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Dumb question about conductors in electric field

  1. Aug 19, 2015 #1
    I've read multiple times that electric field inside of the conductor is always equal to zero because the inner field produced by electrons that moved to one side and left protons on another balance the outer field.
    I've also read about Faraday cage.

    But I have a few questions about this:

    1) So it means that current can only flow on the surface of conductor no matter what?How can electric field be produced on the surface if it is always zero inside then?

    Or maybe this is only for electrostatics?But I don't understand how Faraday cage works if clearly there is current on it.

    I've also read that dielectrics polarize when they are placed inside of the electric wave but the dipoles are not able to create an inner field that completely balances the outer field,they say that it can only weaken it but not balance it.

    2) K,sure,but then it means that in dielectric current can also flow through the inside of the dielectric unlike at the conductor?I don't understand this...

    Sorry for dumb questions guys.
     
  2. jcsd
  3. Aug 19, 2015 #2

    mfb

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    To a good approximation. Real conductors are not perfect, and time-dependent fields lead to deviations as well.
    Current flows in the inside as well. You just don't get charge concentrations there.

    The Faraday cage works fine with currents on the surface only.
    Right.
     
  4. Aug 19, 2015 #3
    I've read from some page of the site that this works only for electrostatics,when charges are in equilibrium,so if there is current flowing through conductor,there WILL be electric field inside of the conductor.I assume that's correct,right?

    But I cannot understand how can they explain how Faraday Cage works with electrostatics if the cage is clearly under the current,it is flowing through the cage.
     
  5. Aug 19, 2015 #4

    mfb

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    If the conductor has a finite resistance and some current is flowing, there is an internal field, yes. This field keeps the current flowing. It is observable as voltage drop at this resistor.
     
  6. Aug 19, 2015 #5
    That's right,but how can the Faraday cage principle be explained with electrostatics if there is an obvious current going through the cage,so it means that it is supposed to get inside of the cage,no?
     
  7. Aug 20, 2015 #6

    mfb

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    Who suggested that?
    You can describe the shielding of static fields with electrostatics (where you ignore any transients and look at the equilibrium situation only), but not the shielding of time-dependent fields.
    The current flow can happen at the outside only, it depends on the cage.

    I don't think mixing cables and Faraday cages helps here.
     
  8. Aug 20, 2015 #7
    They always explain it the same way on every single page:

    "There is an external (outer) electric field that (as an instance) is pointed to the right,electrons on the other hand move to the left and protons stay there where they had been before the electrons moved (at the right side) and so electrons with protons create their own electric field that is opposite to the outer electric field,thus canceling it in a matter of split seconds"

    But there WAS current inside it's just it was cancelled very,very quickly,that's what makes me puzzled.
    If you attach a battery to the cage there is a constant current going through the cage,it just never ends.It's not like it's an open circuit,it is definitely closed but the body still does not get electrocuted.That's what I can't understand.

    There IS a current inside but it gets cancelled by the internal field in split second,but if there is a generator then the current is constant,no?
     
    Last edited: Aug 20, 2015
  9. Aug 20, 2015 #8

    mfb

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    Not necessarily "inside" in the same way as "in the interior of the material".

    Again, I think mixing cables and cages just leads to confusion.
     
  10. Aug 20, 2015 #9
    I know right but I've seen this video where a man was wearing full metal equipment (I think it was Aluminium or something) and well he was being zapped with the Tesla coil but he felt nothing,that's what I don't understand.
    The current clearly goes through the entire equipment including the inner part where his body is kept but he feels no pain of whatsoever.

    Same with the Faraday cage - touching it from the inside deals no harm to you.

    Even in electrostatics it clearly says that it happens very,very quickly but it is still a current.
    I'm sincerely sorry for annoying you,it's just this makes no sense to me...
     
  11. Aug 20, 2015 #10

    mfb

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    No it does not. It is limited to the parts that are good conductors - the suit around the man.
    Yes, because there is no static net charge on the inside.
    In electrostatic nothing "happens", there are no timescales involved, otherwise it is not electrostatics.
     
  12. Aug 20, 2015 #11
    So the fact that Faraday cage does not conduct electricity from the inside can only be explained via electrostatics and not via electrodynamics?

    It's just if we look at it another way via electrodynamics there is current...it's just it seems like this electrostatic explanation is some kind of an excuse,they all explain it the same way but ignore the fact that there is indeed current flowing through the entire cage but somehow there is no electric field inside even though there is current...

    Maybe I'm dumb but is not in reality there is current?It is almost impossible to prove the electrostatic explanation in real life,they make it look like it's so perfect...
     
  13. Aug 20, 2015 #12

    mfb

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    How did you get that impression?
    Electrodynamics is more powerful than electrostatics. Electrostatics is a special case of electrodynamics, it can be a useful simplification in many cases.
    Both allow to explain why there are no net charges on the inside (this has nothing to do with current flow!).
    Both allow to explain why the current flow through the human is tiny (but you don't need a cage for that).
     
  14. Aug 20, 2015 #13
    Could you give me your own explanation of why there is no net charge inside the cage with both electrodynamics and electrostatics then?I just cannot understand why it has nothing to do with the current flow if it clearly exists inside for a brief moment.

    The electrostatics explanation is rather simple,especially the part where it clearly states that the inner field balances the outer field but it was the inner field that moved the electrons to one side in first place.

    I'm truly sorry for annoying you,it's just this explanation does not answer why a man in metal costume feels no current when it zaps his costume,even if the inner electric field balances the outer field,I mean that it happens after a very short period of time,so there was a current for a very brief moment but only after that moment it was balanced,the current disappeared.
     
  15. Aug 20, 2015 #14

    mfb

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    If your external field does not change, the charge configuration in the conductor quickly (nanoseconds) reaches an equilibrium. And we don't care about nanoseconds here.
     
  16. Aug 20, 2015 #15

    Dale

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    Are you familiar with circuits? What happens when you put a big resistor (person) in parallel with a short circuit wire (metal equipment)?
     
  17. Aug 20, 2015 #16
    Well I'm kinda familiar with them...well the connection in this circuit is neither parallel nor series,so current does not split,no?So the current is just supposed to go by the wire to the resistor and then come back to the generator by the other side of the wire,no?But since the circuit is short then it's gonna be happening very rapidly.Oh wait...does this somehow have something to do with the fact that there is less current in resistor because it's resistance is much higher than wire's?But is not it still going through the resistor just with much lower power?

    It's just what I don't understand is that you guys say that conductor's electrons reach equilibrium very quickly so we just ignore this current and pretend that it does not exist inside but is not current actually flowing like this?It's just the potential difference is being constantly created by the battery,thus not allowing the current to stop flowing,like a never ending cycle.

    I DO understand why electrons move to the positive side of the outer field,I do understand why they create an internal electric field that cancels the outer field thus electric field inside of the conductor is zero but what I cannot understand is how can it be like that if there is obvious current flowing through the conductor,is not this basically supposed to mean that current cannot flow inside of the conductor at all if it flows so rapidly that we ignore it?
     
  18. Aug 20, 2015 #17

    Dale

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    The connection is parallel. The suit is a short circuit for any current that would go through the person.
     
  19. Aug 20, 2015 #18

    Dale

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    Yes. This type of circuit is called a current divider. The current splits between the two paths according to the ratio of their conductances. So the highly conductive suit takes almost all of the current, while the much less conductive human is unharmed.
     
  20. Aug 21, 2015 #19
    Makes sense,i understand that due to Ohm's law human body takes much less current since the most current is at the low resistant part of the suit,the costume itself.

    But here is the hard part that my brain is not able to comprehend:

    Someone is inside of the Faraday cage or inside of the suit,the current goes through the suit or the wire (so charges ARE indeed moving and we cannot just say that the electric field inside of the conductor is zero cause there IS current) , that someone touches the cage from the inside or his entire body is touching the suit,why does not that someone feel current going through his body then?Why does that someone feels the current only when he/she/it touches it from the outside and not inside?

    There is indeed current going through the metal costume/cage so the charges are NOT in equilibrium,so we cannot assume that electric field inside of the conductor is zero,so we cannot assume that this example is about electrostatics because there is indeed movement of charges.

    How can this be explained when we assume that current is moving through the conductor AKA metal suit or Faraday cage?
     
  21. Aug 21, 2015 #20

    Dale

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    Because then they are in series with the suit instead of in parallel.
     
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