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Faraday cage

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Entrance to a Faraday room

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.

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. 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).

An external electrical field causes the charges to rearrange which cancels the field inside.

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.

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

  • Mobile phones and radios may 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.
  • 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 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 (via text message using cell phones) in the country by lining every exam room with a Faraday-like cage.[1]
  • Cars and airplanes. When lightning strikes an airplane or a car the electric currents induced on it are forced to travel on the outer skin of the vehicle's body. Other signals such as cell phone signals penetrate the skin of cars and airplanes because the wavelength of the signal is significantly smaller than the holes present on the surface of the conductor (the windows). The BBC television program Top Gear once sat the presenter Richard Hammond in a car while it was struck by a simulated lightning bolt of 800,000 volts at the Siemens High-Voltage lab in Berlin. [2]
  • 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 their experiment at the time.
  • 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".
  • Antistatic bags form a Faraday cage around the enclosed electronic components.
  • Faraday cages have been built into wearable suits, allowing high-voltage workers to sit directly on power lines.[3]

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.

References

  1. ^ http://www.theregister.co.uk/2006/09/25/exam_cages/
  2. ^ http://www.youtube.com/watch?v=ve6XGKZxYxA
  3. ^ http://www.glumbert.com/media/highpower

See also

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