Plasma
En physique, le plasma décrit le quatrième état de la matière (autre que solide, liquide, gaz).
C'est un milieu constitué de particules neutres, d'ions et d'électrons.
La transformation de gaz vers plasma (gaz ionisé) ne s'effectue pas à
température constante pour une pression donnée, avec une chaleur latente de changement d'état,
comme pour les autres états, mais il s'agit d'une transformation progressive. Losqu'on chauffe un gaz suffisamment,
les électrons des couches extérieures peuvent être arrachés lors des collisions entre particules, ce qui forme le plasma. Globalement neutre, la présence de particules chargées donne naissance à des comportements inexistants dans les fluides, en présence de champs électromagnétiques par exemple.
Cet état est le plus représenté dans l'univers, dans les étoiles, les nuages de gaz et aussi l'ionosphère terrestre. A une autre échelle, on trouve également des plasmas dans les néons et les propulseurs spatiaux.
Voir physique des plasmas.
The word plasma has a Greek root which means to be formed or molded (the word plastic shares this root) and has a few definitions: a green type of chalcedony (rare - first used in 1772); the fluid part of blood, lymph or milk (first used in 1845); an ionized gas (first used in 1928).
In Physics and Chemistry, plasma is an energetic state of atomic matter where some or all electrons in the outer orbitals have become separated from the atom. The result is a collection of ions and electrons which are no longer bound to each other. A plasma is sometimes called an ionized gas. The term plasma is generally reserved for a system of charged particles which is large enough to behave collectively (Debye shielding) so that a collection of a small number of charged particles are not usually called plasmas.
Typical characteristics of a plasma:
- Plasma shielding lengths of electric potentials usually are small when compared to plasma physical lengths λD << L
- Large number of particles within a sphere with a radius of the Debye length, λD.
- Mean time between collisions usually are long when compared to typical plasma oscillations τ >> 2π/ω
Three lower energy phases of matter are solid, liquid, and gas. Plasmas are the most common form of matter, comprising more than 99% of the visible universe.
Everyday forms of plasma are the sun and stars, which are plasmas heated by nuclear fusion, fluorescent and neon lights, lightning, the Aurora borealis, solar wind, and interstellar nebulae
Sir William Crookes, an English physicist, identified a fourth state of matter, now called plasma, in 1879. The word 'plasma' was first applied to ionized gas by Dr. Irving Langmuir, an American chemist and physicist, in 1929.
See plasma physics for active research topics.
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In Life sciences, plasma refers to blood plasma.
In geology, plasma is a variety of chalcedony.