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' law again

  1. Mar 2, 2006 #1
    why is it that a gaussian surface cannot pass through discrete charges ... but can pass through a continous charge distribution??
  2. jcsd
  3. Mar 2, 2006 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    I believe technically, the Gaussian surface must enclose the charges, because Gauss's law applies to the enclosed charge.



    For Gaussian surfaces -

    One can put a Gassian surface just outside a distributed charge.
  4. Mar 3, 2006 #3
    i mean ... yes, the gaussian surface should enclose charges .. but the surface should not pass through a discrete charge.... but it can pass through a continous charge distribution ... why is that???
  5. Mar 3, 2006 #4
    id assume its fairly hard for it to pass through a point charge as you take the "size" of the charge to be 0 and therefore it cannot have anything pass through it, a continous charge distribution however has a size and then the surface can pass through it
  6. Mar 3, 2006 #5
    may i know what is distribution charge?
  7. Mar 3, 2006 #6


    User Avatar
    Science Advisor
    Homework Helper

    The electric field of a continuous charge distribution is everywhere defined, while the field of a point charge doesn't exist right at the location of the point charge. I believe that singularity is what invalidates the use of Gauss' law when the surface passes through it.
  8. Mar 3, 2006 #7
    thanks for the replies ... maybe it is due to the fact that the field due to a charge does not exist at the place of the charge.

    Thanks once again!
  9. Mar 3, 2006 #8


    User Avatar
    Staff Emeritus
    Science Advisor

    Simply a collection of charges, but mathematically we treat them as 'uniformly' distributed rather than discrete units.

    In a line conductor, the charge is 'linear charge density' or charge/unit length (e.g. C/m).

    On a surface conductor, one takes of charge/unit surface (e.g. C/m2) area as though the charges were 'smeared' on the surface. In a conductor, the net charge occurs near the surface because this represents least energy, and the charge units (electrons) are mobile.

    In a dielectric, the charges are much less mobile and one can have a volumetric distribution (even non-uniform) of charge.

    See - http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elefie.html#c1

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook