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Homework Help: Expression for Electric Field Outside Sphere

  1. Feb 13, 2012 #1
    1. The problem statement, all variables and given/known data

    The charge desnsity within a uniformly charged sphere of radius R is ρ = ρ0 - ar2, where ρ0 and a are constants and r is the distance from the center. Find an expression for a such that the electric field outside the sphere is zero.

    2. Relevant equations

    Electric Field outside a sphere
    E = KQ/r2

    3. The attempt at a solution

    I'm having trouble setting up this problem, I'm not quite sure where to start. If someone could help me I would very much appreciate it.
  2. jcsd
  3. Feb 13, 2012 #2


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    Hello Ryan129. Welcome to PF !

    You will need to integrate the charge density over the whole volume of the sphere & use Gauss's Law to tell you haow muck total charge you need to get that E-field outside of the sphere.

    There are other ways to do it.
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