ABSTRACT
People are very familiar with the solid, liquid, and gaseous states of matter, but much less with plasma, the fourth state of matter, which is reached at sufficiently high temperatures and energy densities. The plasma state (Figure 20.1) is an ionized gas comprising molecules, atoms, ions (in their ground or in various excited states), electrons, and photons. It is electrically conductive since there are free electrons and ions present, and is in local electrical neutrality, since the numbers of free electrons and ions are equal.
