thanks again, Mark
  7. Nov 18, 2004 #6
    No worries :biggrin:
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