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I've just started looking at plasmas and I have some confusion.

A metal is a lattice of positive ions bathed in a sea of delocalised electrons and conducts electricity. A plasma is a 'gas' of free ions and electrons and can also conduct electricity. Is a metal a solid-state plasma?

Are all ionized gases a plasma? Even if they have not been heated?

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There are several sorts of plasmas. An ionized gas is a sort of plasma, also called thin plasma.

Thin plasmas, while physically different from a metal, share a surprisingly large number of properties with it:

  • They both have two separate charge carriers, cations and electrons.
  • On both, the cations have a negligible contribution to current and the electrons form a gas.
  • They have similar dispersion relations (leading to similar "local" Ohm's law in both).

It's because cations don't contribute much that they seem similar, in spite of one being a gas and the other a solid.

When you heat a thin plasma, for a while it's still an ionized gas. As it gets hotter, you get the energy to rip more and more electrons from atoms, but the nuclei remain untouched. However, cations start to contribute more so properties differ noticeably from a metal. It's the field of magnetohydrodynamics.

When a plasma is hot enough to rip nucleons from the nucleus, it becomes a thermonuclear plasma, and its properties change again. This is the sort of plasma the Sun is made of.

Heat such a plasma again (a lot!) and you have the energy to rip quarks and gluons from the nucleons. This is the quark-gluon plasma.

Overall, you get a different sort of plasma each time you become able to extract a new sort of particle from the system. The similarity with a metal only exists at low energy.

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  • $\begingroup$ Is there a name or a measure to differentiate between thin plasma with more or less electrons ionised from the atoms? $\endgroup$
    – Jojo
    Commented Jul 8, 2022 at 6:58
  • $\begingroup$ @Joe Not that I know of, but I'm not a MHD specialist. $\endgroup$
    – Miyase
    Commented Jul 8, 2022 at 7:00
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    $\begingroup$ Just to add: The Jellium model for the electron behaviour in a metal essentially equates the metal to a plasma, as in it assumes the electrons and ions to be uniformly randomly distributed and considerations are often focused on electrons only (due to mass differences) $\endgroup$
    – Nox
    Commented Jul 8, 2022 at 9:57
  • $\begingroup$ Thank you for this useful answer :) "An ionized gas is a sort of plasma, also called thin plasma" - so is a thin plasma both a plasma and a gas? Are "plasma" and "gas" mutually exclusive, or can something be both? What's the difference - plasmas contain free electrons/charged ions/spare nucleons/spare quarks, whereas a gas simply does not? $\endgroup$
    – Jake Levi
    Commented Aug 16, 2023 at 16:09
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    $\begingroup$ @JakeLevi Technically, a gas is made of electrically-neutral particles, so a plasma isn't a gas. However, in practice, a plasma is commonly described as "a mix of two electrically-charged gases, one made of electrons and one made of cations". Let's say it's an extended definition of the word "gas". $\endgroup$
    – Miyase
    Commented Aug 16, 2023 at 16:55
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To add a bit to the answer by Miyase:
Metal here means a phase of matter (not a type of chemical element) - where the ions form a lattice, while some electrons (conduction electrons) are free to move around. The difference between such a metal and an ionized gas of the atoms of the same type is like the difference between ice, water and water vapor - they are all made of H2O, but they are different phases of the same substance.

Having said that, electrons inside metals can be indeed viewed as a kind of plasma - in particular, the resulting wave-like excitations are called plasmons.

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    $\begingroup$ ... and cosmologists call everything except H and He "metals" $\endgroup$
    – Carl Witthoft
    Commented Jul 7, 2022 at 14:19

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