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Really need help with electricity question

  1. Oct 14, 2005 #1
    For those who have griffiths its problem 3.41 (its not exactly the same but that's what this question is modelled after)

    Alright so I was going through how to solve a problem in my textbook when I got stuck midway through. The approach I had taken wasn't working and I wasn't quite sure where to proceed from where I was. Here's the question.

    "The average of the field E inside a ball Vr of radius R due to all the charges within the ball, is

    Eave = 1/4piepsilon p/R^3 (*)

    we will demonstrate this in the following way "

    a.) show that the average field due to a single charge q at a point r inside the ball is the same as the field at r due to a uniformly charged ball with pdensity = - 3q/(4piR^3)

    Alright so I began the question by finding the average field using the formula (and I now know I probably couldn't do this) then by comparing this to the electric field of a uniformly charged sphere.

    I found the dipole moment (using the charge distribution) and substituted into the eqn. for the average electric field inside the sphere. What I got was

    E = 1/4(pi)epsilon (-3qr)/R^3

    Then I did the same thing for the sphere (solved for Qenc and applied Gauss' Law) and got the same answer.

    (note: is that what the field will be at r in the ball given the charge density?)

    then part b says

    B.) Express the Answer to A in terms of the dipole moment.

    Now this question threw me off track as there isn't really an "answer" to A. So I thought it must mean what I found the electric field to be. Substituting that in thought simply takes me to the thing I'm trying to prove (*).

    C.) Then prove the (*) for an arbitrary charge distribution applying the superposition principle to B.

    This is where I have no clue how to proceed. I imagine that what we want to do is use the fact that we have the average field for a point charge is equal the field in a uniformly charged sphere and apply the principle of superposition to the point charges.

    However I don't really have an idea of what I should be doing.

    I think my biggest problem with the question is that I don't really know how to take the average of the electric field. Its equal to the electric field divided by the volume integral over the region (from what I can remember from calculus) but I'm lost and need an idea of where I should be going.

    Any help with this would be greatly appreciated (I've been trying all sorts of things for over 4 hours and haven't gotten anywhere- I think its probably something pretty obvious but it doesn't feel that way right now.)
    Last edited: Oct 14, 2005
  2. jcsd
  3. Oct 15, 2005 #2


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    Science Advisor
    Gold Member

    Look up dipole moments. The solution is, however, greatly simplified if you are at the center of the sphere.
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