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Force in a vacuum

  1. Jul 2, 2010 #1
    I understand that there are 4 forces: Gravity, Electromagnetic and the two nuclear ones. So, which one is responsible for the force that pushes air (for example) into a vacuum?

    Am I correct in assuming its electromagnetic? Like... there is some sort of electromagnetic potential between the air molecules even at room temperature, or even far below room temperature? Or is it a more entropic reason. Or is it somewhere in between?
  2. jcsd
  3. Jul 2, 2010 #2
    You could state your question simpler - if I got you right, you want to know which elementary force explains air pressure.

    It is indeed electromagnetic interactions that explain it, like it explains the majority of the observed "physical" or "contact" forces on macroscopic level - i.e. push/pull, friction, tension and elastic.

    At very close distances, electron shells of gas molecules repulse each other, causing molecules to bounce off each other, in the same way that they bounce off the container's walls.
    From there on, it is entirely a statistical/entropy effect that the molecules will get distributed in such a way to equalize the pressure caused by these bounces evenly throughout the available volume.
  4. Jul 2, 2010 #3
    Thermal :wink:

    Actually depends on what level of description you want to look at it from. The force that allows thermal collisions, and prevents you from walking through walls, is electromagnetic. Of course the way it gets expressed in practice also depends on the other forces at some level.
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