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: Gauss's Law and E fields

  1. May 10, 2008 #1
    So I'm trying to understand a couple things regarding this.

    lets say we have a plate thats charged to Q. and we want to find the E field of a point .1 meters above the plate. when i looked at the solution, it never took the .1 meters into account when calculating the strength of the E field.

    my roommate says that E field should be the same and thus the distance was irrelevant. so whether it was .1 meters or 1 meter above the plate, the E field would be the same.

    but that doesnt make sense to me when you think about the equation for an electric field E= kq/r.

    can anyone resolve that contradiction for me? :(
  2. jcsd
  3. May 10, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    He's right, at least if you're talking about the field from an infinite sheet of charge. Or a finite sheet of charge at distances much smaller than the dimensions of the sheet. Of course, if the sheet of charge is only 10 cm square, the field 10 m away certainly will depend on distance.
    That's the field from a point charge--a very different configuration from an infinite sheet of charge. (Of course, as you get far enough away from a finite sheet of charge the field begins to look more and more like the field from a point charge.)

    You might want to browse through this site: http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/gaulaw.html#c4"
    Last edited by a moderator: Apr 23, 2017
  4. May 11, 2008 #3
    Yep, because the e-field lines will be 100% perpendicular to the charged (supposedly infinitely wide) plate.
    i.e. going straight up or straight down, so if you bound a gaussian cylinder above and through the plate, the field will go through only the circular discs at the end of the cylinder (and not the 'side walls' of the cylinder) - so it doesn't matter how high the cylinder is (drawing a picture would help heaps)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook