Hot Air: Does It Rise or Is It Heat Transfer?

[nolanp2]

everybody knows that hot air rises. but on thinking about heat as the knetic energy of molecules, heat transfer just becomes another word for collisions between molecules (looking at a gas) right?

so surely hot air, in it's attempt to rise from the bottom to the top of a container, would collide with colder air and hence heat these molecules and cause them to rise instead.

so basically what I'm wondering is does hot air rise, or is it just the heat that rises?

 

[ranger]

Does reading this answer your question?

 

[Astronuc]

everybody knows that hot air rises. but on thinking about heat as the knetic energy of molecules, heat transfer just becomes another word for collisions between molecules (looking at a gas) right?

so surely hot air, in it's attempt to rise from the bottom to the top of a container, would collide with colder air and hence heat these molecules and cause them to rise instead.

so basically what I'm wondering is does hot air rise, or is it just the heat that rises?

Heat transfer by molecular or atomic collision is consider conduction. Heat transfer by bulk fluid transport is considered convection, and it is driven by buoyancy, or the difference in gravitational potential between regions of air of different densities.

 

[Sojourner01]

I intensely dislike the explanation that 'hot air is less dense and so is displaced by colder air'. It's a good approximation, but it is most definitely not the way real gases actually behave. It implies that real gases float around in chunks that obey classical mechanics, and that a given air molecule 'belongs' to a chunk of gas that you define as warmer than another chunk. Clearly this isn't true. Please, for the love of god, look at the statistical viewpoint.

 

[russ_watters]

Sojourner01, it depends on the context, but it largely is true. That's how weather works, for example. A thundercloud is a large "bubble" of warm air, for example, and wind is caused by the displacement when that air rises.

 

[Astronuc]

I intensely dislike the explanation that 'hot air is less dense and so is displaced by colder air'. It's a good approximation, but it is most definitely not the way real gases actually behave. It implies that real gases float around in chunks that obey classical mechanics, and that a given air molecule 'belongs' to a chunk of gas that you define as warmer than another chunk. Clearly this isn't true. Please, for the love of god, look at the statistical viewpoint.

I was driving down a freeway one night in Houston with the window open. In an instant, the air temperature went from the 70's to the 40's - within feet. It was like going across a boundary plane it was that distinct.

 

[Newbie says Hi]

I was driving down a freeway one night in Houston with the window open. In an instant, the air temperature went from the 70's to the 40's - within feet. It was like going across a boundary plane it was that distinct.

Sure it wasn't the ac kicking in? :)

 

[Sojourner01]

Sojourner01, it depends on the context, but it largely is true. That's how weather works, for example. A thundercloud is a large "bubble" of warm air, for example, and wind is caused by the displacement when that air rises.

It's a good model but it's not a good explanation. Surely you understand the difference between rationalising a useful model and providing a qualitative explanation of what is actually happening? - even if an analytical solution of the 'real' scenario is beyond the ability of the student in complexity and/or not that useful in solving quantitative problems.

 

[nolanp2]

i don't like the explanation of changes of density because from what little kinetic theory I've done gravity on particles is ignored to derive equations such as pV=nRT etc, and obviously density doesn't mean much when looking at individual atoms. i would rather some kind of explanation along the lines of what Sojourner seems to be argueing, even if i can't understand it now at least i'd be given a start in the rite direction.

any rough explanation would be appreciated, this questions regularly pops into my head and is quite annoying!

 

[nolanp2]

i suppose my question could be rephrased as how does kinetic theory explain convection?

 

[russ_watters]

i don't like the explanation of changes of density because from what little kinetic theory I've done gravity on particles is ignored to derive equations such as pV=nRT etc, and obviously density doesn't mean much when looking at individual atoms.

What does density have to do with gravity? Density is mass per unit volume.

And don't forget, gravity is what causes the "p" in that equation...

i would rather some kind of explanation along the lines of what Sojourner seems to be argueing, even if i can't understand it now at least i'd be given a start in the rite direction.

I'd like to hear that explanation too...

The bulk fluid transport model may not work in every case, but it does work very well in an awful lot. "Clearly this isn't true" doesn't seem to me to be true...

Heck, there is one example where it is almost perfectly true: in a hot air balloon! A hot air balloon is a near-perfect control volume and the statistical mechanics view won't help you at all there.

IMO, the statistical mechanics view leaves too many unanswered questions - it just plain doesn't explain what is actually happening. It may provide a two-levels-down "why", but it doesn't explain, for example, why a candle flame in a still room creates a very laminar smoke pattern. For that, you need fluid mechanics in addition to the bulk-mass transfer model. Maybe I'm missing something about the statistical model, but it seems to me that it would predict more mixing in places where it isn't seen. It is, however, required for explainng things like why there is very little hydrogen in our atmosphere.

 

[nolanp2]

i know that using density to explain it will justify a lot of a fluid's behavior. but surely you could view density at a point as the number of atoms within a certain distance of an atom at the center of this point.

now you are saying that denser sections fall under gravity faster than less dense sections. And so by looking at the system using my view of density, your model suggests that the more (roughly) independent particles you allow fall under gravity the faster each particle will fall, which can't be true.

your explanation gives a good model but it is the why in particular that i am looking for an explanation of.

 

[Mentz114]

Consider a column of monatomic gas in thermal equilibrium. All the atoms are moving randomly with velocities given by Maxwells distribution. The hotter the gas, the faster the atoms move. The mean free path of the atoms varies with pressure, so the ones at the bottom collide more frequently
than the ones at the top.

Suppose we now disturb the equilibrium by heating the gas slightly - in the middle of the column. The hotter atoms are moving faster than those in the colder sections above and below. But the gas above has a greater mean free path, so the hotter atoms will penetrate ( diffuse) further in the 'up' direction than the 'down' direction.

QED ?

[Added]
I agree with Russ, above, more than this is needed to explain the rapidity of some phenomena. Perhaps if we throw in some Brownian motion concepts, like minute pressure fluctuations, that create bubbles we can get there. See Einstein's original BM paper.
( OK, I've managed to mention E and M so I'll stop).

M