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Hot air

  1. Dec 24, 2005 #1

    daniel_i_l

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    I was thinking to myself, why does cold air go to the bottom of the room and the hot air to the top? At first the answer looked simple, the cold air is "denser" than the hot air. But if you think about it, at the molecular level the only difference between hot air and cold air is that the molecules of the hot air are further apart an are more energetic (please correct me if I'm wrong). If the fact that they are further apart (less dense) has to do with it, It seems like gravity pulls down on the air molecules makeing the bottom of the room have less that the top - this obviously isn't true cause if it were, this would also happen in a room starting with one temp. , the bottom would get cold and the top hot due to gravity. So it sees as if the reason that the hot air goes up is because of the fact that it iis more energetic - but what does that have to do with it?
     
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  3. Dec 24, 2005 #2

    russ_watters

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    Your instincts are better than your logic on this one.....

    Air pressure is higher at the bottom of a room than at the top (just not very much) and simple buoyancy is why rooms stratify.

    I'm not sure what you mean with that second-last sentence, though....
     
  4. Dec 24, 2005 #3

    daniel_i_l

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    Thanks russ, but lets look at a single molecule, why would a more energetic particle want to go to lower pressure?
    In that sentance a meant that if gravity was pulling the particles down making it more dense than the bottom, then the temp. of the room wouldn't matter.
     
  5. Dec 24, 2005 #4

    Tide

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    daniel,

    A single particle picture is inappropriate because the mean free path of molecules near STP is very short. It is virtually impossible for a hot or cold molecule to traverse the height of a room without colliding (exchanging momentum and energy) with another molecule. You really do have to think in terms of fluids.
     
  6. Dec 24, 2005 #5

    daniel_i_l

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    I guess my question applies to fluids to, if it is all one substance with particles that have different energies, why should the seperate out? Shouldn't they just crash around untill there're all the same? Why would the particles at the top have higher energy? Maybe the energetic ones don't "move" up to the higher pressure but rather the particles closer to the top get higher energies because there there's lower pressure? That takes me back to the second-to-last sentance of may first post.
     
  7. Dec 24, 2005 #6

    Tide

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    There's two parts to that. The first one has already been mentioned, namely, the hotter fluid is less dense and buoyancy carries it to the top. However, given enough time, top and bottom can come into thermal equilibrium so the temperature is a constant from top to bottom. In that case, the density is still greater at the bottom and less at the top since there must be a pressure gradient to support the gas against the pull of gravity.
     
  8. Dec 24, 2005 #7

    daniel_i_l

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    Thanks Tide, just one more thing, you say the hot air goes to the top cause it's less dense, but if we start with the hot on the bottom and the cold on top, as the hot goes up and cold goes down, then inbetween the hot particles there're cold ones so how can we still say that there's "dense" air and "less-dense" air? What makes the hot particles continue up and the cold down? Also, since the hot has more energy, shouldn't it hit the cold ones up as they're coming down ?(maybe this dosesn't stop the cold from comeing down cause it's denser?) And, what does this have to do with temp., would the same thing happen If there was dense and less-dense (same temp.) air in the room?
    I guess my main problem is, if the hot particles don't really travel all the way up (as you said in your first post) what makes the hot go to the top? Why doesn't the top just get less dense than the bottom - what does it have to do with the energy?
     
  9. Dec 24, 2005 #8

    Tide

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    daniel,

    That's a lot of questions! I think you're asking for the details of the mechanisms by which the transport occurs. There's basically three of them. At the molecular or microscopic level, there is diffusion which tends to even out density gradients via (very large numbers of) collisions. Momentum and energy are exchanged in this process. On the large scale (macroscopic) convection takes place with relatively large volumes of cold and hot air sliding past each other. In between, there is turbulence which is a kind of chaotic state between simple convection and diffusion. The details of each can get fairly complicated and I'll leave it to experts in the respective areas to jump in with their insights.
     
  10. Dec 24, 2005 #9

    Mk

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    Here's a good time to ask. Where does a candle flame point in the microgravity of SPACE, the last frontier. Does it point... up??
     
  11. Dec 24, 2005 #10

    Tide

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    It doesn't! Most likely a candle would extinguish itself promptly with the buildup of carbon dioxide around it unless air around it is convected artificially.
     
  12. Dec 25, 2005 #11

    Mk

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    Wow. Cooool...
     
  13. Dec 25, 2005 #12
    Last edited: Dec 25, 2005
  14. Dec 25, 2005 #13

    DH

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    at first, from the molecular view, hot air and cold air is defined by temperture.
    hot air has higher temperture than cold air. The temperture presents the average kinetic energy of a molecular gas. Kinetic energy is proportional to temperture. Higher temperture has higher kinetic energy --> move faster and more active. because the gas moves faster --> hot gas will try to move due on it higher kinetics energy and the cold gas is slow due to its lower kinetics energy. Because, the gas molecular still effect by gravity --> the lower kinetics energy can't move to the higher position while the higher kinetics move faster and so that its kinetics energy will go higher because the kinetics energy convert to potiential energy.
    --> that's all
     
  15. Dec 25, 2005 #14

    daniel_i_l

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    Thanks DH, thats the kind of answer I was looking for - the connection between the hight and energy.
    But I see one problem in that explanation (that hopefully someone can resolve) , if the more energetic particles go higher because they have more KE to turn into PE, the by the time they've gotten to the top they'll have conserved all of the extra KE (the difference between the the hot KE and the cold) that they had into PE and now they will have the same temp. as the cold?
    Also, this doesn't mean that a more energetic particle can't move down, it will just be faster as it gets to the bottom than the cold particles, or maybe the particles are moveing in many directions and the hotter ones have a better chance of getting to the top than the colder ones? But still, that explains why the cold ones go to the bottom but not why the hot ones, after using their KE to go up (and then become cold?) don't fall back down. So now my two questions are:
    1) If the reason that the hot ones go to the top is that they have more energy to use, then why, after useing that energy, don't they turn cold?
    2) what keeps an energetic particle from falling anyless than a less energetic one?
    Thanks in advance for any help!
     
  16. Dec 25, 2005 #15

    Tide

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    DH,

    Those are good arguments though it doesn't quite get to the point with respect to hot/cold air in, say, a room. The "hot" atoms may be moving at, e.g. 400 m/s while the "cold" ones at 200 m/s. This would hardly make a difference with regard to being able to make it to a higher position or not being able to make it to a higher position in the room.
     
  17. Dec 25, 2005 #16

    DH

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    sorry, it's typing mistake
    I have to say like that:
    the lower kinetics energy can't move to the higher position while the higher kinetics move faster and so that its kinetics energy convert to potiential energy.
    When the molecular gas go to the higher possition, the kinetics energy convert to potential energy, so that it's kinetics energy will decrease. By the view of a people who learn physics, we know that the temperature and kinetics energy relate to others by the forumlar: K = 3/2nRT --> when kinetics energy decrease, the temperture will decrease.

    Here is another view: if we put two molecular gas 1 hot and 1 cold in the space, which one will move far than other?
     
  18. Dec 26, 2005 #17

    daniel_i_l

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    So then if the hot air got to the top only with the help of it's extra KE, after is loses it shouldn't it now be at the same temp. as the cold air? I guess you could say that the hot air had a lot more KE then the cold air - is this correct?
     
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