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B Question about understanding conductors for EM course

  1. May 21, 2016 #1
    So in conductors, the electrons will distribute themselves to the surface via repulsion forces. But why do we say that the electric field inside is zero? If I put a positive charge inside, clearly it will move in some direction from the electric field of generated from the electrons. Also, are the electrons actually stationary on the surface (i.e. have a net zero force due to each other), or are they moving but have a net effect of zero?
     
  2. jcsd
  3. May 21, 2016 #2
    It appears you are conflating two different experiments/conditions?
     
  4. May 21, 2016 #3

    Dale

    Staff: Mentor

    It is only zero in the electrostatic condition. I.e. given that a system is electrostatic then ##J=0##. In a conductor ##J=\sigma E## so ##E=0##.
     
  5. May 21, 2016 #4
    Okay, so the electrons repel each other by repulsion due to each other's E fields, then end up stationary at the surface of the conductor. At this point, the parallel E fields along the surface cancel out to 0, and all that's left are the perpendicular ones. So since there are no electrons below the surface, there is no E field below the surface?
     
  6. May 21, 2016 #5

    Drakkith

    User Avatar

    Staff: Mentor

    The electrons only arrange themselves on the surface if there are excess electrons. IE, if the conductor is negatively charged. If your conductor is an infinitely long cylindrical wire, the e-field from the electrons cancels out in every direction except radially outwards from the surface. Inside the e-field cancels out in all directions, including radially. If it didn't cancel out, you'd have an e-field set up inside the conductor and the charges would move until the e-field is zero.
     
  7. May 21, 2016 #6
    So in a neutrally charged conductor, the electrons are still attached to the atoms?
     
  8. May 21, 2016 #7
    They could cancel out like this? i.e. field lines bend upwards- won't the bottom parts contribute to a net downwards field too?
    Figure_20_04_05a.jpg
     
    Last edited: May 21, 2016
  9. May 21, 2016 #8
    Alright I understand now; There is a net zero electric field inside a conductor in static equilibrium. If you bring in an electron, it will throw off all the other electrons, and they will rearrange themselves. On a related question, shouldn't the electric field right on the surface of a conductor be zero?
    e18b0f39-785a-4280-b7ef-0749cf6727d7.gif
    Shouldn't there be a discontinuity at x=r?
     
  10. May 22, 2016 #9

    Drakkith

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    Staff: Mentor

    Most are, yes. Some are free and move about the conductor. The exact number depends on the material the conductor is made out of. While these free electrons are able to move around, their net motion cancels out overall, so you don't see current flow or charged areas in a neutral conductor.

    That only works for two charges. There are MANY individual charges. So many that there isn't anywhere for the field lines to "bunch up" like you see in that picture.

    I don't think so. On the surface means that it isn't inside the conductor, so the electric field shouldn't be zero there.
     
  11. May 22, 2016 #10
    So only the excess (net charge) distributes themselves on the surface, while the net charge inside is 0. This is why there is no net E field in the conductor.

    The E field inside a neutral conductor is also 0 because there is a net zero charge meaning a net zero field.
     
  12. May 22, 2016 #11

    Dale

    Staff: Mentor

    Again, only in the electrostatic case.
     
  13. May 22, 2016 #12
    Yes. Looking at this picture though, there seems to be also perpendicular field lines extending into the conductor (from the surface charges)- do these cancel out each other too?
     
  14. May 22, 2016 #13
    Great!
     
  15. May 22, 2016 #14
    If I try to apply Gauss's law here, the Gaussian surface will be right on the charges, so there seems to be no enclosed charge?
     
  16. May 22, 2016 #15
    The "surface" is an idealization - something that is infinitely thin and yet contains any excess charges assumed to be present. What happens exactly at the surface is more of a philosophical question. For instance, if the surface is "right on the charges," then some parts of the charges are within the surface and some parts are outside. That would lead to a transition regime to replace the vertical dashed line in your plot.
     
  17. May 22, 2016 #16
    How do we calculate the field at a point on the surface? So would Gauss's law still hold, and we would use the radius of the sphere?
     
  18. May 22, 2016 #17

    Dale

    Staff: Mentor

    It is discontinuous on the surface.
     
  19. May 22, 2016 #18
    Yes, over any reasonable distance scale, but the OP is asking - maybe unnecessarily - about the E field in a small region at the "surface."
     
  20. May 22, 2016 #19
    Right, it's much clearer now. A quick question, why do we experience electric shocks (i.e. when touching metals) if the human skin is not a great conductor? Lets say I build up a net positive charge from friction, then approach a doorknob. My finger's electric field will induce a negative charge on the doorknob, and when I touch it the electrons flow through my finger, and I feel a shock. Skin is not a great conductor, also the rubber shoes are not great either.
     
  21. May 22, 2016 #20
    Okay I understand now, but one thing is still incomplete- the field lines would also point inwards as well as outwards. These inward lines add up to a field inside right?
     
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