Hello Physics Forums! I've been educating myself on electrostatics, electronics, and electromagnetics and I have come to the topic of plasmas. I understand that plasmas are essentially charged gases - but their behavior is so disparate from regular gases that they are classified as the fourth state of matter. For instance, plasmas will form organized structures, unlike gases or liquids. Apparently plasma particles interact with one another quite differently than do gases. They will exert forces upon one another without coming into physical contact .. I guess my question boils down to - what is the minimum density in order for a charged gas to start behaving like a plasma? Is the interstellar medium always considered a plasma as long as it is ionized? I'm a complete neophyte, but I would think that at very low densities there won't be a significant difference between gases and plasmas! If the particles are spaced apart far enough their electric fields won't affect one another that much, am I right? Any intelligent input is really appreciated.