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Homework Help: Flux Through A Distant Disk

  1. Feb 4, 2013 #1
    1. The problem statement, all variables and given/known data
    A small disc or radius R with surface normal
    k is placed a distance z from a point charge −q,( k hat pointing along the z-axis which goes through the charge and the center of the disk.)

    Assuming that R ≪ z, derive an expression for the electric flux ΦE passing through
    the disc.

    Hint: Think of the correct approximation to make before starting the question.

    2. Relevant equations

    ΦE= q/ε

    3. The attempt at a solution

    I'm struggling to think of the approximation to simplify this problem. I can only think that with z much greater than R the disk would be a point charge but that wouldn't help at all. Any help to get started on the problem would be appreciated.
  2. jcsd
  3. Feb 4, 2013 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Think about the area of the disc, and how that compares with an entire sphere radius z centred on the charge.
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