Electrical discharge types
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Electrical discharge is the result of electrical breakdown of an insulator.
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Spark
A spark is a sudden and momentary electric current that flows between two objects at different electrical potentials. Sparks create a shock wave which is rapidly converted to an impulsive noise in the air - sparks make a distinctive snap or bang. Thunder is created from the much larger shock waves created by lightning. A spark may evolve into an electric arc if sufficient follow-through energy is available from the external circuit. The voltage required to produce a spark in a gas is given by Paschen's law.
Arc
An arc is a continious current flowing between two objects at different electrical potentials. An arc can occur when a spark bridges a gap and sufficient current is provided to continue flow through the resulting ionized gases and/or ionized electrode materials. An arc can also be initiated via an ohmic contact between electrodes held at relatively low voltage (i.e., tens of volts as in arc welding) provided that sufficient current is available to maintain a highly ionized, conductive path through the air.
Corona
Corona is a partial breakdown of air or other gas. In air, it can produce significant amounts of ozone.
Streamer
Larger pulses of displacement current can further ionize and heat the air at the point of initial breakdown. This forms a very conductive "root" of hotter plasma, called a leader, that projects outward. The plasma within the leader is considerably hotter than a corona discharge, and is considerably more conductive. It has properties that are similar to an electric arc. The leader tapers and branches into thousands of thinner, cooler, hairlike discharges called streamers. The streamers look like a bluish haze at the ends of the more luminous leaders, and it is the streamers that actually transfer charge from leaders to nearby space charge regions. The displacement currents from countless streamers all feed into the leader, helping to keep it hot and electrically conductive.
Brush
A brush discharge is a type of corona discharge that takes place between two electrodes in a nonconducting medium like air and is characterized by nonsparking, faintly glowing branch like structures composed of ionized particles.
Brush discharges can for example occur between charged plastic and a conductor. The maximum energy associated with brush discharges is unlikely to exceed 4 mJ. Such discharges may cause ignition of a combustible gas mixture but are less likely to do so than an electrostatic discharges between two conductors.
Colour
Sparks (in particular long sparks) tend to reflect the spectra of the elements within the gaseous media that have the lowest ionization energy. During brief sparks, electrode materials don't contribute to the color except in small cathode and anode spots (the active electrode-gas interface). An arc, however, causes comparatively larger incandescent anode and cathode spots to develop on each electrode. Electrode material readily evaporates from these regions, then becomes ionized within the arc, coloring it. Lightning is a brilliant blue white from ionized oxygen and nitrogen, while an arcing power line or transformer is typically green from ionized copper.
