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Homework Help: Capacitance and Other Traits of a Coaxial Cylinder

  1. Apr 1, 2012 #1
    This is for an electromagnetic theory class.

    1. The problem statement, all variables and given/known data

    Prompt can be seen here

    2. Relevant equations

    J = I/A, J = σE, V = Ed, C = εA/d

    3. The attempt at a solution

    I'm having trouble mostly with parts i and ii, as the rest are fairly simple to acquire after figuring out those two. Here's what I have so far:

    A = (b-a)L
    J = I/((b-a)L)
    dE = (I/((b-a)L))d∅ (Here I'm honestly not sure what to integrate with, however my best guess is d∅, so after integrating nothing changes)
    E = V/d
    I = (Vσ(b-a)L)/((π/2)r)

    At this point I'm not sure if that is the total current, or just a segment of current that needs to be integrated. Hence I don't know what to do with r.

    Any help would be greatly appreciated.
  2. jcsd
  3. Apr 1, 2012 #2


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    Science Advisor

    Current density in steady state is just J=σE, and you should have no trouble computing E based on the fact that E=-∇V and the form they give you. Note that this part of the problem doesn't depend on ε at all. It's only when you get to surface charges that it will matter.
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