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. In Science and engineering, a conductor is a material which contains movable Electric charges. Such an enclosure blocks out external static electrical fields. In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can Faraday cages are named after physicist Michael Faraday, who built one in 1836. Michael Faraday, FRS ( September 22 1791 – August 25 1867) was an English Year 1836 ( MDCCCXXXVI) was a Leap year starting on Friday (link will display the full calendar of the Gregorian Calendar (or a Leap

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. Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction. This effect is used, for example, to protect electronic equipment from lightning strikes and other electrostatic discharges. Lightning is an atmospheric discharge of Electricity, which typically occurs during Thunderstorms and sometimes during volcanic eruptions or Electrostatic discharge ( ESD) is the sudden and momentary Electric current that flows between two objects at different Electrical potentials The term is

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. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. 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. Computer forensics is a branch of Forensic science pertaining to legal evidence found in computers and digital storage mediums 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. Electromagnetic shielding is the process of limiting the flow of Electromagnetic fields between two locations by separating them with a barrier made of conductive material

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. An electroscope is an early Scientific instrument that is used to detect the presence and magnitude of Electric charge on a body

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. Giovanni Battista Beccaria ( October 3, 1716 &ndash May 27, 1781) Italian Physicist, was born at Mondovì, Benjamin Franklin ( April 17 1790 was one of the Founding Fathers of the United States of America. " Later, the Belgian physicist Louis Melsens (1814–1886) applied the principle to lightning conductors. The Kingdom of Belgium is a Country in northwest Europe. It is a founding member of the European Union and hosts its headquarters as well as those Another researcher of this concept was Gauss (Gaussian surfaces). A Gaussian surface is a closed two-dimensional Surface through which a Flux or Electric field is to be calculated

How a Faraday cage works

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. In Physics, a charge carrier denotes a free (mobile unbound particle carrying an Electric charge. Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. 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 term electromagnetic pulse ( EMP) has the following meanings Electromagnetic radiation from an Explosion (especially a Nuclear

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. In Physics and Systems theory, the superposition principle, also known as superposition property, states that for all Linear systems In Classical electromagnetism, Maxwell's equations are a set of four Partial differential equations that describe the properties of the electric

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. In Classical electromagnetism, Maxwell's equations are a set of four Partial differential equations that describe the properties of the electric 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. Skin depth is a measure of the distance an Alternating current can penetrate beneath the surface of a conductor. 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. A rebar, or reinforcing bar, is a common Steel bar and is commonly used in Reinforced concrete and reinforced Masonry structures 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. A microwave oven, or a microwave, is a Kitchen appliance that cooks or heats Food by Dielectric heating. These ovens can still have a strong effect on electronics operating within the frequency of microwaves such as phones and wireless networks.
• Coaxial cables are in fact data cables wrapped by a hollow, flexible conductor, effectively a Faraday cage. Coaxial cable is a cable consisting of an inner conductor surrounded by a tubular insulating layer typically made from a flexible material with a high Dielectric constant, all
• RFID passport and credit card shielding sleeves are small, portable Faraday cages. Radio-frequency identification ( RFID) is an automatic identification method relying on storing and remotely retrieving data using devices called RFID tags or
• 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. The United States of America —commonly referred to as the National security is the entire scope of measures undertaken by the Governments of Nation-states in providing assurance of national Sovereignty TEMPEST is a Codename referring to investigations and studies of compromising emanations (CE The term electromagnetic pulse ( EMP) has the following meanings Electromagnetic radiation from an Explosion (especially a Nuclear
• 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. ^[1]
• Cars and aircraft. When lightning strikes an aircraft or a car the electric currents induced on it are forced to travel on the outer skin of the vehicle's body. Lightning is an atmospheric discharge of Electricity, which typically occurs during Thunderstorms and sometimes during volcanic eruptions or If you were in a car, and the car were struck by lightning, it is not in fact the rubber tires that would save your life. If the lightning can jump from the ground to the sky, then it can jump from the ground to your car. What actually happens, is by being enclosed by the car's cabin, the lightning travels around you, through the conductive frame of the car. This is because the car forms a Faraday Cage. Other signals such as mobile phone signals penetrate the skin of cars and aircraft because the wavelength of the signal is significantly smaller than the holes present on the surface of the conductor (the windows). In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. 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. Richard Mark Hammond (born 19 December 1969 nicknamed " Hamster " is a British Presenter of Radio and Television, Website
• 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. Discovery Channel is an American Satellite and Cable TV channel (also delivered via IPTV, Terrestrial television and MythBusters is a Popular science Television program produced by Australian firm Beyond Television Productions originally for the
• 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 National Radio Astronomy Observatory (NRAO is a Federally Funded Research and Development Center of the United States National Science Foundation operated Green Bank is a community in Pocahontas County in West Virginia 's Potomac Highlands inside the Allegheny Mountain Range. 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. The term electromagnetic pulse ( EMP) has the following meanings Electromagnetic radiation from an Explosion (especially a Nuclear In this context, the cages are often called "screen rooms".
• Antistatic bags form a Faraday cage around the enclosed electronic components. An antistatic bag is a shipping bag for devices (usually electronic) which can be damaged by Electrostatic discharge.
• Faraday cages have been built into wearable suits, allowing high-voltage workers to sit directly on power lines. ^[2]
• Wifi signals are often confined inside of a building, especially if the building has metal siding. Wi-Fi (ˈwaɪfaɪ is the trade name for the popular wireless technology used
• 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 National Oceanic and Atmospheric Administration ( NOAA) is a scientific agency within the United States Department of Commerce focused on the conditions of the ^[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. 3G is the third generation of mobile phone standards and Technology, superseding 2 Quality levels of shielding also depend upon the types of metals used in the cages as well as the thicknesses.

References

See also

• Michael Faraday
• Frieder Kempe
• Anechoic chamber
• Conductive textile
• Electromagnetic interference

External links

• Faraday Cage Protects from 100,000 V :: Physikshow Uni Bonn
• Physics lecture on Faraday cages from Michigan State University
• Pepro LLC Faraday Cage used in communications shelters