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Gauss's Law and conductors

  1. Jun 29, 2015 #1
    So I am extremely confused at the moment. We had a hollow ring in physics lab today. There was a positive charge (30V) inside the sphere that was no centered and then the ground was attached to the ring itself (0 V). So, I understand that the negative charges unevenly distribute to cancel the charge on the inside. To my understanding, the positive charges on the outside of the ring would be evenly distributed. Now, I'm not sure if this is correct because my TA said this is only true when the ground is not connected. So, what is the difference in distribution for conductors when there is a ground present and when there is none on both the outside and the inside.

    I understand that when the ground is not present, then the ring will have a positive NET charge and there will be a field on the outside of the ring. When the ground is present, it allows the ring to cancel out the charge and there to be no NET charge on the ring. Am I correct? Thanks!
  2. jcsd
  3. Jun 29, 2015 #2


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    30 V is a potential, not a charge.

    Also, your experimental setup is not clear to me. I might be able to draw a mental picture of it, but I fear it may or may not be accurate. Can you describe in more detail exactly what your setup was? Can you explain exactly what you mean by
    Evenly distributed in terms of what?

    When something is grounded, it does not have a predetermined charge. It will obtain the charge necessary to minimise the total energy of the system while maintaining the grounded conductor at zero potential.
  4. Jun 29, 2015 #3
    Sorry, that is what I meant to say with the potential. Anyways, to be more clear this is the setup of the experiment. We had a metal ring that was screwed to a carbon sheet of paper with wing nuts and screws. Then a screw and wing nut was placed inside the metal ring, but it was not centered. The positive lead went to the non-centered screw and wing nut. The ground went to the metal ring (one of the many screws and wing nuts holding it to the paper). The potential differed within the ring, but did not outside of it. My TA was explaining that since the metal is a conductor, the negative charges are free to flow to the inside of the ring to cancel out the point charge inside of it.

    I am confused as to what happens to the outside surface of the ring in this experiment to cause no potential difference at any point. Also, WHAT WOULD happen if the ground was removed. Would there be a net charge if I drew a ring around the metal ring and used Guass's law, because when the ground is present there is no net charge.

    Thanks for your time!
  5. Jun 29, 2015 #4


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    This is wrong. Connected to ground does not mean no net charge, as I already mentioned in the previous post.
  6. Jun 29, 2015 #5
    You're misunderstanding me. Since the ground is present, it allows for excessive positive charges to flow. This allows, when a circle is drawn enclosing the ring with the charge inside the ring, there will be a net charge of 0.
  7. Jun 29, 2015 #6


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    I believe the electrons distribute themselves in such a way as to cancel out the difference in potential.

    Hmm... my understanding is that if you remove the ground after the electron redistribute themselves, then there's no change . If you don't attach a ground in the first place then you will have a net charge.
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