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Coulomb triple integral for a sphere

  1. Jun 21, 2015 #1
    1. The problem statement, all variables and given/known data
    Find the electric field of a sphere of radius R and charge Q outside sphere. Use only a Coulomb integral to do this.

    2. Relevant equations
    I know that I have to use a triple integral to find the E-field. I am just unsure of my whole setup really.

    3. The attempt at a solution
    I tried setting up a triple integral of (0-2pi),(0,R) and then I dont know what to put for the the bounds of the third.
     
  2. jcsd
  3. Jun 22, 2015 #2
    You need to assume some more information. Is the charge distribution spherically symmetric?
    In spherical coordinates, your three coordinates are ##(r,\theta,\phi)##. Do you know what the valid range is for the ##\theta## coordinate? If not, you'd better study up on the spherical coordinate system.
     
  4. Jun 22, 2015 #3

    vela

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    Could you please provide the actual problem statement? Your version is kind of ambiguous.
     
  5. Jun 22, 2015 #4
    Problem statement: find th electric field of a sphere of radius R and charge Q outside the sphere. Do this using BOTH a gauss's law and coulomb integral.

    I have done the gaussian one, which was pretty easy. I just cant figure out the coulomb integral.

    No i have not used spherical coordinates before and it is not a prereq for this course apparently. But if i have to do it then i have to ha

    I appreciate any help.
     
  6. Jun 22, 2015 #5

    vela

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    Is that the problem statement as given to you word for word? It still seems kind of vague. Charge Q outside the sphere? Q could be anywhere then, except on or inside the sphere. Do you mean Q is on the surface of the sphere? If so, is it distributed uniformly? Is the sphere conducting or is it an insulator?
     
  7. Jun 22, 2015 #6
    My teacher said the sphere is conducting. And yes that is word for word. He is very vague in all of his problem statements.
     
  8. Jun 22, 2015 #7
    Wow TCC Engineering Physics II I'm guessing.
     
  9. Jun 22, 2015 #8

    vela

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    If it's a conducting sphere, the charge is going to distributed on the surface, so you only have to integrate over the surface.

    You've probably worked out or seen worked out the electric field due to a ring of charge. Try using that result by splitting up the sphere into a bunch of infinitesimal rings.
     
  10. Jun 22, 2015 #9
    yeah pretty much ha
     
  11. Jun 22, 2015 #10
    Ok so it would only be a double integral and not a triple integral right? because it would just be the area and the volume would not be taken into account.
     
  12. Jun 22, 2015 #11

    vela

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    Right. You're integrating over an area, a two-dimensional surface, so it would be a double integral.
     
  13. Jun 23, 2015 #12
    well i asked my teacher today and he said that would have to assume a non-conducting sphere, (was told by classmate that he said conducting ha). So he suggested the same thing as you to find the E-field due to a coin shape of charge. Which I have, and then assume the sphere is composed of infitesimal rings like that

    When I work it out I end up getting a different answer than my gaussian surface. Its close but only off by 1/(4R)
     
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