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Homework Help: Electric field inside a uniformly charged insulator

  1. Mar 22, 2008 #1
    1. The problem statement, all variables and given/known data
    ok here's the problem: find the magnitude of the electric field inside uniformly charged insulating sphere of raduis R.

    2. Relevant equations
    application of gauss's law..but...

    3. The attempt at a solution
    should i use
    [tex]\phi[/tex] = q[tex]_{encl}\epsilon_0[/tex]
    [tex]\phi[/tex] = q[tex]_{encl}[/tex]/[tex]\kappa[/tex][tex]\epsilon[/tex]
  2. jcsd
  3. Mar 22, 2008 #2
    It becomes slightly different inside a sphere. Electric fields are vectors, and a point inside a sphere is being affected by an electric field in every direction. Heres what you need to know: Any point inside a spherical shell of any thickness receives a net electric field of 0N/C. Same concept applies to electric force and gravity.
    Here is a site to help you out on this:
    Using that, if a point is a radial distance a from the center of the sphere (while inside the sphere), the net electric field at a point inside a sphere would be due to only the the charge within the radial distance (the charges outside the radial distance contributes to the 0N/C).
    Last edited: Mar 22, 2008
  4. Mar 22, 2008 #3
    So, are you in space or a material? Would you want to use the electric constant for free space or not?
  5. Mar 23, 2008 #4
    that is what I'm confuswed about...if the insulating sphere is made of a material other than air, which one should I use on the righthand side of the flux equation? [tex]\epsilon_{0} [/tex] or [tex]\epsilon[/tex]. that is should I take the permittivity constant ([tex]\kappa[/tex]) into account or not?
  6. Mar 23, 2008 #5


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    Homework Helper

    Since you are solving for the electric field strength E, you would just use [tex]\epsilon_{o}[/tex]. If you were asked for the electric flux density D, you would need to be concerned with the electric permittivity of the material. (The hint is that you aren't given a value for [tex]\kappa[/tex] in the problem...)
  7. Mar 23, 2008 #6
    Out of curiosity, what volume are you using for the enclosed charge?
  8. Mar 23, 2008 #7
    a sphere
  9. Mar 23, 2008 #8
    thanks dynamicsolo. I'm starting to see how things are now
  10. Mar 23, 2008 #9
    thats a shape, not a volume
  11. Sep 14, 2009 #10
    If a charge were distributed uniformly on the surface of the balloon(insulator). A point particle with charge q inside is greatest when it is anywhere inside the sphere because the force is zero?

    or when it is near the inside surface of the balloon?
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