What would happen to gas in...

  1. what would happen to gas in....

    Hi there,
    I'm working on some experiments regarding to gases. I want to know what'd happen to gas which is uneder high temprature and high pressure when it enteres into vacuum.

    it would be very kind of you to answer me in detail.
    Many thanks, Yashar.
     
  2. jcsd
  3. CompuChip

    CompuChip 4,299
    Science Advisor
    Homework Helper

    First thing that springs to mind is adiabatic expansion (comparable to what happens in a fire extinguisher)... not entirely sure though.
     
  4. what would happen to gas in....

    Hi there,
    I'm working on some experiments regarding to gases. I want to know what'd happen to gas which is uneder high temprature and high pressure when it enteres into space.
    is it loose temp.? (adiabatic?)
    it would be very kind of you to answer me in detail.
     
  5. Shooting Star

    Shooting Star 1,979
    Homework Helper

    (Please don't multi-post the same thing.)

    If it's a real gas, it will naturally expand and cool down in the process. If it does not radiate heat, the process should be adiabatic. You have to find the adiabatic law for real gases. Van der Waals' eqn of state may be a good approximation.
     
  6. As I read your question and the reply I thought of something very interesting. If the hot gas is released into space, we would naturally think that the gas should cool down. However, heat is proportional to the kinetic energy of the gas molecules and since there is no surrounding particles to which the gas molecules can loose their kinetic energy, I would think that it would retain it's temperature. Afterall the only way in which anything can loose heat is by either giving it's kinetic energy to surrounding particles or emmiting it through electromagnetic waves. I don't believe that the gas would start emmiting waves as it is released into space, would it?
     
  7. Pressure and temperature are inversely proportional. In vacuum pressure roughly = 0. Gases expand to occupy space. You can work out the rest...

    Edit: I think what I wrote is bollocks (haven't slept for about 48 hours). So here's a gas law to cover my arse.

    Pressure * volume = number of molecules * gas constant * temperature.
     
    Last edited: Mar 31, 2008
  8. Shooting Star

    Shooting Star 1,979
    Homework Helper

    For an ideal gas in isolation, your argument is quite correct. However, there are various intermolecular forces in a real gas, which are feeble but attractive when the distances between molecules become large. In order to overcome these forces while expanding, the molecules have to lose a part of their KE, and the gas cools down in the process.

    About the loss by EM radiation, that is also bound to happen. If two molecules collide, an electron may be pushed to a higher energy state, and it'll come to the ground state subsequently by emitting a photon.

    The a/V^2 term in van der Waals’ eqn of state represents the attractive force, which reduces the observed pressure of a real gas, as compared to an ideal gas.

    An ideal gas would not cool down while expanding in space, but any real gas would.

    Pardon, your **** is still showing...it's not number of molecules, but number of moles.
     
  9. russ_watters

    Staff: Mentor

    That isn't correct, Shooting Star (though the reasoning for what actually happens is correct). An ideal gas undergoing adiabatic expansion cools down, as the ideal gas equations predict: http://en.wikipedia.org/wiki/Ideal_gas_law
    http://en.wikipedia.org/wiki/Ideal_gas_law
     
  10. Shooting Star

    Shooting Star 1,979
    Homework Helper

    Russ,

    An ideal gas expanding in space has no reason to cool down. It does not have any intermolecular force to overcome, nor any work to do by pressing on any outside wall. The average KE per molecule stays constant, since there is no way it can lose heat. (I am, of course, not considering heat loss by radiation, which will happen in a real gas.)

    Perhaps I made a mistake by writing the phrase "gas in isolation". I was replyiing to Nerd's comment: "If the hot gas is released into space, we would naturally think that the gas should cool down."

    Some further clarification from
    http://en.wikipedia.org/wiki/Adiabatic_free_expansion:

     
  11. russ_watters

    Staff: Mentor

    I stand corrected. I didn't think that the "without constraint" part was critical, but I guess it is. If you expand a gas through a nozzle in the atmosphere, the atmosphere will absorb some of the energy of the expansion. In free expansion, the gas will literally expand forever. I'm still not 100% clear on this, though, since the page on the ideal gas law includes adiabatic (but not necessarily free) expansion and a temperature change calculation. I guess with free expansion, though, the volume goes to infinity and the pressure to zero.
     
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