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What happens on external surface of the Faraday cage?

  1. Jul 2, 2015 #1

    goodphy

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    Hello.

    Let's say we build the Faraday cage which surround strong radiation such that no radiation can escape to outside world for safety.

    What happens on external surface of the Faraday cage in voltage during shielding? Is it fluctuated?
     
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  3. Jul 3, 2015 #2

    anorlunda

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    Voltage relative to what? You need two points to measure a voltage.
     
  4. Jul 3, 2015 #3

    sophiecentaur

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    What you could get is currents flowing around the outer surface. But they will be very low level if the walls are thick enough and the resistivity of the metal is low enough. There will be substantial currents flowing over the inside surface, of course but they would be highly attenuated on the outside. The main problem with real Faraday cages is the joints (need to be welded to be really effective) and the doors and cable entry points.
    Something that is easy to forget is that a transmitting source inside a metal box may be strongly affected by all the power sloshing around inside the cage (assuming it is not absorbed by a specially designed resistive load). The transmitter would, in effect, be trying to radiate power that it can't get rid of. (A bit like feeding a generator into an open or short circuit when no output power is produced.
     
  5. Jul 3, 2015 #4

    anorlunda

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    Would that be a fair description of the inside of a microwave oven? I used to own one that would melt a hole through the glass tray if it was turned on with no food inside.
     
  6. Jul 3, 2015 #5

    sophiecentaur

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    Yes; that's the sort of thing I meant. A bad mismatch can produce very high levels of standing wave and the voltages in some places can do much damage. People used to say it would knacker the Magnetron but the effect you were seeing must be quite common. As soon as there is some Water or Fat in there, the RF load it represents will dissipate the power and not allow a build up of resonant fields. Modern systems can turn down the power when that condition is detected.
     
  7. Jul 5, 2015 #6

    goodphy

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    Hello.

    I'm sorry to reply lately. Any reference voltage is possible here. As long as voltage measured on certain point of the cage is steady. We could see constant voltage which magnitude is depending on the reference voltage. If not, we could see voltage fluctuation. Let's say we can use some reference voltage such that we can see visible voltage fluctuation if it is there.
     
  8. Jul 5, 2015 #7

    goodphy

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    Thanks for replying.

    Thus I interpreted your reply that radiation taken inside the cage would cause strong current flowing over inner surface of the cage but as they have skin effect, current flowing outside is much smaller thus voltage is not fluctuated much on outside surface? On outside surface what about current which is induced by the charge fluctuation inside the cage? Do you think it is severe if radiation is strong?
     
  9. Jul 6, 2015 #8

    anorlunda

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    Radiation is not a charge carrier. The net charge of the sphere should be unchanged.
     
  10. Jul 6, 2015 #9

    goodphy

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    Time-average should be zero but I'm talking about transient time that radiation is in effect.
     
  11. Jul 6, 2015 #10

    anorlunda

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    The average over the entire surface will be zero, even transiently. If not, where would the charge come from?
     
  12. Jul 6, 2015 #11

    goodphy

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    Well..what about this? If ground point for external surface is only one and voltage is fluctuated over the surface in spatial varying. Thus maybe there is current flow on the ground wire to the point due to induced local voltage rise?
     
  13. Jul 6, 2015 #12

    anorlunda

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    Now you're changing the problem. The OP said nothing about a ground wire. But never mind that. Pay attention to the fact that radiation is not a charge carrier. It can make a neutral region split into plus and minus regions temporarily, but the integral over the surface will remain zero.

    You must define how small charge separation and how short the times you are defining as transient. Displacement of a single electron for a nanosecond is far different than displacing a coulomb for a second.

    You also need to define the frequency and intensity of the radiation, how it is generated, the conductivity and thickness of the cage and the medium inside the cage, to make your question answerable. Saying "strong" is no help.
     
  14. Jul 6, 2015 #13

    goodphy

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    I also wish I would get these parameter precisely, but in practice, It is not easy. Anyway thank for commenting me.

    Okay then I have one quite definite question for you. Does current see net voltage over the surface or they can see local phenomena? I'm asking average voltage over the surface is only thing to drive current or small local change also lead current flow in principle.
     
  15. Jul 7, 2015 #14

    sophiecentaur

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    If you look at this wiki article on skin effect you will see a formula that shows the current drops off exponentially with depth in a conductor. It depends upon the wavelength of the radiation and the resistivity of the metal. To get an answer to your question, you would need to slot in figures for the situation you are envisaging but there are many examples given in articles dealing with skin depth. (Fractions of a millimeter in many cases of RF).
    But the main cause of leakage is unlikely to be skin depth (except for very low frequencies) and more likely to be caused by resistance in joints / seams and the holes through which signals and power are fed in and out.
    An 'ideal' Faraday cage is like a Unicorn.
     
  16. Jul 7, 2015 #15

    sophiecentaur

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    It would be better to talk in terms of Field (Volts per metre) fluctuations over the (imperfectly conducting) surface. In reality, you might need to consider the whole structure but sometimes, 'local' effects could dominate*. The Potential relative to Earth would only be relevant if there were a connecting wire, allowing current to flow away from the box and produce some radiation. But in that case you would be dealing with an 'odd shaped' structure, consisting of both box and wire. But a practical design would probably involve the use of a choke (of some sort) to reduce the currents to Earth to an acceptable level.
    Actually this whole thread seems to be hopping between ideal theory and gritty reality and there is a risk of coming to conclusions that are not valid in reality.

    *em waves can be very tricky and you can find that they sneak along surfaces and round corners, producing radiation in unexpected places. For instance, currents over the edges and onto the back surface can adversely affect the performance of a massive and very directive microwave reflector.
     
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