I guess a stupid question, but the Wikipedia article is not clear on this point. If we want to have a Farday cage to keep out radio waves and similar, does it matter if it is grounded or not?
Not for keeping out electromagnetic radiation, no. I’m trying to come up with a situation where it would matter to something inside the cage, but can’t think of one.
Ditto ZenBeam. In fact, I think you couldn’t tell from inside a Faraday cage whether it was grounded. Said in a way that better helps bring insight to the question, I think experiments conducted entirely inside a Faraday cage would not be able to tell about a distant outside location whether objects there are connected to the cage. “Ground”, after all, refers to an electrical connection to some small part of planet Earth, which itself is partly somewhat conductive (especially the oceans) but partly pretty nonconductive (the crust, rocks).
So the EM radiation causes an electric current that races around the cage and eventually loses its energy to heat due to resistance, and you don’t need to ground the cage?
Some would be lost as heat, but most of it would scatter or reflect off of it. The EM wave doesn’t have any charge, so there’s no charge that you need to bleed off with a ground wire.
If you have a situation where the cage is protecting the inside from lightning or some other charge flow, then grounding could matter. Even then, the field inside will be zero, unless the current flow overwhelms the conductivity of the cage.
Would not the ungrounded cage re-radiate the EM wave much like how a metal detector works(The EM wave would induce a current which would induce a EM wave.)? So the cage would not allow the original EM wave through but allow the re-radiated wave unless grounded.
In my many, many years of electronics manufacturing and quality auditing experience I have never heard of a grounded Faraday cage for storing, handling, or shipping electronic components.
The field inside the cage would be flat, and there would be a gradient of zero (unless you were also generating fields with some power source inside). I think you could look at the surface of the cage as reflective, both for radiation that originates inside it and radiation that originates outside it. You could say any wave induces a current that induces another wave, so it reflects; or you could say it was a reflector, and there is another virtual universe on the far side of it, like you can think of mirrors in optics.
There is a bit of a head-scratcher for me, though - if a wave causes a current which reradiates another wave, is the current so close to the surface that the wave is only reradiated back out on the same side of the cage wall? If the current circulates throughout the depth of the cage wall, why aren’t there equal radiations coming out of both surfaces (inner and outer)? I guess the current must circulate at so shallow a depth that the radiation can only be out of the same surface. I feel ashamed, too, because I should know this, but maybe the coffee hasn’t worked yet…
A Faraday cage doesn’t need to be grounded – a FC or a solid metal container will arrange charges on its exterior to zero out any internal field.
But anyone building a Faraday Cage will almost inevitably ground it as a matter of course. I know I did when I built and repaired them at a job a couple of years ago. You typically have some electronics inside a Faraday Cage that you want protected from the environment outside*, and you want to have a ground for those electronics. You have some sort of isolator for the incoming power, and that requires a ground. So practical Faraday cages usually have a grounding strap. Besides, it’s good engineering poractice – why should you risk a discharge from a floating cage when it’s so easy to eliminate?
*Not necessarily, I realize. You might have a desktop cage that has nothing with active electronics inside, or with a battery source. But I’ll bet 95%+ of Faraday cages are hooked to external power.
If an ungrounded one is hit by lightning, then the closest point to actual earth ground will probably melt from the current flowing through it, but other than that, not much difference that I can think of. Hope your chair isn’t sitting on that point. Major Assburn.
Yes, certainly, but that doesn’t deliver any information about what’s happening inside the cage, and only delivers information about what’s happening external to the cage, equivalent to the EM reflecting off of it. In other words, they find out details about the wave that is being used to try to probe the cage. Ground it, and they would get nothing at all. Totally useless. Even the reflected wave is useless, since it only carries information about the EM beam used to probe in the first place. Again, useless.
The conductive plastic bags you ship them in are Faraday cages, although ungrounded. Not very good ones, either. But, it would take some serious high-voltage to fry the components inside, and the goal is safe shipping, not high-voltage/current resistance. They don’t NEED to be good ones.
You are essentially right. They work as reflectors, with the “perfection” being directly related to the conductivity of the cage material. Gold would be best, but even that is not a “perfect” reflector. That said, aluminum, or even steel window screen is “good enough” to prevent any significant information about what’s going on inside to be detectable outside the cage, even using steel. But put some sort of detector almost in contact with the cage, and some information will leak, for the reason you mentioned. Best material to make a Faraday cage out of would be a super-conductor. Too bad, we ain’t got any. So put THICK concrete walls between the actual cage, and anyone’s detector. It should work.
If you only consider the fields due to the currents on the surface of the cage, those currents radiate into the cage, but the fields they radiate are opposite of the incident field without the cage present. The (total) electric field is the sum of the incident field and the field due to the currents, and this total field is zero in the conductor and inside the cage. For perfect conductors, this current would be on the surface. For real, good conductors, there’s some penetration, but the current is still near the conductor surface.
The earth isn’t that bad of a conductor. If you just stick the ends of a multimeter into the soil you aren’t going to get a very good reading, but power systems back in the old days sometimes used the earth as their return wire (saved quite a bit on wiring costs). In some areas the earth’s conductivity isn’t quite as good as in others, and in some areas how well the earth conducts depends on how wet or dry it’s been in the last few months. That’s why we don’t do it that way any more, but those systems mostly worked. Even today we rely on earth being a good conductor so that we can use it for our safety ground on residential power systems.
That’s probably true, for exactly the reasons you mentioned. Most of the time you are protecting or isolating some sort of equipment, and that equipment needs a power source, and that power source needs to be protected and/or conditioned.
Most purpose built “Faraday cages” that you buy are also made out of copper screen instead of solid metal. As long as the holes in the screen are smaller than the wavelength of the EM frequencies you are trying to protect against, screen works just as well as a solid piece of metal.
Why do you say that? Copper is a better conductor than gold is. So is silver.
It is? I thought satellites and such used gold connections for electronics.
More for corrosion resistance than anything else – gold doesn’t tarnish/oxidize, but copper & silver do.
Wendy Carlos’ New York City recording studio is enclosed in a Faraday cage. She sandwiched brass screen between two layers of wallboard.
Not really intended as a hijack: Does aluminum or steel house siding act like a Faraday cage? I’d think the roof would still be open to EM (usually) but wouldn’t the siding still block a lot of signal anyway?
If the siding completely covered the house then yes, it would make a Faraday cage, but a Faraday cage can’t have holes in it. Since a house has things like windows and doors you’ll still get radio signals into the house. Cell phones and radios and the like will usually work fine inside of a house with metal siding.
In recent years, metal frame homes with metal roofs have become more popular, and one of the complaints about them is that you don’t get decent radio reception inside of one. A lot of steel frame buildings are also close enough to a Faraday cage that you can’t get decent cell phone reception inside of them. My cell phone doesn’t usually work in my office, but if I walk over to the break room (which has a window in it) I can get good reception.
A car’s body (as long as it is metal) also acts like a somewhat imperfect Faraday cage, which is why a car protects you from lightning. Cars aren’t perfect Faraday cages, and there have been cases of lightning coming in through the windows, but overall they are fairly safe, and certainly are much safer than being outside in a thunderstorm.