Talk:Faraday cage
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[edit] Is a magnetic field blocked by a Faraday cage?
For instance, will a magnetic compass in a Faraday cage still point toward magetic north? Suggest such knowledge be added to the main page. Andrew8
—The preceding unsigned comment was added by 203.38.122.195 (talk) 01:34, 12 March 2007 (UTC).
[edit] Faraday cage = a metal enclosure?
Is it just me or is this Faraday cage just a metal enclosure? I suggest someone add to the article that a Faraday cage is actually just a metal enclosure. There seems to be an epidemic in science of easily-reducible-to-a-generalized-name concepts still being called by their original discovers or inventors' last names even one hundred years after they discovered or invented it. I think 30 years of calling a new discovery by the last name of its discoverer is appropriate but not one hundred years plus. Foober 22:18, 18 February 2007 (UTC)
- It's more than just a metal enclosure. It depends upon the frequencies that you are considering - for example bridges are Faraday cages at radio frequencies but certainly not at optical frequencies.Flying fish 02:26, 6 March 2007 (UTC)
- No, it need not be metal. Conducting non-metalic material can also create a Faraday cage. Andrew8 —The preceding unsigned comment was added by 203.38.122.195 (talk) 01:32, 12 March 2007 (UTC).
[edit] Old relocated conversations
The last part of this article is written in German!
- The bit about holes is inaccurate: EM radiation with a wavelength comparable or smaller to the size of the hole gets through easily. That's why the mesh in microwave oven windows has to be so fine. The Anome
-
- I think 'hole' refers to the inside, not to a hole in the mantle. Patrick 14:03 Nov 25, 2002 (UTC)
This whole article has a "subtly wrong" feel to it: it conflates several things as if they are one, such as:
- ground plane
- Faraday cage principle (exclusion of electric field)
- preferential / sacrificial conductor
- skin effect
- RF shielding
Can people with a physics / EE background please take a look at the article and try to sort this out?
Perhaps we need to break this article into several sections:
- Idealised Faraday cage
- Mathematics of the Faraday cage
- Faraday cage in practice
- --> article on Electrostatic shielding and earthing
- --> article on RF shielding and earthing
- Starting to rework: but it's still wrong... hmmm... The Anome
"Since like electrical charges repel each other, they accumulate where they can be furthest apart, on the outside layer of the Faraday cage." ... This is a description of the electrostatic principle that the electric field inside a conductor is zero; it is not applicable to changing EMFs. I'm rewriting the paragraph... --hb
The excited fields oppose the applied fields, in concord with the law of conservation of energy.
- The relationship is not clear to me. Patrick 11:11 Nov 26, 2002 (UTC)
a current that cancels the applied field inside the conductor
- This may create the impression that as long as the electric field is outside, the current flows to cancel it inside; however, in the case of a static field, the charge distribution (+ here, - there) takes care of it, the current is there only very shortly to create this charge distribution. Patrick 11:56 Nov 26, 2002 (UTC)
I agree that the article seems to include stuff that isn't strictly within the original definition of the Faraday cage effect. I added some historical info at the beginning to show where the idea started. Perhaps with more research we can trace how the concept has been broadened to RF etc., and whether the later additions deserve to be part of this article. -- Heron
The concept doesn't have to be "broadened" to include RF. It includes all electrical energy. In practical F Cages, instead of smooth walls, a mesh is used to save metal/cost (or for visibility), and in that case the wavelength of the electrical energy becomes significant vis a vis the mesh size; obviously, the EM energy at the optical wavelengths are meant to get through. If the cage is completely closed and smooth (e.g., a closed sphere), classical EM theory predicts zero field inside everywhere. Of course, quantum considerations change everything...
There has been some back and forth about FCs on (ironically) the tin-foil hat page. Take a look at the discussion (and the talk page) for some more information about skin effect, partial FCs, etc., that could be usefully integrated.
What exactly is the difference between the concept of a Faraday Cage and the Skin Effect? They both seem very very simular, except that the skin effect is based on alternating current.
I have read that the real reason why a person is safe in a car during lightning is the skin effect, as a lighning bolt acts as alternating current, although only one half wave of it.
[edit] Contradiction tag added
The first paragraph seems to contradict itself, defining a faraday cage as shielding from electromagnetic radiation and then talking about Michael Faraday experimenting with electrostatics. Obviously the cage will shield both, but this does not make clear what the definition of the term is (i.e. perhaps it was initially an electrostatic shield and then discovered to also be an electromagnetic shield). Either way, as mentioned above, the article could probably do with a rewrite by someone with a background in the area. --postglock 08:36, 18 July 2005 (UTC)
- I hope interested readers can discern that the Postglock's post was addressed shortly after his post. The article is correct. See also skin effect for the RF (i.e. Electromagnetic version) --Ancheta Wis 00:01, 18 December 2005 (UTC)
[edit] magnetic fields
needs more details on the differences between magnetic, electric, and electromagnetic fields, frequency, near field vs far field, and which of those a faraday cage shields against. and real-life, resistive cages vs ideal perfect conductors. — Omegatron 03:22, 22 December 2005 (UTC)
Is this description correct?
Magnetostatic fields would not be blocked by such a shield, but changing magnetic fields would create eddy currents in the surface that create their own fields and cancel them out, so the magnetic shielding gets more effective at higher frequencies?
Changing electric fields would not be blocked as much as electrostatic (which are blocked perfectly), since the charges have to move around to cancel them out, and charges can't move instantaneously, so the shield would be less effective against electric fields as frequency increases? — Omegatron 16:10, 23 March 2006 (UTC)
[edit] Do true Faraday cages need to be grounded?
It seems that a Faraday Cage requires a completely enclosed cage so that there are clear "inner" and "outer" surfaces. However, one of the links on the bottom goes to a site that talks about grounding a conductive shield and specifically says that the Faraday Cage does NOT have to closed on all sides. It seems to me that if the cage is NOT closed on all sides, then the conductor has only a single surface and the electrical charge is free to propogate to any point on that surface. (The linked site also credits scientists working with Einstein or at least in the same era for creating the first Faraday Cage, but I digress.)
So is this a different property than Faraday's enclosure? Is this more simply, shunting the electrical charge away and dissipating it to ground? Does a Faraday Cage need to be grounded? Is a Faraday Cage more effective either for blocking electrical fields or as an RF shield if the cage is grounded? If the cage is not fully closed on all sides, is it a "Faraday Cage"? --JJLatWiki 16:08, 8 June 2006 (UTC)
- A Faraday cage does not need to be grounded the shield the inside from external EM. The reason cages are connected to an internal circuit's ground is to prevent the cage from capacitively coupling parts of the circuit to each other, and to prevent crosstalk from cables. Connecting the shield to ground causes them to capacitively couple to ground instead. There's a very clear description and picture in ISBN 0-7506-7403-2, which we should mimic. — Omegatron 06:26, 25 November 2006 (UTC)
[edit] Vehicle safety
Added to the list item indicating occupants in a car were safe from lightning. The truth is more conditional than that and cars don't provide complete safety.
See http://www.lightningsafety.com/nlsi_pls/vehicle_strike.html (National Lightning Safety Institute) for example.
-Michael
Question: What does this comment "Faraday cages are not good to use outside in the event of a lightning strike." mean? The cited source does not support the assertion. Perhaps it can be reworded. --69.3.237.169 21:03, 24 November 2006 (UTC)
- Is protection from electrical currents really an aspect of a Faraday cage? I mean, I know that Faraday cages protect against currents, but is that really relevant? The key point of Faraday cages is that they prevent electric fields from penetrating. Protection from currents is related more to the skin effect, no? — Omegatron 07:41, 25 November 2006 (UTC)
[edit] Mesh size; explanation; magnetic fields
The intro says that Faraday cages keep out "electromagnetic fields". Do they indeed keep out magnetic fields? Naively, I would assume that they keep out electric fields, and therefore electromagnetic radiation, but not necessarily magnetic fields (unless the conductor happens to be magnetizable, but that's not part of the definition of a Faraday cage).
The intro gives as core reason for the operation of a Faraday cage that repelling charges will accumulate on the surface of a conductor. While that is true, it doesn't explain how a Faraday cage blocks an electric field. The explanation further down in the article (moving charges creating a field that cancels the original one inside the cage) is much more to the point. I would argue to remove the intuitive explanation in the intro, or replace it with a better one.
I'm also interested in the relationship between the mesh size of a Faraday cage and the wave length of the radiation that it blocks. Does the mesh size have to be significantly smaller than the wave length? My little hand-held radio can receive radiation with wave length of over 3m just fine in a car (whose mesh size as a Faraday cage is smaller than one meter). AxelBoldt 21:40, 19 September 2006 (UTC)
- I don't understand this either, and I should. From what I understand, a Faraday cage keeps out static electric fields but not static magnetic fields. It also keeps out changing electric fields, though not as well, and worse at higher frequencies, due to resistance of the material, and also blocks changing magnetic fields, better at higher frequencies, due to induced eddy currents. And those are all near field. Far field EM I don't know about, and I'm sketchy on the difference between near and far field anyway.
- But that's just a hunch/gut feeling kind of thing. I could be thinking about it wrong. I'm going to look in some books. — Omegatron 22:40, 24 November 2006 (UTC)
Summary of facts from ISBN 0-7506-7403-2:
- Near field shielding
- Electric fields
- Created by high voltage/low current devices, like dipole or monopole antennas, sparks, or high-impedance circuits
- Wave impedance is very high, while conductor's impedance is low, so
- Shielding is perfect for an electrostatic field, diminishes as frequency increases
- Magnetic fields
- Created by motors and loop antennas (and power transformers?)
- Wave impedance is low, while conductor's impedance is low, so
- Not much reflection occurs; shielding must be by absorption
- Needs to be several skin depths thick
- Shielding gets better at higher frequencies
- At low frequencies, even absorption shielding is useless.
- For low frequencies, use high-permeability shielding to re-route the magnetic field lines, like mu-metal
- Electric fields
Summary of facts from ISBN 0-521-37095-7: Low frequency magnetic fields are not shielded well by metal enclosures. Instead, try not to create loops in the circuit layout, and if you do, don't let them cover a large area. Use twisted pairs to connect things, since they minimize loop area and each successive twist picks up the opposite field of the last, canceling out. Can also use mu-metal shielding. — Omegatron 07:11, 25 November 2006 (UTC)
[edit] Recording Studios
I've heard that some recording facilities use a Faraday Cage. Can anyone verify this or provide more information? Mikebritt UTC
[edit] Lightning vs. a car?
A bolt of lightning has over a billion volts and a lot of amps, but if it hits a car, it won't penetrate through it, and kill the people inside? 64.236.245.243 14:35, 1 March 2007 (UTC)
