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 Field inside a nonconducting sheet

  1. Mar 9, 2012 #1
    1. The problem statement, all variables and given/known data

    A huge (essentially infinite) horizontal nonconducting sheet 10.0 cm thick has charge uniformly spread over both faces. The upper face carries +95.0 nC/m2 while the lower face carries -25.0 nC/ m2. What is the magnitude of the electric field at a point within the sheet 2.00 cm below the upper face? (\varepsilon_0 = 8.85 × 10-12 C2/N · m2)

    2. Relevant equations

    electric flux = ∫E*da = q(enc)/ε_0

    3. The attempt at a solution

    I figured this problem should be relatively easy, but the fact that the charge densities are different on each side is throwing me off...

    First, I thought to find the E field at some particular point, I ought to use a Gaussian surface, and my thought was to use a cylinder, sticking in as far as .02m below the upper face. Doing so, gives me E=σA/ε_0*A, and the A's cancel, of course.

    What I'm really confused about is how to take into account the fact that it's only .02m into the sheet. And for that matter, because the charge density is different on each side, I'm not sure how to go about finding the charge density at that particular point of .02m inside the sheet.

    If anyone could help clear some of this up, I'd really appreciate it.

  2. jcsd
  3. Mar 9, 2012 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Principle of superposition.
    You can do it OK as a single sheet of charge right?
  4. Mar 9, 2012 #3
    Thanks for the reply.

    That's right, just confused about the varying charge density. So if I understand you correctly, I should find the field of the one side, the other side, and then again for 2cm in? How do I determine a correct charge density for the point that's 2 cm in, however?
  5. Mar 9, 2012 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Your description says the charge is spread uniformly over the faces only - not penetrating into the sheet. So what you have is two parallel 2D sheets of charge.

    so put the +Q sheet at z=+5cm and the -Q sheet at z=-5cm and work out the field at z=+3cm.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook