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Electric Fields in Faraday cages

  1. Jun 5, 2006 #1
    Lets say, hypothetically of course, that i know somebody that had something that, with that something being charged, was generating a strong electric field(or maybe electromagnetic waves?) that was emitted in all directions for infinity, if this hypothetical friend were to put a faraday cage around the said object, would there be any field, or waves(answer for both please :approve:) outside of the cage, and if not where would they be, and if they were diverted, would there be a way to, (for lack of better words) to capture or harness, said electromagnetism, and use it?
    (P.S. This is not a homework question, Im actually very curious thank you very much!)o:)
  2. jcsd
  3. Jun 5, 2006 #2

    Meir Achuz

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    Apart from a bit of leakage, all the field lines would end at the metal of the cage. There would be currents induced in the metal of the cage.
    These currents could be used to heat the cage or even drive a motor.
  4. Jun 5, 2006 #3
    Thanks, Thats so Awesome, I cant wait to try it out! Youve been an enormous help :rofl:
  5. Jun 5, 2006 #4


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    What kind of answer were you expecting then ?
  6. Jun 5, 2006 #5
    Well... I wasn't being sarcastic! He gave me the exact answer i wanted :smile:

    So this type of answer was Perfect
  7. Jun 27, 2006 #6
    Were the faraday cage a true closed superconductor then no,there would be no static fields or dynamic EM fields inside it (both theoreticaly and practically).
    In reality there are usually no superconducting faraday cages but only very well conducting metal cages.And small magnetic fields and EM fields might be present inside it but attenuations of the fields are very considerable.
    Of course,there is an interesting situation when a hypotheical friend encloses the door of the superconducting box placed in the dynamic EM field.
    That action may capture EM waves inside such box,with eternal wave reflection from the internal walls ...
    Last edited: Jun 27, 2006
  8. Sep 1, 2006 #7
    OK Help Again please

    Ive come back for continuation of help from the previous problem. This time, instead of leaving quite so much ambiguity, i Will be more specific, but in one detail.
    Ok, again with the hypothetical thing producing EM waves in all directions. And lets say I stuck a metal pop-can around it. Would the can turn the EM waves into current? if so how would I use the current? If it was current wouldn't it have to be going somewhere, instead of stuck in the can? Would there be, say someway to use it to power an LED or something low current?

    If the can wouldnt cant turn it to current is there some way I can?
    Is there some way that I can detect if there is any EM waves emmitted from teh thing producing EM waves? I need to know this stuff. thanks guys :wink:
  9. Sep 14, 2006 #8
    Yes,it would.
    Think of antenna theory :radio receivers, TV sets ...
    What do you think how these devices pick EM signals ?:wink:
  10. Sep 17, 2006 #9
    The thing is that your faraday cage, or the metal can, are conductors, i.e. there are freely moving electrons inside the material. Now, if you subject those electrons to an electric field, they will move around according to the field applied and thereby try to counter the electric field (because they will try to reach static equilibrium). This motion of the electrons inside the conductor is what has been refered to as current.
    But be careful: If you try to make the electrons go somewhere else, that is leaving the conductor, you're in for trouble.
  11. Sep 17, 2006 #10
    Time I mentioned the perfect faraday cage again: a cake tin.
  12. Sep 17, 2006 #11
    If I remember correctly, I do not think this is the case. Set up a problem where you have an uncharged sperical metal shell (your faraday cage) and a charged object in the center. Now if you use Gauss's Law, you'll see that in fact there will be an electric field outside your cage. This is for the static situation.

    For the case of electromagnetic waves, you will get a little current in the cage while the electrons rearrange themselves to cancel out the field within the cage, but it shouldn't completely block all the field from going outside the cage
  13. Sep 19, 2006 #12

    Meir Achuz

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    If there is net charge inside the cage, there will be a static E field outside the cage (by Gauss), but no time dependence. This assumes a perfect conductor. For a conductor with finite conductance, there will be leakage of the time dependent fields for high enough frequency.
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