1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Electric potential problem

  1. Feb 6, 2007 #1
    1. The problem statement, all variables and given/known data
    derive an expression for the electrical potential at a distance x measured along the axis from the centre of a circular ring of radius R on which a charge Q is uniformly distributed.hence derive an expression for the electric field strength at this point

    2. Relevant equations
    to be honest im not too sure where to go after:
    V= -⌠E.dl
    any guidance at all would be much appreciated!!!thanks:biggrin:

    3. The attempt at a solution
  2. jcsd
  3. Feb 6, 2007 #2
    It looks like a start, what if the "ring" were simply two charges, each of 1/2Q, and -r,0 and r,0 relative to a test charge at 0,x. What would that look like?
  4. Feb 6, 2007 #3
    This simply involves a lil integration.

    Q is distributed uniformly throughout the ring. This linear charge denstity on it would be lambda= Q/2pi*R.

    Let dV due to each infinitesimally small element (dl) on the ring = (1/4pi*epsilon)*(lambda*dl/(x^2+R^2)^1/2.

    Then just integrate from 0 to 2pi*R.
  5. Feb 6, 2007 #4
    oh i see, so its ok to treat them as 2 separate point charges and then sum the electric potentials at the end.
    doing this i got an expression:
    V= - Q/4*pi*ε(x^2 + R^2)^1/2

    and E= dV/dx
    E = - [Qx]/[4*pi*ε(x^2 + r^2)^3/2]
  6. Feb 6, 2007 #5
    no, what i meant was look at it first as 2 point charges, then 4, then an infinite number spread around the ring, but Mr 4 points the way I was hinting at in post above directly.
  7. Feb 6, 2007 #6
    Quite so. I think your logic would be more useful in some questions. Thanks for that denverdoc.
  8. Feb 6, 2007 #7
    Still getting the hang of helping without doing the work, ie trying to help posters conceptualize w/o telling them how to pursue directly. Sometimes I think I just add to the confusion:grumpy:
  9. Feb 7, 2007 #8
    Nope. I get you loud and clear! Maybe its coz I'm just as confusing!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook