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Find the electric field a distance z above a disc of radius R and surface charge

  1. Oct 19, 2011 #1
    1. The problem statement, all variables and given/known data

    The electric field, E a distance z above a circular loop of charge density lambda, radius r, in the x-y plane centred on the origin, is given by

    E(z)=[lambda z r] i(subscript z)/[2 epsilon0((z^2 + r^2)^(3/2))]

    a) using this, find the electric field, E, a distance z above a disc of radius R and surface charge density sigma (8 marks)
    b) describe the limits of E for the disc at R tends to infinity and z>>0 (4 marks)

    3. The attempt at a solution

    a) E(z)=[sigma z R] i(subscript z)/[2 epsilon0((z^2 + R^2)^(3/2))]

    I don't think that's going to gain me the full 8 marks and I have no idea what to do next. Please help

    b) I can't even make sense of it. How can z>>R if R tends to infinity?
     
  2. jcsd
  3. Oct 19, 2011 #2
    Re: find the electric field a distance z above a disc of radius R and surface charge.

    a) A disk of charge could be considered to be made up a large number of concentric loops of charge. You'd need to sum (think - integrate) the effects of many infinitesimally thin loops to get the field of a disk. To me, looks like you've just replaced r -> R and charge per unit length with charge per unit area which is wrong.

    b) Question says z>>0, not z>>R.
     
  4. Oct 22, 2011 #3
    Re: find the electric field a distance z above a disc of radius R and surface charge.

    but the question does say z>>R, I just checked
     
  5. Oct 22, 2011 #4
    Re: find the electric field a distance z above a disc of radius R and surface charge.

    I made a mistake the first time I typed the question
     
  6. Oct 22, 2011 #5
    Re: find the electric field a distance z above a disc of radius R and surface charge.

    never mind, I just found out that z>>R and z tends to infinity are two separate ques
     
  7. Oct 30, 2011 #6
    Re: find the electric field a distance z above a disc of radius R and surface charge.

    E=[pi sigma z/[2 epsilon0]] ((1/z) -1/(sqrt(R^2 +z^2)))

    so as R tends to infinity

    E=p[i sigma/[2 epsilon0]] z(1/z -0)
    =pi sigma/[2 epsilon0]??
     
    Last edited: Oct 30, 2011
  8. Oct 31, 2011 #7
    Re: find the electric field a distance z above a disc of radius R and surface charge.

    Looks plausible :) not going to thoroughly check it... you should try and convince yourself whether it is right or not. Does it seem sensible to you?
     
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