Faraday cage

From Wikipedia, the free encyclopedia

Image:Elektrisch dode kamer (kooi van Faraday).JPG A Faraday cage or Faraday shield is an enclosure formed by conducting material, or by a mesh of such material. Such an enclosure blocks out external static electrical fields. Faraday cages are named after physicist Michael Faraday, who built one in 1836 and explained its operation.

The electrical charges in the enclosing conductor repel each other and will therefore always reside on the outside surface of the cage. Any external static electrical field will cause the charges to rearrange so as to completely cancel the field's effects in the cage's interior. This effect is used for example to protect electronic equipment from lightning strikes and other electrostatic discharges.

To a large degree, Faraday cages also shield the interior from external electromagnetic radiation if the conductor is thick enough and its meshes, if present, are significantly smaller than the radiation's wavelength. This application of Faraday cages is explained under electromagnetic shielding.

1 History
2 How a Faraday cage works
3 Real-world Faraday cages
4 See also
5 External links

[edit] History

In 1836 Faraday observed that the charge on a charged conductor resided only on its exterior, and had no influence on anything enclosed within it. To demonstrate this fact he built a room coated with metal foil, and allowed high-voltage discharges from an electrostatic generator to strike the outside of the room. He used an electroscope to show that there was no electric charge present on the inside of the room's walls.

The same effect was predicted earlier by Francesco Beccaria (1716–1781) at the University of Turin, a student of Benjamin Franklin's work, who stated that "all electricity goes up to the free surface of the bodies without diffusing in their interior substance". Later, the Belgian physicist Louis Melsens (1814–1886) applied the principle to lightning conductors.

Image:Faraday cage.gif

[edit] How a Faraday cage works

A Faraday cage is best understood as an approximation to an ideal hollow conductor. Externally applied electric fields produce forces on the charge carriers (usually electrons) within the conductor, generating a current that rearranges the charges. Once the charges have rearranged so as to cancel the applied field inside, the current stops.

The cage will block external electrical fields even if the cage contains some charges and an electric field in its interior. This is a consequence of the fact that the Maxwell equations are linear and the superposition principle.

[edit] Real-world Faraday cages

Airplanes act as Faraday cages, protecting passengers from lightning strikes.
Cars (except convertibles or cars made of fiberglass) are also Faraday cages and provide protection against lightning strikes. Passengers should not touch metal objects connected to the outside in the event of a lightning strike. [1]
Mobile phones and radios have no reception inside elevators or similar structures. Some traditional architectural materials act as Faraday shields in practice. These include plaster with metal lath, and rebar reinforced concrete. These affect the use of cordless phones and wireless networks inside buildings and houses.
The cooking chamber of the microwave oven itself is a Faraday cage enclosure which prevents the microwaves from escaping into the environment.
RFID passport and credit card shielding sleeves are small, portable Faraday cages.
Some United States national security buildings are contained in Faraday cages, intended to act as a TEMPEST shield, and possibly also as a mitigation against electromagnetic pulse.
A teacher in the UK has come up with the idea to curb the cheating epidemic in the country by lining every exam room with a Faraday-like cage.[2]