Why Do Molecules Move?

  1. I mean, I know that they move faster the more energy there is, but why do they move at all in the first place?

    Would a single molecule move if gravity wasn't acting on it and it didn't touch anything else?
     
  2. jcsd
  3. mathman

    mathman 6,617
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    The simplest answer is inertia. They have energy and momentum. Although collisions result in transfer, there is no loss, if nothing outside is affecting their motion. Also there are significant differences in what is going on depending on the state (solid, liquid, gas).
     
  4. I'll take a stab at this. A molecule consists of two or more atoms with all kinds of internal, dynamical "vibrations", thermally induced or otherwise inherently present. This arrangement causes a constant, quasi-cyclic natural shifting of molecular position with respect to the immediate "environment"
    Though most readily seen is gases, this occurs in solids as well.
     
  5. I've been a little puzzled by this one actually. I was reading up on The Kinetic Theory of Gases and was a little surprised by what I saw. Can anybody tell me this:

    Is a molecule of hydrogen gas at a fairly ordinary pressure really travelling at 6000 feet per second?

    http://web.lemoyne.edu/~GIUNTA/classicalcs/joule.html

    "Therefore, since it is manifest that the pressure will be proportional to the square of the velocity of the particles, we shall have for the velocity of the particles requisite to produce the pressure of 14,831,712 grs. on each side of the cubical vessel, v = (14,831,712/395.938)1/2 x 32 1/6 = 6225 feet per second..."
     
  6. Doc Al

    Staff: Mentor

    That sounds about right for the rms speed of room temperature hydrogen gas.
     
  7. So I'm surrounded by air travelling at the speed of a bullet. Hmmn.
     
  8. Hootenanny

    Hootenanny 9,677
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    Yes, but what is the mass of a hydrogen /oxygen / nitrogen etc. molecule compared to a bullet? Compare momentums not velocities.
     
  9. So why's it going at 6000 m/s?

    If its all down to internal vibrations, where's the imbalance that causes it to propogate in one particuler direction?
     
  10. russ_watters

    Staff: Mentor

    Yes, don't you feel it?

    [hint: you'd notice if you held your breath and that air suddenly went away...]

    edit: ehh, better yet, just squeeze a basketball or soccer ball. What is pushing back at you?
     
    Last edited: Jun 20, 2006
  11. russ_watters

    Staff: Mentor

    There is no imbalance. Some are going left, some right, some up, some down -- every direction, essentially randomly.

    If, however, you release some air into a vacuum, then there would be an imbalance and the air would flow.
     
  12. So could I just stop a molecule of hydrogen gas, and if I did would it just sit there, presuming its on its own.
     
  13. To completley stop the molecule's movement you would have to reduce it's temperature to 0 K's

    Good luck to you.
     
  14. russ_watters

    Staff: Mentor

    How exactly would you stop it? When it hits something, it bounces.
     
  15. Hootenanny

    Hootenanny 9,677
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    And even at 0K there will still be some movement.
     
  16. So, I'm still not getting what causes the locomotion of molecules.

    If I reduced temperature to 0K, then warmed it up again, what makes it move off in a particular direction?
     
  17. So, I'm still not getting what causes the locomotion of molecules.

    If I reduced temperature to 0K, then warmed it up again, what makes it move off in a particular direction?
     
  18. russ_watters

    Staff: Mentor

    How do you warm it back up again? The answer to that is the answer to your question...
     
  19. Head on collision with another with exactly the right circumstances?
     
  20. mathman

    mathman 6,617
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    Conservation of momentum would simply require the molecule to go at the same speed in the opposite direction.
     
  21. To warm it up you'd have to give it energy which would involve hitting it with other particles or shining a light on it. Either way, something will come in, hit the particle and give it energy. The collision will obey the conservation of momentum and so the direction the particle flies off in will be dictated by the particle which warms it up.
     
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