Register to reply

Why does hot air rise?

by jammieg
Tags: rise
Share this thread:
jammieg
#37
Jun30-04, 11:27 PM
P: n/a
So the less dense hot unit of air is pushed from the bottom by the more dense air because the less dense unit is less affected by gravity? Heat is the cause of the relative difference in density from a hot unit of air to a cool unit which is really just the constant extra pushing of random motion, do I have this right? Why wouldn't gravity only affect the individual molecules alone, why the whole unit, it's as if it was working on both scales, it's as if the unit of hot air was connected in a way with each individual atom linked to the unit otherwise wouldn't each molecule sink as any other cooler molecule, I mean with gravity affecting them equally? Could the gravitational affect be operating on multiple scales, micro and macro, or is this an illusion of a simple mechanism for defying gravity that I still haven't quite grasped?
It's strange how a waft a smoke sticks together for so long...supposing all matter had some sticky property to neighboring matter, what would cause this?
zoobyshoe
#38
Jul1-04, 07:47 AM
zoobyshoe's Avatar
P: 5,641
Quote Quote by jammieg
Why wouldn't gravity only affect the individual molecules alone, why the whole unit...
A dam good question, one that points to the Booda/DubYa/Maxwell's entity as the real mechanism. Something like this: in a gravitational field higher energy molecules are able to do more work against gravity than lower energy ones.

I would think that this could be tested by containing some air in an insulated container with sensitive thermometers at vertical intervals. If this mechanism is the correct explanation, then an initial uniform temperature from top to bottom should change to a hotter reading from the top and a cooler reading from the bottom as the molecules with higher kinetic energy pass that energy to the ones on top from the ones lower down.

It would be fascinating to discover that gravity has been Maxwell's entity all along.
ArmoSkater87
#39
Jul8-04, 12:30 AM
P: 253
PV = nRT
That is the ideal gas law, P=pressure, V=volume, n=moles, R=gas constant, T=temp.
u can minipulate the equation:
(MM)PV = (MM)nRT [MM= molar mass]
(MM)n = mass
(MM)n/V = D = density
P(MM) = DRT
D = P(MM)/RT

As u can see, when T goes up, D goes down
If the air has a lower density, it will rise because of boyant forces
omin
#40
Jul18-04, 10:12 PM
P: 284
Gravity.

Take two bouncy balls. Exert less force on one than the other when you spring them off the ground and see which one rises from the ground up under gravity farthest. The one you launch with greater speed.

The warmer molecules push against gravity harder because they are moving faster, therefore they rise.

I'd say a bubble rising in a glass of beer is mass for mass much warmer than the beer.
Rampantbaboon
#41
Jul21-04, 05:00 PM
P: 13
well, because the weight of the atmosphere causes continous(sp.) pressure increasing as hight above sea level decreases, the pressure at any point directly below another point is higher. Because pressure is defined as a force per unit area there is more force on the point below causing a net force in the upward direction. This is true for all things in air, except the buyant(sp) force is usually negligible. Armo proved that the air is less dense and when and object is less dense than the fluid around it, the net force upward is greater than the weight.
JohnDubYa
#42
Jul22-04, 11:30 PM
P: 1,322
So the entire atmosphere should accelerate upwards, leaving the planet with no atmosphere?
brewnog
#43
Jul23-04, 07:34 AM
Sci Advisor
PF Gold
brewnog's Avatar
P: 2,793
Yes, but we have gravity.
BobG
#44
Jul23-04, 09:46 AM
Sci Advisor
HW Helper
BobG's Avatar
P: 2,284
Loren Booda's explanation would seem to make sense when you're talking about gases. But how would that explain a cork floating on the water?

The difference in density between two gases is due to the extra kinetic energy in the warm gas. Because of the molecules moving faster, one of two things has to happen. They push harder on their 'container' than slow moving molecules or the 'container' expands until the pressure from the fast moving molecules is the same as the pressure from the slow moving molecules (in other words, the warm gas becomes less dense).

The cool air, being more dense, having more mass per unit of volume, experiences greater gravitational force than the warm, less dense air. Relative to each other, the cool air has a net downward force and the warm air has a net upward force. Relative to the surface of the Earth, both have a net downward force (none the less, the Earth does lose a few molecules into space).
kishtik
#45
Aug1-04, 04:00 AM
P: 115
I can't imagine the buoyancy acting to a single molecule (of hot air for example). For a balloon, yes, the pressure is higher at bottom so a net force is exerted to it upwards. Or for a wooden block sunken at sea, when we release it, it goes up because of buoyancy and the difference of pressure (between top and bottom) causes buoyancy. A group of hot air molecules isn't an "object" to feel buoyancy. And the same for alcohol rising through water I think. So where am I wrong? Thinking of "alcohol", yes its density is lower etc. but I couldn't imagine the density difference logic for individual molecules.

I made an experiment in Working Model for the subject (not sure if it represents the truth really). I created a box (which represented a tube), placed small balls in it (for air molecules) at the same height (so they would have same KE when the reach the bottom of the box). Gave full elasticity to the molecules and the box (and because there wouldn't be any force between the balls, I had an ideal gas I thought).

When I clicked the Run button, the "molecules" fell down as I figured, but after a second or so, I realised there were great KE differences between them, and the faster ones were higher. The other thing I realised was the molecules very lover moved VERY slowly, and they seemed like liquid! I had both gas and liquid! Now, was that a reliable simulation? It seemed to me, being liquid or gas and having vapor equilibrium was directly caused by elastic collision laws and gravity (for ideal gases of course)! The higher parts of the box had less number of balls/unit area ratio (like mass/unit volume (so lower density of course)).

It was entertaining so I continued to play with that, and "saw" diffusion etc.
At a point of development of the simulation, I had a mechanism which gave the red balls higher KE and the default yellow ones lower, it was completely a fuse! I noticed that, again the molecules which had higher KE were at higher altitude, but they weren't all red of course.

Love!
zoobyshoe
#46
Aug1-04, 07:51 AM
zoobyshoe's Avatar
P: 5,641
I can see your point. Why should a more energetic molecule weigh less than a less energetic one? What's the difference? The more energetic molecule requires more "territory" so to speak, per unit mass, but at the level of two individual molecules why should that make it weigh less than the other?
HallsofIvy
#47
Aug1-04, 10:26 AM
Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 39,682
It's not true that an energetic molecule weighs more (we're not talking relativistic speeds here!). What is true is that an energetic molecule has more energy (of course!) and so can go to a greater height before it loses all of its kinetic energy (total mechanical energy is conserved). Over a "long" period of time (and for molecules, this might be a fraction of a second), the more energetic molecules, the ones that CAN get higher, will be higher than less energetic molecules.
HungryChemist
#48
Aug2-04, 12:59 PM
P: 144
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?
Yes, Cuz water is not ideal gas.
BobG
#49
Aug2-04, 04:51 PM
Sci Advisor
HW Helper
BobG's Avatar
P: 2,284
Quote Quote by HallsofIvy
It's not true that an energetic molecule weighs more (we're not talking relativistic speeds here!). What is true is that an energetic molecule has more energy (of course!) and so can go to a greater height before it loses all of its kinetic energy (total mechanical energy is conserved). Over a "long" period of time (and for molecules, this might be a fraction of a second), the more energetic molecules, the ones that CAN get higher, will be higher than less energetic molecules.
How are fluid dynamics going to come into play in this (if at all)?

Assuming we're talking about a certain amount of hot air, you have several molecules at approximately the same temperature. The molecules in the center of the hot air mass can't lose their heat to the cooler air very rapidly. In other words the 'hot' energetic molecules(1) in the middle collide with the molecules(2) a little further out, adding back in some of the kinetic energy that molecule(2) might have lost, etc. There's a gradual gradient from the center of the hot air mass to the cooler hot air.

So why wouldn't the hot air mass, as a whole, be considered as an entity that was less dense than its surroundings?

Especially since 'warm' molecules are moving in random directions - not only up, but down?
HallsofIvy
#50
Aug2-04, 08:03 PM
Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 39,682
Quote Quote by BobG
How are fluid dynamics going to come into play in this (if at all)?

Assuming we're talking about a certain amount of hot air, you have several molecules at approximately the same temperature. The molecules in the center of the hot air mass can't lose their heat to the cooler air very rapidly. In other words the 'hot' energetic molecules(1) in the middle collide with the molecules(2) a little further out, adding back in some of the kinetic energy that molecule(2) might have lost, etc. There's a gradual gradient from the center of the hot air mass to the cooler hot air.

So why wouldn't the hot air mass, as a whole, be considered as an entity that was less dense than its surroundings?

Especially since 'warm' molecules are moving in random directions - not only up, but down?
Yes, but the "cool" molecules are also moving down. The only difference is that "warm" molecules can go higher that "cool" molecules

"So why wouldn't the hot air mass, as a whole, be considered as an entity that was less dense than its surroundings?"

They can- but I thought the question was asking WHY that was true.
BobG
#51
Aug2-04, 09:56 PM
Sci Advisor
HW Helper
BobG's Avatar
P: 2,284
Yes, but the "cool" molecules are also moving down. The only difference is that "warm" molecules can go higher that "cool" molecules
Good point.
HungryChemist
#52
Aug5-04, 08:38 PM
P: 144
Quote Quote by BobG
Good point.
Yes...But the Hot molecules could go down also and it will go down deeper than the cold molecules could go.
falseVacuum
#53
Aug6-04, 11:28 AM
P: 8
Although it looks like the question has been answered sufficiently, here's a bit of additional reading on hot vs. cold air
Loren Booda
#54
Aug7-04, 01:04 AM
Loren Booda's Avatar
P: 3,408
Take a mole of molecular oxygen (more dense molecules) and a mole of molecular nitrogen (less dense molecules) and combine them in a vessel so that they coalesce randomly at one atmosphere pressure. What is the time evolution of this system while undisturbed?


Register to reply

Related Discussions
WHY is the sun red during rise and set... Astronomy & Astrophysics 2
Does hot air rise? Classical Physics 12
Rise and Run Introductory Physics Homework 2