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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

    Astronuc

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

    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/gaulaw.html#c2

    Hyperphysics

    For Gaussian surfaces -
    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/gausur.html#c1

    One can put a Gassian surface just outside a distributed charge.
    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/gausur.html#c3
     
  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

    Galileo

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    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

    Astronuc

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    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

    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/gaulaw.html#c4
     
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