1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electric potential 3

  1. Dec 27, 2008 #1
    Electric potential 3 :)

    1. The problem statement, all variables and given/known data

    Hey guys.
    Look at this question, I'm suppose to find how much energy does it take to build this sphere with the new formula.
    The answer suppose to be (3/5) * (kQ^2/R), instead I got (1/10) * (kQ^2/R).
    I tried to solve it again and again but nothing, I couldn't find the problem.
    Any idea where I got this wrong?

    Thanks.

    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Dec 27, 2008 #2

    Defennder

    User Avatar
    Homework Helper

    Re: Electric potential 3 :)

    I don't think [tex]d^3x = d^3 r[/tex]. In this case it should have been [tex]dV=dxdydz[/tex], a triple integral in rectangular coordinates. So in spherical coordinates, where this problem is best solved, it should be [tex]dV=r^2dr \sin \theta d\phi d\theta[/tex].
     
  4. Dec 28, 2008 #3
    Re: Electric potential 3 :)

    Thanks a lot.
     
    Last edited: Dec 28, 2008
  5. Dec 28, 2008 #4
    Re: Electric potential 3 :)

    I'm still getting the same answer, look at that.
    Any idea?
     

    Attached Files:

  6. Dec 29, 2008 #5
    Re: Electric potential 3 :)

    I have used symbol e for epsilon.
    The formula you have used requires integration over all of space - not only over the sphere.
    But the equation for electric field E will be different for inside sphere and outside sphere.
    So you should break the integral into two parts. One from 0 to R and the other from R to infinity.
    You have already done the first part. Do the second part and add the two.
    For the second part,
    E = KQ/r^2 (point is outside the sphere)
     
  7. Dec 29, 2008 #6

    Defennder

    User Avatar
    Homework Helper

    Re: Electric potential 3 :)

    I think you need to tell us more about how the problem is set up. Is it a insulating charged spherical shell? Or a conducting shell? And yes you have to integrate over all space for r, visharad said.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?