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Homework Help: Electric field calculation

  1. Mar 23, 2010 #1
    1. The problem statement, all variables and given/known data
    We have a sphere with radius b, within that sphere there's another sphere with radius a. Between the two spheres we have an electric charge with density A/r. Also, we have a charge Q in the center. We need to find the constant A so that the field between a and b is independent of r (meaning, it's constant for a<=r<=b).

    2. Relevant equations
    Gauss' law.

    3. The attempt at a solution
    I added the scanned pages I've written, it should be understandable. As I said, I used Gauss' law to calculate the field, using a sphere with radius r (a<=r<=b) to calculate the electric flux. After finding the electric field as a function of r, I calculated d(E(r))/dr and demanded that it would be=0, but I can't seem to get a solution for A that's independent of r.

    Attached Files:

  2. jcsd
  3. Mar 23, 2010 #2


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    Homework Helper
    Gold Member

    In your work, you had

    [tex] Qin = \frac{A}{r} \left[ \frac{4}{3}\pi r^3 - \frac{4}{3}\pi a^3 \right] + Q [/tex]

    That's not right. You're going to need to re-evaluate the volume integral. The [tex] (4/3)\pi r^3 [/tex] terms only makes sense if the charge distribution is uniformly distributed about the sphere. You can't just multiply them by A/r. It doesn't work that way. You'll have to re-do the volume integral, integrating A/r' from r' = a to r in spherical coordinates (be careful to use the correct volume differential when performing the integral -- it's not simply dr') .

    Once you have an expression for the electric field, E, don't worry about taking the derivative. Instead, find the electric field at r = a, and call that Ea. Then find Eb, the electric field at r = b. Set Ea and Eb equal to each other and solve for A.
  4. Mar 25, 2010 #3
    How did I not notice that..thanks, that solved it.
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