Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Ionized Gas

  1. Jun 22, 2004 #1
    I'm trying to do some thought experiments involving ionized gas. More specifically, I'm thinking about ionized air. For the sake of argument, lets just imagine that we're ionizing the air by sticking a pin out of a Van de Graaff generator that's positively charged.

    My main question is this: how do the gas molecules actually behave? Do they keep the same rms speed as their non-ionized counterparts? The moment that they're ionized by the needle, I'd imagine that they're accelerated in all directions (except for towards the VDG generator), as they are repelling one another, so would this change its rms speed? Overall, I'm very confused over the whole subject.
  2. jcsd
  3. Jun 22, 2004 #2
    I do not know if some help in terms of ionic production (positve or negative )would help here? A interesting question though when considering the potent smell(ozone) before electrical spark generation :smile:

    Maybe a condensive feature from such a spark leaves telling tales about the nature of that enviroment?
    Last edited: Jun 23, 2004
  4. Jun 23, 2004 #3
    I don't really care about ionic production here. If you want to think of it as being produced by an X-ray, that's fine by me. I'm more interested in how the gas behaves after it's been ionized. Thanks anyway, though.
  5. Jun 23, 2004 #4
    A neutral molecule of air gets close enough to the needle that one or more of the electrons are stolen by the needle at which point that air molecule has a positive charge. The positive ion would then be repelled by the positive field from the Van de Graaff.
  6. Jun 23, 2004 #5
    Right, but I'm trying to consider how the gas as a whole behaves. The movement of individual air molecules will most likely be slowed by the neutral molecules that it slams into along the way. So, will the rms speed of all of the charged molecules remain about the same as the uncharged ones?
  7. Jun 23, 2004 #6


    User Avatar
    Science Advisor

    My intuition says the ions will be faster on average--possibly fast enough to create additional ions from some of their early collisions with neutral molecules. Once an ion is moving slowly enough to be likely to regain neutrality from its collisions with oppositely-charged particles, it won't take long, on average, for it to fall back to essentially the same speed as the neutral molecules. But that's just my intuition.
  8. Jul 10, 2004 #7
    Vrms = sqrt(3RT/M)
    This is an equation commonly used to find the root mean square velocity of gases. From what i see...the speed must be much much faster. It's common knowlegde for all of us here that ionized gas is very hot, therefore, its moving fast. From the equation its clear that as T(temp.) increases, so does the rms of the gas. Well thats how i see it :)
    Last edited: Jul 10, 2004
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook