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Homework Help: Can someone help explain me a problem?

  1. Jan 26, 2012 #1
    1. The problem statement, all variables and given/known data

    A sphere of radius R has a charge Q distributed uniformly over its surface. How large
    a sphere contains 90 percent of the energy stored in the electrostatic field of this charge

    2. Relevant equations

    U= 1/8pi integral E^2dv.

    3. The attempt at a solution

    I was able to derive that the energy to assemble a shell is Q^2/2R. The problem is idk what kind of sphere they are talking about? Do they mean a hollow sphere or one with a charge density?
  2. jcsd
  3. Jan 26, 2012 #2


    Staff: Mentor

    I think they mean solid sphere because otherwise they would say hollow sphere in the question.

    So you have to compute the energy in the initial sphere, take 90% of it and then use it to get a new radius, right?
  4. Jan 26, 2012 #3
    I guess it is a solid one. The thing that confused me was that the initial sphere has to be hollow because there is no charge density on the inside just on the surface. Thus I know I can treat that as a shell.
  5. Jan 26, 2012 #4
    Solid or hollow the charge is on the surface only.
  6. Jan 26, 2012 #5


    User Avatar

    Staff: Mentor

    It could be that they mean what size of a spherical volume of space surrounding the charged sphere contains 90% of the energy of the field. So the "sphere" would be insubstantial, like a Gaussian surface, a designated boundary.
    Last edited: Jan 26, 2012
  7. Jan 26, 2012 #6
    I think I understand now thanks a lot gneill I'll post my answer tmr. I'm pretty tired from working all day.
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