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Homework Help: Magnetic field for parallel plate capacitor

  1. Apr 20, 2008 #1
    Hello hello,

    What is the b-field for a parallel plate capacitor if the current J was found to be in the z-direction? I am told it will be in the z-direction as well. How so?

    Thanks so much!
  2. jcsd
  3. Apr 21, 2008 #2

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  4. Apr 21, 2008 #3
    thanks. i see it now....:-)
  5. Apr 22, 2008 #4


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    B is never in the direction of J.
  6. Apr 22, 2008 #5

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    True. Who said there is a j in between the plates, if that is what you mean?

    It's the dE/dt part which makes B non zero. After all, isn't that why Maxwell introduced the "displacement current"?
  7. Apr 22, 2008 #6
    so I fear if i say too much, i run the risk of being mistaken for getting hw help here. eeeek. (im a newbie to the forums) truth be told...hw for this problem was due last week...it was a discussion with the ta and office hours that got this question rolling in my head. solutions have yet to be posted...so i'd love to keep posting if possible...

    the problem stated that there was a parallel plate capacitor separated by a distance d connected to a vacuum in between. There are two scenarios for the problems in which we are asked to evaluate the H-field: one while connected to a battery of voltage V, one without. The idea is that there is no free current - but the plates are oscillated in such a manner that charge between the plates will vary in time as you increase/decrease the separation distance. Here, V is constant, the other scenario, in which the battery is disconnected it is not.

    Here is where I got confused: I've seen parallel plate capacitors with charge +Q and -Q on the individual plates, and I guess that varying the distance d = d0 + d1sin(wt) would create a current, thus a current density J distributed across the plates. As a result, a time varying H field results, from which I was told was in the z-hat direction [provided that the plates lie in the x-y plane separated by a distance d up the z-axis.] I solved for J - but was told that was in the z-hat direction too.

    I am thinking after all - perhaps I misunderstood the direction J would be in. Perhaps J tangent to the plates, or "flows around the plates"?

    Solutions should be posted soon. Perhaps they will be of some clarification. Just kind of interesting problem....a new one definitely and one I just kept turning over in my head...

  8. Apr 22, 2008 #7


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    Staff: Mentor

    Thread moved to Homework Help (where it belongs).
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