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Insulator Conductor Concept Question

  1. Sep 9, 2008 #1
    For insulators the excess charge stays on the outside of the object.
    For conductors the excess charge travels through the sea of electrons until the net charge inside the object is zero with the rest of the charges pushed to the outer edges of the conductor.

    So with both the excess charges are on the surface? How can I tell the difference between an insulator and a conductor? And why does the electric field from the charges inside the conductor only go through one side of the conductor whereas an insulator has electric fields from the charges on the inside going through all sides where the electric field is perpendicular to the sides or faces of the insulator?

    Thanks for the help.
    Stephen
     
  2. jcsd
  3. Sep 9, 2008 #2

    LowlyPion

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    You might want to read about electrical conduction:
    http://en.wikipedia.org/wiki/Electrical_conduction

    Charge transport in an insulator is quite low, which is of course why they are employed.
     
  4. Sep 9, 2008 #3
    I read through the article but it really did not answer my question:

    So with both an insulator and a conductor the excess charges are on the surface? How can I tell the difference between an insulator and a conductor? And why does the electric field from the charges inside the conductor only go through one side of the conductor whereas an insulator has electric fields from the charges on the inside going through all sides where the electric field is perpendicular to the sides or faces of the insulator?


    Anybody have any ideas?
    thanks
    Stephen
     
  5. Sep 9, 2008 #4

    Dick

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    Charges in an insulator don't have to be at the surface. They can also be inside the insulator if they were placed there. There is no electric field inside a conductor at steady state because if there were then the charges would be moving, and it wouldn't be at steady state.
     
  6. Sep 10, 2008 #5
    Do my definitions of the insulator and conductor describe how the charges are dispersed in the material:
    For insulators the excess charge stays on the outside of the object.
    For conductors the excess charge travels through the sea of electrons until the net charge inside the object is zero with the rest of the charges pushed to the outer edges of the conductor. Is the net charge of the conductor zero, with charges on the surface? Do the charges on the surface affect the net charge of the conductor?
     
  7. Sep 10, 2008 #6

    Dick

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    That's fine for the conductor. And, of course charges on the surface affect the net charge. The ARE the net charge. For an insulator charges stay where ever you put them. If you happen to deposit them on the surface, fine. But you could put them inside the object as well and they will stay there. They aren't mobile. With real materials your mileage may differ. Electrons that are bound to atom in the neutral material may be completely immobile, but excess added electrons may have some mobility.
     
  8. Sep 10, 2008 #7
    so in a conductor do the sea of electrons push the excess charge to the surface, allowing the net charge inside to be zero?
     
  9. Sep 10, 2008 #8

    Dick

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    The excess charge tries to move around to minimize potential energy coming from the electrical repulsion between the charges. The solution to that is to move it to the surface and spread it out.
     
  10. Sep 10, 2008 #9
    once the charges are spread out, is the net charge zero?
     
  11. Sep 10, 2008 #10

    Dick

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    No. If you have an excess charge, spreading it out won't make it zero. How could it?
     
  12. Sep 10, 2008 #11
    so if you charge an insulator and a conductor

    the charge will stay on the outside of the conductor where you charged it.
    The charge will go through the conductor's sea of electrons until it is spread through the surface of the conductor?
     
  13. Sep 10, 2008 #12
    the charges in an insulator are bound. they only move a very short distance. nevertheless the net effect of all the electrons moving this very short distance is exactly the same as the effect in a conductor except that not all of the electric field is cancelled.
     
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