Faraday cage: Difference between revisions

Faraday cages work because an external electrical field will cause the [[electric charge]]s within the cage's conducting material to be distributed in a way that cancels out the field's effect inside the cage. This phenomenon can be used to protect sensitive [[electronic equipment]] (for example [[RF module|RF receivers]]) from external [[radio frequency interference]] (RFI) often during testing or alignment of the device. Faraday cages are also used to protect people and equipment against electric currents such as [[lightning]] strikes and [[electrostatic discharge]]s, because the cage conducts electrical current around the outside of the enclosed space and none passes through the interior.

Faraday cages cannot block stable or slowly varying magnetic fields, such as the [[Earth's magnetic field]] (a [[compass]] will still work inside one). To a large degree, however, they shield the interior from external [[electromagnetic radiation]] if the conductor is thick enough and any holes are significantly smaller than the [[wavelength]] of the radiation. For example, certain [[computer forensics|computer forensic]] test procedures of electronic systems that require an environment free of [[electromagnetic interference]] can be carried out within a screened room. These rooms are spaces that are completely enclosed by one or more layers of a fine metal mesh or perforated sheet metal. The metal layers are grounded to dissipate any electric currents generated from external or internal electromagnetic fields, and thus they block a large amount of the electromagnetic interference. See(see also [[electromagnetic shielding]]). They provide less attenuation of outgoing transmissions than incoming: they can block [[electromagnetic pulse]] (EMP) waves from natural phenomena very effectively, but especially in upper frequencies, a tracking device may be able to penetrate from within the cage (e.g., some cell phones operate at various radio frequencies so while one frequency may not work, another one will).

In 1755, [[Benjamin Franklin]] observed the effect by lowering an uncharged cork ball suspended on a silk thread through an opening in an electrically charged metal can. Franklin recorded that, "the cork was not attracted to the inside of the can as it would have been to the outside, and though it touched the bottom, yet when drawn out it was not found to be electrified (charged) by that touch, as it would have been by touching the outside. The factbehavior is singular." Franklin had discovered the behaviorthat of what we now refer to as a Faraday cage or shield (based on Faraday's later experiments which duplicated Franklin's cork and can).<ref>Krauss, J. D. (1992) ''Electromagnetics'', 4th ed., McGraw-Hill. {{ISBN|0-07-035621-1}}</ref>{{failed verification|reason=not in other books by Kraus[sic]]]|date=February 2024}}

If a charge +Q is placed inside an ungrounded Faraday shield without touching the walls (let's denote this charge quantity as +Q), the internal face of the shield becomes charged with −Q, leading to field lines originating at the charge and extending to charges inside the inner surface of the metal. The field line paths in this inside space (to the endpoint negative charges) are dependent on the shape of the inner containment walls. Simultaneously +Q accumulates on the outer face of the shield. The spread of charges on the outer face is not affected by the position of the internal charge inside the enclosure, but rather determined by the shape of outer face. So for all intents and purposes, the Faraday shield generates the same static electric field on the outside that it would generate if the metal were simply charged with +Q. See [[Faraday's ice pail experiment]], for example, for more details on electric field lines and the decoupling of the outside from the inside. Note that electromagnetic waves are not static charges.

If the cage is [[ground (electricity)|grounded]], the excess charges will be neutralized as the ground connection creates an [[equipotentialElectrical bonding (electricity)|equipotential bonding]] between the outside of the cage and the environment, so there is no voltage between them and therefore also no field. The inner face and the inner charge will remain the same so the field is kept inside.

[[File:Skin depth by Zureks-en.svg|thumb|350px|Skin depth vs. frequency for some materials at room temperature, red vertical line denotes 50&nbsp;-Hz frequency:{{ubl

In the case of a varying electromagnetic fields, the faster the variations are (i.e., the higher the frequencies), the better the material resists magnetic field penetration. In this case the shielding also depends on the [[electrical conductivity]], the magnetic properties of the conductive materials used in the cages, as well as their thicknesses.

A good ideaexample of the effectiveness of a Faraday shield can be obtained from considerations of [[skin depth]]. With skin depth, the current flowing is mostly in the surface, and decays exponentially with depth through the material. Because a Faraday shield has finite thickness, this determines how well the shield works; a thicker shield can attenuate electromagnetic fields better, and to a lower frequency.

* Faraday bags are portable containers fabricated with metalicmetallic materials that are used to contain devices in order to protect them from electromagnecticelectromagnetic transmissions for a wide range of applications, from enhancing digital privacy of cell telephones to protecting credit cards from [[RFID skimming]].

* Automobile and airplane passenger compartments are essentially Faraday cages, protecting passengers from electric charges, such as lightning.

* A [[booster bag]] (shopping bag lined with [[aluminiumaluminum foil]]) acts as a Faraday cage. It is often used by shoplifters to steal [[radio-frequency identification|RFID]]-tagged items.<ref>{{Cite news | last=Hamill | first=Sean | title=As Economy Dips, Arrests for Shoplifting Soar| date=22 December 2008 |newspaper=[[The New York Times]] | url=https://www.nytimes.com/2008/12/23/us/23shoplift.html | access-date=12 August 2009}}</ref> Similar containers are used to resist [[RFID skimming]].

* [[Elevator]]s and other rooms with metallic conducting frames and walls simulate a Faraday cage effect, leading to a loss of signal and "dead zones" for users of [[cellular phone]]s, [[radio]]s, and other electronic devices that require external electromagnetic signals. During training, firefighters, and other first responders are cautioned that their two-way radios will probably not work inside elevators and to make allowances for that. Small, physical Faraday cages are used by electronics engineers during equipment testing to simulate such an environment to make sure that the device gracefully handles these conditions.{{citation needed|date=August 2018}}

* A [[microwave oven]] uses a partial Faraday shield (on five of its interior six sides) and a partial Faraday cage, consisting of a wire mesh, on the sixth side (the transparent window), to contain the electromagnetic energy within the oven and to protect the user from exposure to microwave radiation.<ref>{{cncite web | title=What keeps microwave radiation from leaking out the oven door? | website=The Straight Dope | date=September4 November 2003 | url=https://www.straightdope.com/21343269/what-keeps-microwave-radiation-from-leaking-out-the-oven-door | access-date=1 June 20232024}}</ref>

* Electronic components in some music instruments, such as in an [[electric guitar]], are protected by faradayFaraday cages made from copper or aluminum foils that protect the instrument's electromagnetic pickups from interference from speakers, amplifiers, stage lights, and other musical equipment.

* Some buildings, such as prisons, are constructed as a faradayFaraday cage because they have reasons to block both incoming and outgoing cellphonecell phone calls by prisoners.<ref>{{cite magazine |magazine=[[AARP Magazine]]

|title=REDACTED FOR PUBLIC INSPECTION: Contraband phone task force status report |date=April 26, 2019|publisher=[[CTIA (organization)|CTIA]]}}</ref> The exhibit hall of the [[Green Bank Observatory]] is a Faraday cage to prevent interference with the operations of their [[radio telescope]]s.<ref>{{cite web |title=2023 Science Center Offerings |url=https://greenbankobservatory.org/visit/science-center-schedule/ |publisher=Green Bank Observatory |access-date=6 May 2024}}</ref>

* [https://www.youtube.com/watch?v=mUWxYesR5Wo Faraday Cage Protects from 100,000 V :: Physikshow Uni Bonn]