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Homework Help: Gauss' Law problem

  1. Oct 17, 2007 #1
    1. The problem statement, all variables and given/known data

    An insulator in the shape of a spherical shell is shown in cross-section above. (see attached .gif) The insulator is defined by an inner radius a = 4 cm and an outer radius b = 6 cm and carries a total charge of Q = + 9 mC (1 mC = 10-6 C). (You may assume that the charge is distributed uniformly throughout the volume of the insulator).

    What is Ey, the y-component of the electric field at point P which is located at (x,y) = (0, -5 cm) as shown in the diagram?

    2. Relevant equations
    I cant do this I am totally stuck.

    3. The attempt at a solution

    OK so obviously gauss law.
    I chose sphere inside the sphere, of radius 4cm for my gauss surface
    i need to calc the charge in this enclosed sphere.

    So charge in the sphere should be:

    Qin = (r^3 / (R^3 - r^3))*Q

    = 7.2e-5

    this much is wrong but dont know why
    please help --thanks

    Heres the drawing on flickr:

    Attached Files:

    Last edited: Oct 18, 2007
  2. jcsd
  3. Oct 18, 2007 #2


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    Homework Helper

    can't see the attachment... but I assume (0, -5cm) is located at a radius of 5cm from the center? If I'm wrong about this let me know.

    So using Gauss' law... what is the volume of the insulator enclosed within a radius of 5cm?
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