Talk:Faraday cage

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[edit] Are electromagnetic waves (aka light) blocked by a Faraday cage?

Are electromagnetic waves blocked by a Faraday cage? Electromagnetic waves like the ones used by cell phones and microwave ovens are essentially light in different frequencies. They are different from a electric field. Would loss of reception of cell phone in elevator and microwave oven be right examples of a Faraday cage?

Yes, and no. The difference between light and say cell phone microwaves, as you pointed out, is in the frequencies. Now, the wavelength and frequency have a relationship (in electromagnetic waves), which can be stated as, "wavelength multiplied by frequency is equal to the speed of light" (you can rearrange this equation to solve for wavelength or frequency). Ok so getting to the point: the holes in the faraday cage are too small for the wavelengths to pass through. So, being that light has much, much smaller wavelengths than say microwaves or radio waves or whatever else might be used for communication, it passes through the faraday cage easily. In conclusion most forms of electromagnetic waves are blocked by faraday cages, and I would even venture to say that 'all' traditional forms of communication using electromagnetic waves are blocked by a faraday cage (now as a small point there have been attempts to make a communication device which utilizes lasers to pass the information on, and these would not be blocked by a faraday cage, but these devices are usually considered impractical because it is necessary for there to be a line of sight for the device to work). Hope this answers your question. Mjs072 (talk) 02:23, 3 April 2008 (UTC)

[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

No. Magnetic field is not blocked by a Faraday cage. I discovered this the hard way as a student when someone re-located a CRT computer monitor in an adjacent lab. So their computer monitor was 2-3 feet away from the experiment area, but on the opposite side of a wall -- took forever to figure out where the interference was coming from....

—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)


Faraday cages do not block DC or slowly-changing magnetic fields. A magnetic compass will work inside a Faraday cage.
From <a href="http://en.wikipedia.org/wiki/Maxwell%27s_equations">Maxwell's Equations</a>, however, a changing magnetic field creates an electric field (and vice versa), and the walls suppress the electric field. They therefore also suppress the magnetic field. Whether the changes are "fast" (and therefore suppressed) or "slow" (and transmitted) is set by the wall materials' thickness and <a href="http://en.wikipedia.org/wiki/Skin_depth">skin depth</a>.
Faraday cages large enough to people, furniture, etc. are often called "screen rooms".
They're also used in pulsed high-voltage experiments to protect sensitive electronics from electromagnetic pulses created by the experiment.
I'll note all this in the article.SMesser 01:09, 6 June 2007 (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

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)

I can't argue with any of that.
  • Static magnetic fields are not shielded at all by a Faraday cage.
  • Far fields are those that have an E and an M component at right-angles to each other and to the direction of propagation, like EM waves in space. The wave impedance of free space is 377 ohms, while that of a conductor is low (milliohms, and a complex number [1]). If you short out the E component with a conductor, the M component gets clobbered too, since the two components generate each other. In near fields the two components aren't related in a simple way, so you have to consider them separately.
  • The attenuation of a hole is proportional to the ratio of wavelength to diameter. There isn't a sharp cutoff. This source [2] (table on p2) shows that at half a wavelength you get -3 dB, so you shouldn't be surprised to get some leakage.
  • "For circular apertures which have diameter much smaller than a wavelength, the radiation coupled through is proportional to the sixth power of the radius." [3] (right at the end), hence the microwave oven mesh. This seems to contradict the previous point, so I suppose the two statements apply to different ranges of λ/d.
  • A thought experiment. If the cage were superconducting, it would keep out very LF magnetic fields, and even switching on an external DC electromagnet would count as changing the magnetic field, so the eddy currents induced in the cage would circulate for ever, cancelling out the field. --Heron 17:05, 26 May 2007 (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)

In general it will not, yes. This lightning safety sheet calls a car the second safest place to be in a storm, after a proper building. Just make sure you don't touch any conductors connected to the metal body of the car. Algebraist 16:57, 14 November 2007 (UTC)

[edit] Fry's Electronics

The article mentions that small electronic parts at Fry's are kept in a cage, inferring that the cage is used as a Faraday shield. What I've seen at any Fry's I've ever been to is nothing but a security cage made of chain-link fence. There would be no reason to keep parts in a Faraday cage; there would however be a reason to keep them in a secure area; CPU's and memory are valuable and easily stolen. 24.165.87.76 03:01, 14 May 2007 (UTC)

[edit] Faraday Suit

I saw a request for a picture of a Faraday Suit. This is a Russian "Radar Suit" that I photographed at Hack Green Secret Nuclear Bunker museum some years ago. Not sure if it is what is needed for the page. --jmb 15:12, 8 July 2007 (UTC)

Russian "Radar Suit"
Russian "Radar Suit"

Is this a faraday suit? The image is Creative Commons, so is suitable for uploading here. MarašmusïneTalk 17:00, 29 July 2007 (UTC)

[edit] Comments

My thoughts: To get a "Faraday's cage effect" it can have big holes and must not be metal as long as it has low resistance (metal helps both because of thet the charges inside the metal extremely quickly re-distributes which in total makes the field 0 inside the cage, but also might help because of the low resistance since you get a voltage division vs the source), and it certainly does not have to be grounded. See my comments on "Antistatic bag" too. --88.131.22.138 (talk) 14:59, 11 March 2008 (UTC)

[edit] Microwave oven as shield?

At a business that was located across a two-lane street from one cell providers antenna, several employees including myself did an experiment on our lunch break that involved sticking the phone inside of the microwave, with it powered off the whole time, and seeing how many bars remained and/or calling the phones. Most phones went to out of service. Two rang, including the phone that matched the service provider on the antenna farm across the street. The matching-provider phone kept 100% of its bars during the entire time it was inside the microwave.

The cellular band (1.8, 1.9GHz) isn't too far from the 2.5GHz used to actually cook the food. It does a pretty mediocre job of keeping these larger wavelengths out, but yes it did in fact block them somewhat. Just not as well as would be expected if the device were designed to do such, considering how frequently the signal is blocked unintentionally.

It seems like we should be able to get a citation on exactly how much of a shield a microwave oven is, or at least ought to be, considering that we are allowed to have 1W WiFi amplifiers on bookshelves and these ovens are usually between 400 and 1200 watts; I would think that somewhere, there ought to be a published guideline for shield effectiveness. —Preceding unsigned comment added by Zaphraud (talk • contribs) 20:27, 9 May 2008 (UTC)