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!

Gauss' Law problem! Help please infinite sheet with charge density?

  1. Feb 21, 2013 #1
    Gauss' Law problem! Help please!!! infinite sheet with charge density?

    In the figure below, a small circular hole of radius R = 1.80 cm has been cut in the middle of an infinite, flat, nonconducting surface that has uniform charge density σ = 4.50 pC/m2. A z-axis, with its origin at the hole's center, is perpendicular to the surface. In unit-vector notation, what is the electric field at point P at z = 2.56 cm?

    Figure
    http://www.chegg.com/homework-help/...infinite-flat-nonconducting-surface--q1088630

    Teachers Solution:
    "The correct electric field can be gotten by adding the infinite charged plane with uniform
    density σ to the disk with radius R and opposite charge density -σ."

    I understand how to solve the problem but I don't understand why the charge density of the disk is negative? So why is it -σ??? HELP!
     
  2. jcsd
  3. Feb 21, 2013 #2

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Re: Gauss' Law problem! Help please!!! infinite sheet with charge dens

    The hole can be imagined as a circle with zero charge density. But you can make zero charge density by putting a circle with opposite charge onto the plate, making the charge of unit area equal to σ+(-σ)=0

    ehild
     
  4. Feb 21, 2013 #3
    Re: Gauss' Law problem! Help please!!! infinite sheet with charge dens

    so if the surface has a uniform charge density, then the net charge on the surface is zero?
     
  5. Feb 21, 2013 #4

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Re: Gauss' Law problem! Help please!!! infinite sheet with charge dens

    If the added surface has uniform charge density -σ, the net charge density on the circular area is zero.

    ehild
     

    Attached Files:

    Last edited: Feb 21, 2013
  6. Feb 22, 2013 #5
    Re: Gauss' Law problem! Help please!!! infinite sheet with charge dens

    got it, thanks!
     
  7. Feb 22, 2013 #6

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Re: Gauss' Law problem! Help please!!! infinite sheet with charge dens

    You are welcome:smile:

    ehild
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Gauss' Law problem! Help please infinite sheet with charge density?
Loading...