Faraday Cage - Forearm Crutch - Medical Arm Crutch Manufacturer
History
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 (17161781) 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 (18141886) 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) [from ].
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.
The effectiveness of a Faraday cage or shield is dependent upon the wavelength of the electric or electromagnetic fields it is intended to shield. Effectiveness of shielding also depends upon the types of metals 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.
See also
Frieder Kempe
Anechoic chamber
Conductive textile
Electromagnetic field
Electromagnetic interference
Hoffman box
References
^ "Faraday, Michael - ninemsn Encarta". Archived from the original on 31 October 2009. http://www.webcitation.org/5kwc3quLs. Retrieved 20 November 2008.
^ J. D. Krauss, Electromagnetics', 4Ed, McGraw-Hill, 1992, ISBN 0070356211'
^ Hamill, Sean (22 December 2008), "As Economy Dips, Arrests for Shoplifting Soar", New York Times, http://www.nytimes.com/2008/12/23/us/23shoplift.html?_r=2, retrieved 12 August 2009
External links
Faraday Cage Protects from 100,000 V:: Physikshow Uni Bonn
Physics lecture on Faraday cages from Michigan State University
Make a Faraday Cage Wallet
Michael Faraday: The Invention of Faraday Cage background and related experiment
Top Gear's Richard Hammond is protected from 600,000 V by a car (a Faraday Cage).
Categories: Electrostatics | Michael FaradayHidden categories: Articles needing additional references from July 2008 | All articles needing additional references
by: gaga
Guilin Rice Noodle Outer End Of The Final, "dressed As" Rice Noodle Upcoming - China Medical Medical billing and coding training Buy Flexeril drug with overnight shipping Hst For Physicians Doctors And Medical Practitioners In Canada Fosamax and Other Defective Drugs Shijiazhuang Pharmaceutical Generic Drugs Tens Of Millions Of Sales Of The 20 Million Beneficiaries Structure And Functions Of Various Types Of Drug Treatment Center Gunnar Peterson - China Axillary Crutches - China Medical Arm Crutch Effective Medical Claims Management with Medical Billing Outsourcing Five Ways To Increase Your Medical Spa's Revenues The Development Of Tcm Injections Of Pharmaceutical Excipients Used - Traditional Chinese Medicine, Driver Drug Testing Regulations The Necessity of Drug Rehabilitation Program