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Why does hot air rise?

by jammieg
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jammieg
#1
Jun23-04, 08:39 PM
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Why does hot air rise?
What I'm really getting at is why should the speed of kinetic motion of the individual atoms of heated air rise compared to it's slow moving neighbor, is it merely because it bounces around more often and so all air competes for dominance upward such that the fastest moving air must go up? I mean when I watch smoke rise I think well these must be some heavy particles in that smoke and so they should go down but instead they go up because it's warmer than the surrounding air...seems odd to me, but then my thermodynamics knowledge is basic maybe that's why or maybe I'm too philosophically trained to accept this answer and be done with it.
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chroot
#2
Jun23-04, 08:44 PM
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Hot air is less dense and experiences a buoyant force, just like a bubble of air in water.

- Warren
Loren Booda
#3
Jun23-04, 11:04 PM
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According to statistical mechanics, where air has greater kinetic energy it has a greater probability to occupy a higher gravitational potential than less energenic air. That is, the system tends toward maximal entropy.

Ian
#4
Jun24-04, 09:34 AM
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Why does hot air rise?

Chroot is correct, hot air rises because it occupies a greater volume per unit mass of air than cooler air. It is simply an application of Archimedes principle.
Incidentally, if the volume remained constant as the temperature rose, then according to GR/SR since energy is a mass equivalent the hotter body would sink rather than rise.
quartodeciman
#5
Jun24-04, 11:46 AM
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Jammieg has posed an excellent question.

There are quick answers to it, but then there must follow deeper and more detailed answers IMO. Why doesn't the less dense hotter air simply diffuse into the denser cooler air across the given boundary layer? Why doesn't the system just slowly move to thermal uniformity without material exchange?

There is something called a temperature gradient that is supposed to tell much of the story. I haven't found any really good explanatory text online about this subject (free convection).

Jammieg has posed an excellent question.
chroot
#6
Jun24-04, 02:09 PM
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There are, in general, two forms of heat transport in play here: radiative transport, and convective transport. When the temperature gradient is low, radiative transport is most efficient, and heat "diffuses" without any mass moving around. When the temperature gradient exceeds a threshold, however, convection becomes more efficient and heat is exchanged by the movement of mass.

- Warren
krab
#7
Jun24-04, 04:30 PM
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A related question, also very interesting, is Why do cold and hot water regions not self-mix? We've all experienced these when swimming in a quiet lake in the summer. In particular, one can dive to the bottom of a deep lake and discover many layers of different temperature. Why are there layers instead of a more or less constant gradient? and why does the warmer layer not warm the colder one? I've never seen a good explanation of this.
RAD4921
#8
Jun24-04, 05:37 PM
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I agree with chroot and the others. Cold air is denser and heavier than hot air. I also agree that hot air has more "kinetic energy" which in turn makes the atoms take up more space and therfore makes it less dense and lighter.
napoleonmax
#9
Jun24-04, 08:37 PM
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Hmmmmm..... I'm not sure it has to do with buoyancy (though this would appear to make sense). I remember reading somewhere the fire system used on the International Space Station and it said the flames do not rise in space but rather moves in all directions. Therefore, I think Loren Booda's explanation makes the most sense.
napoleonmax
#10
Jun24-04, 08:41 PM
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Quote Quote by krab
A related question, also very interesting, is Why do cold and hot water regions not self-mix? We've all experienced these when swimming in a quiet lake in the summer. In particular, one can dive to the bottom of a deep lake and discover many layers of different temperature. Why are there layers instead of a more or less constant gradient? and why does the warmer layer not warm the colder one? I've never seen a good explanation of this.
I think that layers of cold and hot DO mix, just not necessarliy at a very fast rate. The laws of thermodynamics state that heat moves form more dense to less dense, so hotter regions must get colder and vice verse. However, in a lake, the top is getting much more sunight than the bottom and therefore absorbs more heat than it can transfer, causing layers of hot and cold in water.
Janitor
#11
Jun24-04, 10:03 PM
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After one experiences the sight of the snowy top of a mountain ridge while sweating in the heat of the desert floor below, one might naively assume that heat falls rather than rises. This strikes me as one of those situations like when someone finishes explaining why an airplane wing generates lift because of the shape of the airfoil, and then some other guy pipes in with, "Yeah, but I was at an airshow last Saturday, and doggone it if there weren't some airplanes flying upside down!"
Moe
#12
Jun25-04, 05:11 AM
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You can't fly upside-down forever, just for short periods of time.
The temperature profile of our atmosphere is quite jaggy, at first it drops with altitude, then it starts to rise again. The reason for that are different layers that absorb, reflect and radiate heat. At first it gets colder because you are moving away from the earth, which stores heat fairly well, that is why you can have snow on a mountain overlooking a desert. The fact that the air gets thinner (=less dense, less total heat capacity) might play its part as well.
JohnDubYa
#13
Jun25-04, 12:24 PM
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I think Loren's explanation is the most accurate.

Systems want to minimize potential energy, which is why rocks fall when released. In order for a particle to increase its potential energy, it must have sufficient kinetic energy that it can transform into potential energy during the process.

The hotter the air, the more likely an individual molecule will have sufficient kinetic energy that it can transform into potential energy (which means raising its height).

So it is a statistical property.

I try not to use the concept of buoyant force in such explanations, because it tends to beg the question. Instead, think in terms of the inherent desire of systems to minimize potential energy. Helium balloons rise because the potential energy of the room is less after it has risen than before. (Light volume of gas moves up, heavier volume of gas moves down to replace it.)
kuenmao
#14
Jun25-04, 12:28 PM
P: 76
I don't get why mechanical energy has to do with it rising...can somebody elaborate a bit on that?
Anyway, what I think(which may be wrong) is that since both the hot air and cold air are at the same pressure(atmospheric pressure), by the ideal gas laws, the same mass of hot air would occupy more space than the cold air. That means that the mass per unit volume, which is density, is smaller. By fluid mechanics, less dense objects float on top of denser objects, and so the hot air floats.
Anything wrong with that?
chroot
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Jun25-04, 02:36 PM
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Quote Quote by Moe
You can't fly upside-down forever, just for short periods of time.
Wrong. In a proper aircraft, you can fly upside-down all you like, forever. The problem with some small aircraft is that they make use of gravity in their fuel systems, and the engines will be starved for fuel after a long period of inverted flight. In a properly design aerobatic airplane, however, you can fly inverted forever.

- Warren
krab
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Jun25-04, 03:10 PM
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Quote Quote by kuenmao
I don't get why mechanical energy has to do with it rising...can somebody elaborate a bit on that?
Anyway, what I think(which may be wrong) is that since both the hot air and cold air are at the same pressure(atmospheric pressure), by the ideal gas laws, the same mass of hot air would occupy more space than the cold air. That means that the mass per unit volume, which is density, is smaller. By fluid mechanics, less dense objects float on top of denser objects, and so the hot air floats.
Anything wrong with that?
That's the best explanation so far.
chroot
#17
Jun25-04, 03:16 PM
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Isn't that what I said in the first response to the thread?!

- Warren
kuenmao
#18
Jun26-04, 04:51 AM
P: 76
Yeah chroot, you were right, but you could've elaborated a bit...kinda confusing to just say that it's less dense.


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