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Homework Help: Confused on Capacitor problem.

  1. Apr 2, 2008 #1
    1. The problem statement, all variables and given/known data
    An axon is the relatively long tail-like part of a neuron, or nerve cell. The outer surface of the axon membrane (dielectric constant = 5, thickness = 1 10-8 m) is charged positively, and the inner portion is charged negatively. Thus, the membrane is a kind of capacitor. Assuming that an axon can be treated like a parallel plate capacitor with a plate area of 4.20 10-6 m2, what is its capacitance?

    2. Relevant equations
    C = k * Eo * A / d

    3. The attempt at a solution
    I used C = 5 * 8.85*10^-12 * 4.20*10^-6 / 1*10^-8 = 1.9E-8 F

    However this is not the correct answer and I am perplexed as to why. Is Eo not 8.85*10^-12 or is d not just the thickness?
  2. jcsd
  3. Apr 2, 2008 #2
    So is this a parallel plate capacitor with with a dielectric in between? If so then you should be able to just use

    [tex]C = \epsilon_r C_{vac}[/tex]

    where [itex]\epsilon_r[/itex] is the dielectric constant. I think you may have used the wrong electric permitivity.
  4. Apr 2, 2008 #3
    Maybe your units are incorrect.
  5. Apr 2, 2008 #4
    I think the permittivity is correct, 8.85e-12 is the vacuum permittivity, and it was multipled by 5, the dielectric constant.

    If you are doing this as a web-based problem, it's probably a matter of significant digits. Use more.
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