Classical plasmas occur over a wide range of densities and temperatures - the density varies over 30 orders of magnitude and the temperature can vary over 7 orders of magnitude. The following picture illustrates some of the laboratory plasmas investigated by the Associated Plasma Laboratory of the National Space Research Institute. It also shows some of the natural plasmas studied in several divisions of the Atmospheric and Space Sciences Coordination of the Institute.
Plasma is a gas consisting of charged ions and electrons (Irving Langmuir introduced the word plasma in 1928 to describe an ionized gas). In each atom of a neutral gas the number of negatively charged electrons, which are orbiting the nucleus of the atom, is the same as the number of positively charged protons in the nucleus. However, losing one or more of the orbiting electrons ionizes the atoms. The figure illustrates plasma of free ions and electrons formed by fully ionizing the hydrogen gas. Over a large volume the plasma is quasi-neutral, meaning that the number of free negative charges is equal to the number of free positive charges.
Losing electrons ionizes the atoms in a gas. Ionization takes place and is maintained because the gas:
Plasma is also called the "fourth state of matter", complementing solids, liquids and gases (this description was first used by William Crookes in 1879). The illustration shows how matter changes its state by the addition of thermal energy.
Plasmas possess all the dynamical properties of fluids, like turbulence. Since they are formed of free charged particles, plasmas conduct electricity. They both generate and respond to electromagnetic fields, which leads to collective effects. This means that the motions of all other particles influence the motion of each charged particle. Collective behavior is a key concept in the definition of plasma.