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Equilibrium in pent up energy during decompression.

  1. Aug 2, 2011 #1
    I understand the atmospheric equilibrium concept... but not the why behind it. In other words, when you shoot a window out in space station what factors decide that the higher state of compression of the two burst out into the lower until a balance is established?

    Side note: A question popped into my head while typing the first question. What causes the atmosphere of our planet to stay put (remain stable)? I want to say gravity and temperature, but figured I'd throw it out there anyway.
  2. jcsd
  3. Aug 3, 2011 #2


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    Welcome to PF!

    Hi Ntstanch! Welcome to PF! :smile:

    Pressure is defined as force per area.

    Force is rate of change of momentum, so the https://www.physicsforums.com/library.php?do=view_item&itemid=80" when molecules bounce off it (basically, roughly twice the actual momentum).

    So if one side of the window has higher pressure, that means more momentum is hitting it on that side than on the other side

    Remove the window, and the molecules that would have hit it will go straight through to the other side …

    that means momentum has been transferred to the side with lower pressure, and that keeps happening until the pressure is the same.
    Yes, the molecules at the top of the atmosphere will escape if their speed (which depends on temperature) is high enough to exceed https://www.physicsforums.com/library.php?do=view_item&itemid=154" (which depends on gravity).

    For hydrogen and helium, it is, for oxygen and nitrogen it isn't.
    Last edited by a moderator: Apr 26, 2017
  4. Aug 3, 2011 #3
    I apparently took too long trying to clarify my question... which was fairly short, but I tried to refine it for like an hour and it logged me out. So, I'll just stick to the original intro of my response and ask again later. *quick facepalm*

    Thank you, both for the response and the uncommonly friendly welcome. :)
  5. Aug 4, 2011 #4
    Right, going to make this short (I hope).

    I was asking more about why the force is there and how it evacuates itself. An example I could think of was to compare a water balloon in zero-gravity having a small hole put in it, and how (spatially) you can see the contraction of the rubber, which was stressed out of it's normal equilibrium, as it presses back into its normal state while at the same time ejecting the pressurized contents.

    Though with a solid structure (a space station) that physically and spatially apparent force isn't really there. It's obvious that the compression into the area applies pressure to the walls of the station, and you can increase or decrease that pressure... but when you "shoot out the window" what forces cause, on an atomic level, everything to flow out into the lower pressure area?
  6. Aug 4, 2011 #5


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    The air in the room is under pressure. It exerts a force on the walls.

    Think of what you'd see if the airlock had a large clear window made out of a flimsy sheet of acrylic. The acrylic would bow outward under the pressure, right? Now imagine what would happen if you knocked that flimsy piece of acrylic out. The air, under pressure, would pour out.

    How is air pressure measured? In PSI - pounds per square inch. If the pressurized room were at a pressure of a mere 1.1 atmospheres inside, and 1 standard atmo outside, that's a differential of 0.1 atmosphere - 1.47 pounds per square inch.

    That means that large, flimsy acrylic window is experiencing a pressure of almost 1 1/2 pounds for every square inch. If it were a square ten inches on a side, the air would be pushing on it with a sum total of 147 pounds - the weight of a person, trying to push that window out of its seals.
    Last edited: Aug 4, 2011
  7. Aug 4, 2011 #6
    Sorry, I often have trouble being clear with my questions. I'll try again using the atomic-structure illustration thing I recall from a chem book I had for a class a few years ago (from memory). The book showed a large cube with a bunch of equal sized smaller cubes inside and illustrated compressing forces by connecting the cubes with a spring like design. So you could imagine the cube as a set of box shaped links formed into a larger cube where each connection had a spring like element in relation to the whole. And when you compress it, like compressing gases into the space station, it contracts due to the external forces applied to it, though it also pushes back, and when you release (kick out the acrylic) it bounces back into a balanced form.

    Though for me this doesn't explain how all of the compressed gases manage to evacuate. Outside of the fact that they will, in every case, do so.
  8. Aug 4, 2011 #7


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    They won't. They flow only until they reach equilibrium. If the other chamber is vacuum and of equal volume, the whole system will stabilize at half pressure (same particles, twice volume). If the other chamber is empty space, then it will stabilize at zero pressure (same particles, infinite volume).

    But it does take time. Each atom bounces around inside the original chamber losing energy but eventually it will fly right out the open airlock. As pressure drops, atoms bounce off each other less and less. The pressure drops asymptotically to zero. i.e the last few atoms may take a looooong time to leave. This is why it is virtually impossible to create a high vacuum anywhere other than space. You can only get arbitrarily close to vacuum. There's no way to get tohse last few (million) atoms out of the chamber.

    Perhaps the key point to recognize is that atoms of a gas are always in motion, flying around the room, bouncing off walls and other atoms.
  9. Aug 4, 2011 #8

    I think you are missing something in your thought processes. You have to remember that atoms and molecules all have energy and are constantly vibrating and pushing and shoving each other. By compressing any gas you are putting more and more energy into it. Once you decompress the gas this energy gets released. If it is released slowly over time then you have a controlled release if it is all released suddenly like a sudden rupture in any gas (not gasoline) tank you have an explosive release of the energy. You can even compress a gas until it becomes liquid that means you have a tremendous amount of energy stored up and if released suddenly, POW!:

    http://pipeline.corante.com/archives/2006/03/08/how_not_to_do_it_liquid_nitrogen_tanks.php [Broken]

    Last edited by a moderator: May 5, 2017
  10. Aug 5, 2011 #9
    I am aware of these things (don't take that as me being ungrateful for the responses).... I suppose I may be wondering more so about the distribution of energy at the atomic level and how their interactions seem to involve more of a fluid like interaction in an electromagnetic attraction/repulsion sense. Instead of the (more or less) pool ball ricochet interaction model that I'm used to.

    This is the last question I'll ask for tonight (My summer job = constant sun and brutal work, which doesn't help me in being efficient with my questions).

    If you were to compress our galaxy down to half its original size, what would be the most notable and observable consequences? And if you released whatever was compressing the galaxy, or poked a hole in the thing compressing it, what would occur as it reverted back to it's natural state?
  11. Aug 5, 2011 #10


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    But it is a pool ball interaction, not a fluid one …

    the gas is mostly empty space, and the collisions and migrations do proceed one at a time!
    Sorry, this question makes no sense (you can't "compress a galaxy"). :redface:
  12. Aug 5, 2011 #11
    Yeah... I think I need a couple more semesters in me to really ask these types of questions in detail. Right now I don't know enough about what I'm asking to form a proper question. However I do enjoy asking approximately as many questions as I can without wasting peoples time.

    As for not being able to compress a galaxy... all I have to say is, "Not yet". They told Einstein you can't ride a beam of light, and years later he rode one, and then crashed, and it killed him. :tongue2: (trying to copy some humor from a movie called 'Dinner For Schmucks')
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