I know some physics but have always been bothered by the term “fourth state of matter” with respect to plasma.
I was reading about the solar wind, or “star vomit” as we call it around the house, and thought that a sparse, electrically neutral plasma ought to behave like an ideal gas. I may be way off base here (Go Cubs!) but the intermolecular interactions should be small. When there are collisions the ionized constituents rebound from mutual positive charge. How is this different from collisions in an ideal gas?
In an ideal gas molecules collide and transfer momentum through their mutually surrounding electron clouds. I suppose you could go all QM here and draw a Feynman diagram but I just think the fuzzy balls don’t like each other. Since EM is mediating the interaction in both cases how did the idea of an entirely different fourth state come about? Do dense plasmas behave wildly differently than sparse plasmas? Help me out here.
One major difference is that plasma, being made up of charged particles, has very high electrical conductivity. So plasma affects - and is affected by - magnetic fields. If the solar wind was not ionized, it would flow right through the earth’s magnetic field and fall everywhere on the earth.
The main reason that plasma is referred to as a fourth state of matter is that, to go from a gas to a plasma, you need to add energy which doesn’t go into increasing the temperature. But this doesn’t necessarily stop you from calling a plasma a “gas”: Many materials have multiple states, with phase transitions between them, that are all the same “phase of matter”. For instance, there are something like seven different forms of water ice. All of them are solid, but they have different crystalline structures, and it takes energy to change from one to another. The only reason that solid, liquid, gas, and sometimes plasma get special billing is that a wide variety of matter can assume those forms.