# FeaturedB Warm air goes up...reason on a microscopic scale?

1. Jan 21, 2018

### Wrichik Basu

I was wondering about the microscopic reason warm air rises up, while cold air comes down. I am aware of the macroscopic reason - density changes. But what happens microscopically? Decrease in density means that the gas molecules are widely spaced out, but their mass remains the same. Then why does warm air go up?

2. Jan 21, 2018

### BvU

Hi,

Microscopically, it's all collisions, collisions and collisions.
Basically 'hotter gas' means 'higher kinetic energy of molecules'.
Faster molecules go further and redistribute kinetic energy in the 'slower' volume.
Faster molecules exert more 'pressure'

All from the kinetic theory of gases, which google to find something you like to study from

(PS I do hope some real expert like @Chestermiller puts me right if I claim too much )

3. Jan 21, 2018

### Wrichik Basu

And the slower molecules just come down when displaced from their position?

4. Jan 21, 2018

### BvU

You can't have read all this stuff in the link yet ? A bit heftier is e.g. this pdf. But a textbook might be more suitable.

'just come down' doesn't describe it in a conceptually responsible manner. It's probably right statistically, though.

Microscopically, gravity is just a puny effect. But for the huge number of molecules involved it only works in one direction: 'down', so macroscopically it sure counts.

5. Jan 21, 2018

### Wrichik Basu

I know kinetic theory of gases. I know that the molecular speeds and collisions increase with increase in temperature. I just wanted to know why the increased collisions and velocity pushes the molecules up.

6. Jan 21, 2018

### BvU

The density of a hotter gas is lower macroscopically. But you were asking about microscopic effects.
'They need more room' is unsatisfactory, I suppose ?

7. Jan 21, 2018

### Staff: Mentor

The decrease in density is the reason. Temperature is a statistical phenomenon so you need a decent number of molecules. If you have a large enough number of molecules to have a well defined temperature then you also have a well defined density.

8. Jan 21, 2018

### Wrichik Basu

It's satisfactory.

What happens to the gas which is cool? It needs less room. That's why it comes down?

9. Jan 21, 2018

### Wrichik Basu

So you're saying that kinetic theory of gases explains change in density, and density, in turn, explains the rising of hot gases, right?

10. Jan 21, 2018

### Drakkith

Staff Emeritus
Hmmm. Might if have something to do with the fact that the pressure at the bottom of a mass of air is higher than at the top? Would the warm air molecules be able to better transfer their kinetic energy to the cooler air molecules near the top than the bottom because of this pressure difference? I assume that the warm mass of air that's rising is not composed of the same air molecules as it rises, but is the result of the net transfer of energy to air molecules progressively higher and higher up.

11. Jan 21, 2018

### sophiecentaur

You can't really have hot or cold molecules. A mass with molecules that are, on average, faster, will be hot etc.. Hot and Cold are Macroscopic terms.
I feel that the OP is under a bit of a misapprehension that better understanding things necessarily has to involve a microscopic approach. The statistics of a situation are highly relevant to what happens and the behaviour of an individual part of a system may tell you nothing useful.

12. Jan 21, 2018

### Drakkith

Staff Emeritus
Sorry, Sophie, but I don't see how my post uses the terms hot and cold (or warmer and cooler) in an incorrect or inaccurate way. I'm still talking about the statistical behavior of large numbers of molecules, a situation where hot and cold apply just fine.

13. Jan 21, 2018

### Staff: Mentor

I agree with that, but what I was saying is that the concept of temperature only makes sense with a whole bunch of molecules, and once you have enough molecules to have temperature then you also have enough molecules to have density.

14. Jan 21, 2018

### rumborak

Drakkith, I think your intuition about the pressure differential is right. In zero gravity hot air doesn't "move", it only slowly equalizes in temperature.

I suspect the higher average kinetic energy gives the "hot" molecules more opportunity to escape both up and down, whereas the "cold molecules" have less opportunity to do so, feeling gravity more in comparison. This probably results in a sort of "sorting algorithm", where hot molecules slowly percolate up.

15. Jan 21, 2018

### sophiecentaur

I don't think I'm being picky to pick on the use of hot or cold as a way to describe a molecule. That was what you appeared to b e doing in your post. Rather than 'hot molecules', you could use "hot gas with mostly faster molecules in it" but it is the gas that's hot and not the molecules
We're in the region of the Maxwell's Demon which is a though experiment in which there is a trap door which lets fast molecules through one way and slow molecules through the other way - thus separating all the 'hot' molecules from all the 'cold' molecules. Thermodynamics doesn't allow that.
I remember a guy describing the three electron beams in an old CRT tube as having red green and blue electrons in them. That was almost allowed into a BBC TV Science programme script until I got agitated about it. Same (not false) dichotomy.

16. Jan 21, 2018

### sophiecentaur

But the molecules are likely rather to transfer their momentum to nearby molecules than to move by themselves. Gas conducts heat faster than diffusion of different gases. And conduction works downwards too. This is why I am not happy with trying to deal with the 'small' in statistical processes.

17. Jan 21, 2018

### Drakkith

Staff Emeritus
That's certainly not what I intended.

Ah, but the gas is composed of a large number of molecules, so saying "hot molecules" should be synonymous with "hot gas" in this case.

I'm not so sure. I'm thinking along the same vein as sophie, in that individual molecules are mostly randomly moving about and it is their net transfer of energy upwards that leads to the hot mass of rising air, not because the original molecules in the warm air are all moving upwards.

18. Jan 21, 2018

### Stephen Tashi

That's an interesting question and to consider it seriously, you would have to model a microscopic situation.

For starters, suppose we have two containers having the same dimensions In one container, we have one slow moving particle and in the other container a fast moving particle. As the particles move and bounce off the walls of the container, does one spend more time near the top of the container than the other? If the container was tall and the slow moving particle was very slow, it might never bounce up near the top of the container.

For another simplistic situation, consider the case when the slow moving particle does have enough energy to hit the top of the container and bounce off of it. Consider particles that are bouncing straight up and down. As time passes, which of the particles spends more time in (say) the top half of the container? I haven't computed the answer , but it amounts to combining the analysis of two elementary physics problems - i.e. "A ball is shot upward from the ground with initial velocity V0 ..." and "A ball is thrown straight down from a height H with an initial velocity -V1..". (The question isn't which particle spends the greater percentage of its "own" time in the top half of the container, the question is which particle has spent more time in the top half of the container after a long time T has elapsed. )

19. Jan 21, 2018

### rumborak

What you are describing is heat conduction. However, air actually being a pretty good insulator, it could not account for the often drastic and fast changes.
Nah, hot air rising is likely to be due to convection instead. Even in the atmosphere, that's how wind comes about after all.

20. Jan 21, 2018

### Drakkith

Staff Emeritus
Hmm. Good point.

21. Jan 22, 2018

### sophiecentaur

I think this is the nub of our disagreement. You are implying that the word 'molecules' increases the understanding of the phenomenon. We're in the same neck of the woods as when people want to discuss circuit theory in terms of 'electrons', as if that will help them in any way at all. The microscopic has its place but that place is not everywhere. If I throw a brick at you and I describe its trajectory, would it help in any way at all to discuss the molecules of the brick, rather than using macroscopic Newtonian Physics.
Of course, I realise that you know the basics but I always try to look at these questions from the point of view of someone who doesn't. Your approach to this particular problem could serve to promote the mistaken impression that the microscopic is the only really valid approach. But just look at General Relativity. So far, it has not been reconciled with QM, which is the ultimate in microscopic approaches.

22. Jan 22, 2018

### Drakkith

Staff Emeritus
No, I'm just using "hot molecules" in lieu of "molecules of the hot mass of air".

It often does as long as the person asking understands that while you can talk about what the an individual or small number of charges in a wire are doing, the equations and components that are encountered in basic circuit theory are described and formulated in terms of the net behavior of a large number of charges.

This is silly. The OP literally asked what's going on at the molecular level, so I don't know why you're berating me for some imagined slight against scientific education when all I've done is say "cooler air molecules" instead of whatever you personally prefer. I appreciate your dedication to wanting to teach people the correct terms and avoid inaccuracies and pitfalls, but I can't help but feel as if you're twisting what I've said all the way to the breaking point and blowing it well out of proportion.

23. Jan 22, 2018

### sophiecentaur

Sorry. I am 'berating' the idea and not the man.
I would say that the correct answer is that there is not a one step answer at the molecular level and I think the OP should have been given that message from the start (or at least a massive caveat). The relationship between PV and T can be derived from a particle model as can the density but, once you've got the macroscopic model, surely you stick with it. When a gas departs from Ideal, due to the effect of Van Der Waal's forces, any PVT calculations can still be done. Where is one supposed to stop in the use of microscopic models? Would every physics calculation have to include lower than statistical treatment in order for it to be 'acceptable'? I guess my Engineering background can be blamed for my being prepared to have my scientific life full of black boxes but the onion approach to learning seems to work very well; you only need to peel away another layer when the one you are at starts to let you down.

24. Jan 22, 2018

### Wrichik Basu

@Drakkith and @sophiecentaur hope that the misunderstanding has been resolved, because it doesn't look good when two esteemed members of PF don't agree over something

From all the discussions, I've come to this conclusion: I should explain change in density using kinetic theory, and then use density for explaining the rest.

Thanks for the explanations. I've once again received a proper explanation at PF, and thanks to all for that

Last edited: Jan 22, 2018
25. Jan 22, 2018

### Khashishi

Consider a bunch of microscopic dots, moving in random directions in a box. Consider an imaginary surface cutting the box into a top and bottom half. The dots in the bottom half move faster than the dots in the top half. So, there are more dots that move across the surface from the bottom half to the top half than vice versa.

But, as more bottom dots start to invade the top half, collisions between the bottom and top dots pushes some more top dots into the bottom half.