Plasma
Plasma (physics), in physics, usually gaseous state of matter in which a part or all of the atoms or molecules are dissociated to form ions (see Ionization ). Plasmas consist of a mixture of neutral particles, positive ions (atoms or molecules that have lost one or more electrons), and negative electrons. A plasma is a conductor of electricity, but a volume with dimensions greater than the so-called Debye length exhibits electrically neutral behavior. At a microscopic level, corresponding to distances shorter than the Debye length, the particles of a plasma do not exhibit collective behavior but instead react individually to a disturbance, for example, an electric field. On the earth, plasmas usually do not occur naturally except in the form of lightning bolts, which consist of narrow paths of air molecules of which approximately 20 percent are ionized, and in parts of flames. The free electrons in a metal can also be considered as a plasma. Most of the universe, however, consists of