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Why don't neutral atoms distort the electric field?

  1. Sep 14, 2014 #1
    Suppose I have a positively charged conductor with a cavity. There's a positive charge inside the cavity and the system has reached electrostatic equilibrium. Then there's negative charge surrounding the cavity and positive charge on the surface of my conductor. If I apply gauss law to the interior of the conductor I conclude that the electric field is zero at every point of my gaussian surface, and if it weren't so, then charged particles would be accelerating or whatever.

    But my question is: why don't the atoms in the interior affect the electric field? I mean, even if the atoms are neutral (consider now a non-conducting material) they have charges inside that should somehow alter the electric field in the neighborhood.

    What's going on?

    Thanks.
     
  2. jcsd
  3. Sep 15, 2014 #2

    Simon Bridge

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    ... didn't you say there was a positive charge inside the cavity?
    If your gaussian surface enclosed the positive charge, then wouldn't the flux through the surface be non-zero?

    Quite right - the electric field from "neutral" atoms is not exactly zero everywhere. That is an approximation, which holds very well at distances greater than around 10-10m.

    Try this: treat a neutral atom as a shell of negative charge surrounding an equal positive charge ... place a gaussian surface around the entire thing and work out the field.

    However
     
  4. Sep 15, 2014 #3
    If there is a positive charge inside my cavity, and the system has reached electrostatic equilibrium, then there is also negative charge surrounding the interior surface of the cavity so as to make the electric field inside the conductor zero by gauss's law.
     
  5. Sep 15, 2014 #4
    http://www.physics.sjsu.edu/becker/physics51/images/23_20ConductingTube.JPG [Broken]

    Like (c)
     
    Last edited by a moderator: May 6, 2017
  6. Sep 15, 2014 #5
    I'm starting to think that it's more practical to assume that the only things that exist are swarming protons and electrons for these problems. Is this the idea in classical electrostatics?
     
  7. Sep 16, 2014 #6

    Simon Bridge

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    In the diagram you showed me, the brown area is a solid conductor - there is no field inside the conductor.
    The white area in the middle is a cavity - there is certainly an electric field inside the cavity.
    The diagram is not quite accurate - diagram (c) should have more "+" signs on the outer surface than the others.
    But I think I'm clear on what you are talking about now.

    Classical electrostatics treats charges as infinitely divisible fluids.
    This gets modified slightly when you realize that, for normal solids, the mobile fluid is the negatively charged one - but for most purposes it does not matter.

    Electrons and protons belong to the particle model of charge - they are not the only charged particles, so just talk about positive and negative charge unless otherwise specified.

    IRL: atoms can become polarized as well as charged, and they carry magnetic as well as electric fields, thisall makes things tricky ... as you advance in your education you will learn how to handle these things in more detail. Meantime you'll notice that the electrostatics you are doing involves quite long distances and very large numbers of atoms so the small effects of individual atoms will tend to average out.
     
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