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Plasma is an important phase of matter, which makes up about 99% of “ordinary” matter in the Universe.

Plasma was first discovered by William Crookes (1832–1919) in 1879, but it wasn't called “plasma” until 1928, when Irving Langmuir (1881–1957) coined the term. It is sometimes called the “fourth state of matter.” Unlike gases, solids, or liquids, plasma has a very large component of ions (Figure 3.20). The electrons in the outer orbitals are stripped away at high temperatures, and the result is a collection of ions and electrons. The free charged electrons mean that plasma responds strongly to electromagnetic fields, which partly explains the complex patterns it can adopt when exposed to such fields.

Figure 3.20 The structure of plasma compared to other states of matter.

Hot plasma is typically at a temperature of thousands of Kelvin. An example is gas in the Sun's atmosphere where high temperatures are able to ionize it. Other examples are plasma generated by atmospheric gases exposed to lightning strikes, or the ionization of atmospheric gases by solar particles, forming the northern lights, Aurora Borealis. Plasma can also be generated at relatively low temperatures. Everyday examples of cold plasma include the glow discharge of gas in neon lighting or fluorescent bulbs, plasmas, which are typically produced at temperatures of ∼300–1000 K.