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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 electric fields. Faraday cages are named after the English scientist Michael Faraday, who invented them in 1836.[1]

A Faraday cage's operation depends on the fact that an external static electrical field will cause the electrical charges within the cage's conducting material to redistribute themselves so as to cancel the field's effects in the cage's interior. This phenomenon is used, for example, to protect electronic equipment from lightning strikes and other electrostatic discharges.

Faraday cages cannot block static and slowly varying magnetic fields, such as Earth's magnetic field (a compass will still work inside). To a large degree though, they 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 screen room. These screen rooms are essentially work areas that are completely enclosed by one or more layers of fine metal mesh or perforated sheet metal. The metal layers are grounded to dissipate any electric currents generated from the external electromagnetic fields and thus block a large amount of the electromagnetic interference. See also electromagnetic shielding.

The reception of external radio signals, a form of electromagnetic radiation, through an antenna within a cage can be greatly attenuated or even completely blocked by the cage itself.

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. A closely related concept is that of the Gaussian surface, discovered by Carl Friedrich Gauss.

Although this cage effect has been attributed to Michael Faraday, it was Benjamin Franklin in 1755 who observed the effect by lowering an uncharged cork ball suspended on a silk thread through an opening. In his words, "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 fact is singular." Franklin has discovered the behavior of what we now refer to as a Faraday cage or shield (based on one of Faraday's famous ice pail experiments which duplicated Franklin's cork and can).[2]

Operation

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

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.

Effectiveness of shielding of a static electric field depends upon the geometry of the conductive material. In the case of a non-linearly varying electric field, and hence an accompanying varying magnetic field, the faster the variations are (i.e., the higher the frequencies), the better the material resists penetration, but on the other hand, the better it passes through a mesh of given size. In this case the shielding also depends on the electrical conductivity of the conductive materials used in the cages, as well as their thicknesses.

Examples

  • The shield of a screened cable, such as the coaxial cable used for cable television, protects the internal conductors from electrical noise.
  • A booster bag (shopping bag lined with aluminum foil) acts as a Faraday cage. It is often used by shoplifters to steal RFID-tagged items.[3]
  • The cooking chamber of a microwave oven is a Faraday cage, made to contain the RF signals of the cavity magnetron which cooks the food.
  • Some linemen wear Faraday suits, which allows them to work on live, high voltage power lines without risk of electrocution. These suits work because they are not grounded. The suit prevents electrical current from flowing through the body, and have no theoretical voltage limits, as linemen have successfully worked even the highest voltage (Russian 1150Mv) lines safely.

See also

References

  1. ^ "Faraday, Michael - ninemsn Encarta". Archived from the original on 31 October 2009. Retrieved 20 November 2008. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  2. ^ J. D. Krauss, Electromagnetics, 4Ed, McGraw-Hill, 1992, ISBN 0070356211
  3. ^ Hamill, Sean (22 December 2008). "As Economy Dips, Arrests for Shoplifting Soar". New York Times. Retrieved 12 August 2009.
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