1) What is a plasma?

Sometimes, plasma is described, as the fourth state of matter. This definition, however, is not exact, for the following reason. The transitions between the three “usual” states of matter can be described by the so-called phase diagrams, as the one shown below for water.


Fig.1: Phase diagram for water


For plasmas, such a graph does not exist.

So, what is a plasma then?

Usually, matter is made of atoms, that is protons and neutrons forming the nucleus, and electrons orbiting around them. These atoms, can bind to form molecules (e.g. H2O), or solids (such as NaCl).

In the case of a plasma, the atoms are ionized (that means, they have lost some or all of their electrons). So, we can imagine something more or less similar to a electron-ion “soup”, a gas consisting of positive and negative charges moving around, NOT forming atoms or molecules!



Finally, we must note, that the same word is also used in medicine (blood plasma). However the meaning there is completely different!