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Is the intersteller medium a charged gas or a plasma?

  1. Mar 25, 2014 #1
    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.
  2. jcsd
  3. Mar 25, 2014 #2


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    membraineBound, Welcome to Physics Forums!

    Yes, plasmas are considered to be the "fourth state of matter". [(Edit)Yes, the interstellar medium is a plasma.]

    When gaseous ions and electrons move freely and independently we call that a plasma. Of course charged particles do obey the attraction and repulsion laws, so, if the energy source that ionized the gas is removed the plasma will tend to neutralize itself.

    Here are two websites that explain plasmas much better than I can:


    After you've read over these sites, and if you have some specific question(s) or doubt(s), then come right back here and ask them. Here at Physics Forums there are many members educated, experienced, and qualified in this area who are ready and willing to assist a true "searcher" like you.

    Cheers, Bobbywhy
  4. Mar 25, 2014 #3
    The definition of a (gasseous) plasma is any gas which contains ions. It therefore follows that any gas which contains ions behaves "like" a plasma (since it is a plasma). I don't understand your comment that gases and liquids don't form organized structures. You seem to be working from a definite definition of "organized structure" which I do not share. (I neither agree nor disagree with your assertion. Since I don't understand what you mean, I find it void of meaning.) Since you understand some electrostatics, you know that electric force is proportional to the reciprocal of the distance squared. When the temperature of the gas is high enough (see https://en.wikipedia.org/wiki/Kinetic_theory) any force exerted between ions will be negligible. So, it is not only the concentration (pressure) of the gas, but its temperature as well. Your question can't really be answered. Why? Because it depends on the context whether a mixture of molecular hydrogen, atomic hydrogen, and various ions of both is considered a "gas" or a "plasma". (Thus, I've contradicted my original definition). It will depend on the context in which the gas is being considered. From a spectroscopic perspective if subtantially all the hydrogen is unionized, it will be called molecular hydrogen, otherwise it will be called hydrogen plasma or ionized hydrogen gas. Generally, my experience (as a chemist, not a physicist) is that unless the flow of the gas, or its electrical/magnetic properties are specifically being considered, ionized hydrogen is termed a gas rather than a plasma. This is about terminology; terminology depends on context and isn't really of much interest, once you understand the concepts. My point is: what difference does it make whether it is called ionized gas or a plasma?
  5. Mar 25, 2014 #4


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    abitslow, You wrote in post #3 above “The definition of a (gasseous) [sic] plasma is any gas which contains ions.”

    This is not the correct definition of a gaseous plasma.

    From Wikipedia: “A plasma contains charged particles: positive ions and negative electrons or ions.”

    http://en.wikipedia.org/wiki/Plasma_(physics [Broken])
    Last edited by a moderator: May 6, 2017
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