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Homework Help: Find charge field within coaxial cables

  1. Apr 15, 2013 #1
    1. The problem statement, all variables and given/known data
    two very long coaxial cylinders. Inner has radius a and is solid with charge per unit length of λ. Volume is also uniform but not defined by a parameter. Outer is hollow with inner radius b and outer radius c. Outer cylinder is a conductor with charge per unit length of -2λ. Find E(r) for all r in terms of given parameters. you can also view attached picture.

    2. Relevant equations

    3. The attempt at a solution
    I'm hung up right away on what to do with the inner radius from 0->a Ive got the left side of the equation I think, but I don't know how to get qenclosed in terms of r, if it needs to be. I expect to integrate on that part. Anywho here's where I'm at.


    I've got λ=dq/dh where h represents the length of the cable being considered. All the examples seem to use charge density rather than charge per unit length, using this gives me:

    E2∏rh=(1/ε0)∫λdh which is not in terms of r. I suppose I could still integrate it and cancel h on both sides, let me know if that's wrong.



    Is this wrong for any reason? It doesn't feel right to not use the radius on the right side.

    Attached Files:

  2. jcsd
  3. Apr 15, 2013 #2
    Let's consider the cable's length finite, L. The total charge of the inner cylinder would be Q = λL. Applying Gauss' theorem ∫E*dA = λL/ε°, since E is uniform and perpendicular to dA: E∫dA = λL/ε° <=> E(2πrL) = λL/ε° <=> E = λ/2πε°r.
    The units on both sides are also ok, so your formula is right.
    Last edited: Apr 15, 2013
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