Plasma (physics)

Plasma (from Ancient Greek πλάσμα 'moldable substance')[1] is one of the four fundamental states of matter. It consists of a gas of ions  atoms or molecules which have at least one orbital electron stripped (or an extra electron attached) and, thus, an electric charge. It is the most abundant form of ordinary matter in the universe,[2] being mostly associated with stars,[3] including the Sun.[4][5] It extends to the rarefied intracluster medium and possibly to intergalactic regions.[6]

Top: Lightning and neon lights are commonplace generators of plasma. Bottom left: A plasma globe, illustrating some of the more complex plasma phenomena, including filamentation. Bottom right: A plasma trail from the Space Shuttle Atlantis during re-entry into Earth's atmosphere, as seen from the International Space Station.

Plasma was first systematically studied by Irving Langmuir in the 1920s.[7][8] It can be artificially generated by heating a neutral gas or subjecting it to a strong electromagnetic field. The presence of charged particles makes plasma electrically conductive, with the dynamics of individual particles and macroscopic plasma motion governed by collective electromagnetic fields and very sensitive to externally applied fields.[9] The response of plasma to electromagnetic fields is used in many modern technological devices, such as plasma televisions or plasma etching.[10]

Depending on temperature and density, a certain amount of neutral particles may also be present, in which case plasma is called partially ionized. Neon signs and lightning are examples of partially ionized plasmas.[11] Unlike the phase transitions between the other three states of matter, the transition to plasma is not well defined and is a matter of interpretation and context.[12] Whether a given degree of ionization suffices to call a substance 'plasma' depends on the specific phenomenon being considered.

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