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Faraday cages protect against EM radiation and charges.

As far as I understand, this effect happens through the induction of an electric field, which produces a current inside of it, which then counteracts any incoming current. So, my question:

If I replaced part of a wire in my circuit with a Faraday cage, say + at the feet and - at the top, would the interior still be protected, or would the current, given that the skin effect applies, move through both the outer surface and the inner surface? Are there any differences between DC/AC?

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  • \$\begingroup\$ I think you mean it protects against discharges, right? A charge is.. a charge, you can't really protect against it. \$\endgroup\$ Apr 15 at 14:53
  • \$\begingroup\$ In essence, you're asking us how to apply Maxwell's equations under boundary conditions. Could you elaborate on how far you've gotten with that? \$\endgroup\$ Apr 15 at 14:55
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I replaced part of a wire in my circuit with a faraday cage, say + at the feet and - at the top, would the interior still be protected, or would the current, given that the skin effect applies, move through both the outer surface and the inner surface?

Assuming "part of Circuit" meant connected to a battery /generator then you cannot protect(from the electric field due to battery or generator) the material (metals) inside the Faraday's cage because an electric field would exist inside the Faraday's cage to satisfy Laplace's equation and boundary condition , although when connected to battery or generator material inside Faraday's cage still protected from electric field due to outside charges and we can prove it using Superposition theorem

And are there any differences between DC/AC?

In case of DC(battery) you'll get steady electric field inside the cage at steady state while in case of AC (generator) you'll get an alternating electric field at steady state .

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  • \$\begingroup\$ This is incorrect. In the electrostatic case, an extrernal electric field does not penetrate into the interior of a Faraday cage, even when there are wires extending from the surface of the cage to the interior. \$\endgroup\$ Apr 15 at 17:11
  • \$\begingroup\$ @Math Keeps Me Busy, why would you called (or assume) an state electrostatic case ,if a battery is connected to cage and Current flowing through surfaces due to potential difference that is what op asking and this is definitely not an electrostatic Case \$\endgroup\$
    – user215805
    Apr 15 at 18:08
  • \$\begingroup\$ If the current through a conductor is "small", then E field in that conductor is small. When the E field in a conductor is small, any net charge remains on the surface of the conductor. If a conductor is hollow, and has an inside and outside surface, then, unless there is a net charge in the hollow space, then there is no charge on the inside surface. Even if wires extend from the outside surface to the inside, they don't bring any charge with them. A battery inside a Faraday cage, will not be affected by external fields external to the cage, even if 1 of its terminals is attached to the cage \$\endgroup\$ Apr 15 at 18:33
  • \$\begingroup\$ @Math Keeps Me Busy , what my intention was during writing this answer that metals inside Faraday's cage cannot be protected from Field of battery or generator but it will protect from electric field due to outside charges (which i didn't mention) \$\endgroup\$
    – user215805
    Apr 15 at 19:00
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    \$\begingroup\$ I'm pretty sure the OP is concerned about protection from electric fields due to outside charges. But I agree the question is vague. Clearly, if you have a battery inside a Faraday Cage, there will be an electric field caused by the battery. And, if you have a Van de Graaff generator inside a Faraday Cage, you can use it to destroy ESD sensitive devices. \$\endgroup\$ Apr 15 at 19:15
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Note: Every conductor has resistance. The current in a lightning strike is very large (thousands of amperes). If the Faraday cage is not sufficiently conductive, there may be a voltage drop between where current hits the cage and where it leaves. That voltage drop could cause some current to flow through alternate paths inside the Faraday cage. For the purposes of this answer, I will assume that the Faraday cage is sufficiently conductive to protect it's interior from currents that are likely to strike it. If it is not sufficiently conductive, well, all bets are off.

If a circuit consists only of components completely inside of the Faraday cage, plus the Faraday cage itself, then the components inside of the Faraday cage will be protected from electrical discharges originating outside of, and some distance away from, the cage.

The cage could be used, for example as the "ground" of a circuit, as is often the case in airplanes

However, if the circuit has any part that extends beyond the cage, then this protection will be lost. For example, a circuit in a Faraday cage which has a ground wire that extends outside of the cage to earth, (but is not bonded to the cage itself) would not be protected. Potentially, a discharge could pass from a source (such as lightning) to the cage, then jump to the ground wire (possibly through circuitry inside the cage) and continue to the earth.

On the other hand, a circuit which is entirely enclosed, but contains a battery or other electrical source of power, will have protection. For example, the electronics in airplanes struck by lightning are typically unaffected. (The "Faraday Cage" of a commercial airplane is designed to be highly conductive, and tested for the currents it is likely to experience in a lightning strike.)

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Edit: The same principle applies to changing electromagnetic fields. If a circuit contained within a metal box is adequately isolated from external electromagnetic fields by that box, then including that box into the circuit, for example by making it a ground "net", will not compromise the isolation afforded to that circuit.

Nor will driving a current through a Faraday Cage from an external source adversely affect anything inside the Faraday cage (of course always with the proviso that the cage is adequately conductive so that no voltage develops across the cage, and there is no appreciable heating of the cage as the current flows through it.)

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  • \$\begingroup\$ This is definitely a correct answer for a completely different question which op didn't asked \$\endgroup\$
    – user215805
    Apr 15 at 18:35
  • \$\begingroup\$ @user215805 Please read the edit I made. Your answer is wrong \$\endgroup\$ Apr 15 at 18:51
  • \$\begingroup\$ @user215805 thank-you for editing your answer. \$\endgroup\$ Apr 16 at 1:14
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Faraday cages are used all the time in RF circuits to isolate the ingress and egress with sufficient attenuation. It also adds some pF from the body as a capacitor plate but this prevents proximity effects of placement from modulating or detuning the resonant LC circuits.

All TV tuners were done this way to output on the IF as well as the demodulator to convert to baseband chroma and lumina signals with sync. Often they use flanged lids for access.

We also used them on 1 GHz ceramic hybrids, so you can expect to see them on WiFi routers. There is a company that specializes in the corner soldered tin plated thin plates. We made them from half etched brass in a PCB shop for prototypes and tinned them in the lab then breakout like a tabbed array of same boards to be folders and soldered onto a thick track with propane microtorch with a lid or just a tuning hole for test point access.

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This four page article Shielding Effectiveness of Microwave Cable Assemblies:

https://www.gore.com/system/files/2018-11/MWR-0740-GORE-REPRINT-MAR-2018.pdf

Before delving into shielding effectiveness, let’s define the term. A shield is a conductive barrier that envelops and isolates an electrical circuit. For a microwave coaxial cable, the isolated electrical circuit is the center conductor, dielectric and outer conductor. Because of the skin effect, at microwave frequencies the return current on the outer conductor travels through a thin layer of the inner diameter of the outer conductor. This leaves the remaining portion of the outer conductor as the shield. Shielding effectiveness is defined as the ratio of the RF energy incident on one side of the shield to the RF energy transmitted to the opposite side.

The inner region of the outer conductor provides the circuit function of signal conductor and the outer region of the outer conductor provides the function of shield (cage) for the signal flow in the inner and outer conductors.

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