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!

Electrostatics problem (Spherical surface)

  1. Nov 17, 2009 #1
    1. The problem statement, all variables and given/known data
    Find the electric field a distance z above the center of a spherical surface of radius R, which carries a uniform charge density "sigma" Treat the case z<R(inside) as well z>R (outside). express your answers in terms of the total charge q on the sphere.[hint:use the law of cosines to write r interms of R and "theta". be sure to take the positive square root:
    sqrt(R^2+z^2-2Rz)=(R-z) if R>z and (z-R) for R<z]


    here is the solution for the problem:
    http://img689.imageshack.us/img689/2492/questionr.jpg [Broken]

    even with the solution i have problem understanding....
    what i don't understand is:
    1) how did he get dq=sigma*R^2*sin(theta)d(theta)d(phi)
    2)the reason for taking cos(curlyphi) ?
    my guess: to get the vertical component? since horizontal components cancels out
    3)is there a different way looking at this problem? i remember in college there is a simpler way doing this kind of problem....
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Nov 17, 2009 #2

    Delphi51

    User Avatar
    Homework Helper

    1. R*sinθ*dφ and R*dθ are the length and width of the area da where the charge dq lies.
    2. You are correct.
    3. Yes, you could use Gauss' Law. This problem must be at a point in your course where you have not taken Gauss' Law - and it is good practise!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook