Electric arc
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[[Image:Lichtbogen 3000 Volt.jpg|thumb|right|A 3000 volt electricity arc between two nails]]
[[Image:Arcing pickup shoe.jpg|thumb|right|Electricity arcs between the power rail and electrical pickup "shoe" on a [[London Underground]] train]]
[[Image:CaOmelt1.jpg|thumb|right|An electric arc can melt [[calcium oxide]]]]
An '''electric arc''' is an [[electrical breakdown]] of a gas which produces an ongoing [[Plasma (physics)|plasma]] [[Electrostatic discharge|discharge]], resulting from a current flowing through normally [[Electrical conductance|nonconductive]] media such as [[air]]. A synonym is '''arc discharge'''. The phenomenon was first described by [[Vasily V. Petrov]], a Russian scientist who discovered it in [[1802]]. An archaic term is '''voltaic arc''' as used in the phrase "voltaic arc lamp".
==Overview==
The various shapes of electric arc are [[emergence|emergent properties]] of nonlinear patterns of [[current (electricity)|current]] and [[electric field]]. The arc occurs in the gas-filled space between two conductive [[electrodes]] (often made of [[carbon]]) and it results in a very high [[temperature]], capable of [[melting]] or [[Evaporation|vaporizing]] most materials. An electric arc is a continuous discharge, while a similar electric [[spark]] discharge is momentary. An electric arc may occur either in direct-current circuits or in alternating current circuits. In the latter case, the arc may re-strike on each half cycle of the current. An electric arc differs from a [[glow discharge]] in that the current density is quite high, and the voltage drop within the arc is low; at the [[cathode]] the current density may be as high as one million amps per square centimeter. <ref> A. H. Howatson, ''An Introduction to Gas Discharges'', Pergamon Press, Oxford pgs. 80-95</ref>
==Uses==
Industrially, electric arcs are used for [[welding]], [[plasma cutting]], for [[electrical discharge machining]], as an [[arc lamp]] in [[Movie projector|movie theater projectors]], and [[Followspot]]s in [[stage lighting]]. [[Electric arc furnace]]s are used to produce [[steel]] and other substances. [[Calcium carbide]] is made in this way as it requires a large amount of energy to promote an [[endothermic]] [[Chemical reaction|reaction]] (at temperatures of 2500 [[Celsius|°C]]).
Low-pressure electric arcs are used for lighting, e.g., [[fluorescent lamp|fluorescent tubes]], mercury and sodium [[Street light|street lamps]], and camera flash lamps.
Electric arcs have been studied for [[electric propulsion]] of spacecraft.
==Undesired arcing==
Undesired or unintended electric arcing can have detrimental effects on [[electric power transmission]] and [[Electric power distribution|distribution]] systems and [[electronics|electronic]] equipment. Undesired arcing in electrical [[contactor]]s can be suppressed by various devices, including:
* immersion in [[Transformer oil|oil]], [[inert gas]] or [[vacuum]]
* [[arc chute]]s
* [[magnetic blowout]]s
An electric arc has a non-linear relationship between current and voltage. Once the arc is established (either by progression from a glow discharge <ref>Princples of Electronics By V.K. Mehta ISBN 8121924502 pages 101-107</ref> or by momentarily touching the electrodes then separating them), increased current results in a lower voltage between the arc terminals. This negative [[impedance]] effect requires that some positive form of impedance to be placed in the circuit, if it is desired to maintain a stable arc. This property is the reason uncontrolled electrical arcs in apparatus become so destructive, since once initiated an arc will draw more and more current from a fixed-voltage supply until the apparatus is destroyed.
==See also==
* [[Arc transmitter]]
* [[Arc welding]]
* [[Arc lamp]]
* [[Spark gap]]
* [[Vacuum arc]]
==References==
<references/>
== External links ==
{{Commonscat|Electric arc}}
* [http://www.arcadvisor.com/index.html Arc Analysis]
* [http://www.rmcybernetics.com/research/resonance/plasma.htm Unusual Arcing Photos]
* [http://arcmelt.velp.info/ Some more info about making electric arcs] using a [[welder]].
* [http://205.243.100.155/frames/longarc.htm Videos of 230,000 volt 3-phase "Jacobs Ladder" and unintentional 500,000 volt power arc]
* [http://www.cirris.com/testing/voltage/arc.html High Voltage Arc Gap Calculator] to calculate the length of an arc knowing the voltage or vice versa
[[Category:Electric arcs| ]]
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