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Gases are kept in the air by their large kinetic energy

  1. Sep 29, 2004 #1
    gases are kept in the air by their large kinetic energy, no? if we observed one particle in the atmosphere, however, would we not observe it to fall in a parabolic arc(in between collisions, which keep it propped up) much as a macroscopic particle would?

    this just gets me thinking because-with singular particles in a box, wouldnt the particle go towards the bottom of the box? or... if at that level the collision is elastic, it would go back to its original height , but there have the lowest kinetic energy

    is that why its cold in high places?
  2. jcsd
  3. Sep 29, 2004 #2
    You neglected the fact that there are so many air particles(All kinds of mloecules) so that the "air particles" you mentioned would be in a medium of similar density. It's like pouring water into water; water does not sink through water like matter with high densities.
    It's just that air has a very low density, so it doesn't seem that obvious.
  4. Sep 29, 2004 #3
    i said "in between collisions, which keep it propped up". aside from the collisions.
  5. Sep 29, 2004 #4


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    It's cold in high places for a couple of reasons. One is radiational cooling and another is that gases do work to expand as they climb thereby cooling.
  6. Sep 29, 2004 #5


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    Essentially yes, that is correct. But you cannot neglect the temperature. Since the average velocity of the molecules in a gas is determined by the temperature, if the temperature is high enough, the molecule may well have a velocity greater the escape velocity. So the parabolic path could take the molecule into space and free of the earths gravitational field. This is one reason why we do not have free Hydrogen or Helium in our atmosphere. The fact that H loves to react, will be another reason we do not find it. However even if it were not reactive any free H would be lost to space.

    It is possible of fill an open container with a heavy gas, such as Ar or CO2.
  7. Sep 29, 2004 #6
    I think kuenmao said it right:
    Just think of air (gasses in general) as a low density liquid.

    I had this feeling often when flying on a plane and watching the wonderous weird cloud formations, if you think of the air atmosphere as a liquid sea covering the whole earth, it doesn't look so weird anymore (yet beautiful anyway :-).
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