Definitions

Faraday cage

Faraday cage

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.

An external static electrical field will cause the electrical charges within the conducting material to redistribute themselves so as to 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 any holes are significantly smaller than the radiation's wavelength. For example, certain computer forensic test procedures of electronic components or systems that require an environment devoid of electromagnetic interference may be conducted within a so-called screen room. These screen rooms are essentially labs or work areas that are completely enclosed by one or more layers of fine metal mesh or perforated sheet metal. The metal layers are connected to earth ground to dissipate any electric currents generated from the external electromagnetic fields, and thus block a large amount of the electromagnetic interference. This application of Faraday cages is explained under electromagnetic shielding.

History

In 1836 Michael 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, 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. Another researcher of this concept was Gauss (Gaussian surfaces).

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.

If a charge is placed inside an ungrounded Faraday cage the internal face of the cage will be charged (in the same manner described for an external charge) to prevent the existence of a field inside the body of the cage. However, this charging of the inner face would re-distribute the charges in the body of the cage. This charges the outer face of the cage with a charge equal in sign and magnitude to the one placed inside the cage. Since the internal charge and the inner face cancel each other out, the spread of charges on the outer face is not affected by the position of the internal charge inside the cage. So for all intents and purposes the cage will generate the same electric field it would generate if it was simply charged by the charge placed inside.

If the cage is grounded the excess charges will go to the ground instead of the outer face, so the inner face and the inner charge will cancel each other out and the rest of the cage would remain neutral. A Faraday cage is capable of completely stopping an attack using electromagnetism such as an EMP.

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 superposition principle and the fact that the Maxwell equations are linear.

A Faraday cage will not shield its contents from static magnetic fields. However, rapidly-changing magnetic fields create electric fields in accordance with Maxwell's equations. The conductors cancel the electric fields and therefore the changing magnetic fields as well. The wall materials' thickness and skin depth set the frequency at which the cage suppresses electromagnetic fields. Static or slowly-changing magnetic fields penetrate the cage; rapidly-changing ones do not.

Real-world Faraday cages

  • MRI scanners are typically housed inside Faraday cages.
  • Mobile phones and radios may have no reception inside some 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 partial Faraday cage enclosure which prevents the microwaves from escaping into the environment.
  • Electronic components that can be damaged by static charges, such as integrated circuits and computer cards, are shipped in Faraday cages consisting of special bags made of an electrically conductive plastic, called antistatic bags.
  • Coaxial cables are in fact data cables wrapped by a hollow, flexible conductor, effectively a Faraday cage.
  • RFID passport and credit card shielding sleeves are small, portable Faraday cages.
  • Some United States national security buildings which house a Sensitive Compartmented Information Facility 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 cheating (via text message using mobile phones) in examinations by lining every exam room with a Faraday-like cage.
  • Cars and aircraft function as Faraday cages when struck by lightning. The metal frame and outer skin of the vehicle cause the electrical charge to travel safely away from the occupants. This differs from a popular urban legend that claims that a car's tires cause the lightning strike to reach the ground. However, radio and cellular phone signals can still reach inside the vehicle since their wavelengths are significantly smaller than the windows and other openings in the vehicle's conductive frame.
    • The role of a car as Faraday cage was demonstrated by Richard Hammond in an episode of the BBC television program Top Gear. At the Siemens High-Voltage Lab in Berlin, Germany, Hammond sat in a car that was being struck by simulated lightning of over 800,000 volts, and reported during the experiment that he felt nothing.
  • The Discovery Channel television show MythBusters used a Faraday cage made from a brass mesh to "cancel out" radio signals that might have interfered with the consistency of an experiment.
  • In scientific environments such as the National Radio Astronomy Observatory in Greenbank, West Virginia, Faraday cages are used to enclose computer equipment rooms that, despite being vital to the cause, interfere with experiments involving radio astronomy. The cages block the electromagnetic waves that skew data and could damage radio telescopes. Pulsed high-voltage experiments also use such Faraday cages to protect sensitive electronics from the experiments' electromagnetic pulses. In this context, the cages are often called "screen rooms".
  • Faraday cages have been built into wearable suits, allowing high-voltage workers to sit directly on power lines.
  • Wifi signals are often confined inside of a building, especially if the building has metal siding.
  • The internal metal lining of most consumer electronics, as well as the metal case of most personal computers, act as a Faraday cage to reduce interference to and from other devices.
  • A manufactured home with aluminum siding will contain many of the physical characteristics of a Faraday cage, with the exception of windows and flooring. This type of structure is noted for blocking NOAA Weather Radios from receiving accurate signals, especially if the receiver is improperly placed in the manufactured home.

The effectiveness of a Faraday cage or shield is dependent upon the wavelength of the electric or electromagnetic fields it is intended to shield. This explains why a microwave oven, for example, can perform such shielding from the observer peering through the metal mesh screened "window" at the front of the oven to watch the cooking process take place. The holes are sized such that the waves within the oven cannot pass through even though visible light which has a much shorter wavelength easily passes through the holes. This also explains how cell phones have improved in building performance using the higher frequencies (shorter wavelengths) of EMFs than the earlier predecessors, notwithstanding improved digital modulation algorithms in so called 3G handsets today and later standards forthcoming. Quality levels of shielding also depend upon the types of metals used in the cages as well as the thicknesses.

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