Hot Air Rising

[Zilali]

Ok i'm sure theres a simple answer for thsi but here you go.

Why does hot air rise, I understand heat energizes the molecules giving them more kinetic energy but why do they move upwards?

Logically they must be lighter but does that mean they lose mass, as i'm sure E=MC2 is nothing to do with this i'm assuming they don't.

Anyway can someone explain?

[Drip Curl Magic]

I don't really have a definate answer to this. But, I know that hot air... as apposed to cold air, is more spread out from all the motion of the molecules. I would guess that the cold air kind of clumps together to create a mass of heavier molecules. Hot air spreads out... so there would be less hot air molecules per cubic inch than a cubic inch of cold air, right? Based on this, it only makes sence that hot air would be lighter, since you can fit more cold air into a tighter space. which would make cold air more dense.

[Zilali]

Hmm so your saying that molecules with kinetic energy are less likely to form intermolecular bonds like van der waals with other molecules, makes sense i suppose.

[cwes99_03]

Originally Posted by Zilali

Hmm so your saying that molecules with kinetic energy are less likely to form intermolecular bonds like van der waals with other molecules, makes sense i suppose.

No, if intermolecular bonds are forming then you no longer have a gas but another state of matter (excluding of course those molecules that are gases).

[Erasmus00]

If we are talking about classical systems, then what we need is two different results to explain this.

First, lets consider how high a particle can rise into the air if it started from the ground with a velocity straight up. The easiest way to approach this is the conservation of energy:

E at bottom = E at top.

At the bottom, our energy is entirely kinetic, and the formula is 1/2mv^2. At the top, we aren't moving and our energy is entirely potential, mgh.

The maximum heigh is then h = (v^2)/2g.

So we see particle with higher velocities rise higher.

Now, classical thermodynamics tells us that the average translational energy of a gas goes as follows

Avg E = 3/2 kT.

1/2 m avg(v^2) = 3/2 kT.
avg(v^2) = 3mkT.

So, if you have trouble following the math, a quick summing up. Hotter particle have more energy (thats what it means to be hotter). Because they have more energy, they are able to rise higher into the air. This also explains why air gets colder as it rises, because its kinetic energy is converted to potential as you move up in the atmosphere. Lower energy means lower temperature.
-Will

[Zilali]

interesting, thanks

[cwes99_03]

Originally Posted by Erasmus00

If we are talking about classical systems, then what we need is two different results to explain this.

First, lets consider how high a particle can rise into the air if it started from the ground with a velocity straight up. The easiest way to approach this is the conservation of energy:

E at bottom = E at top.

At the bottom, our energy is entirely kinetic, and the formula is 1/2mv^2. At the top, we aren't moving and our energy is entirely potential, mgh.

The maximum heigh is then h = (v^2)/2g.

So we see particle with higher velocities rise higher.

Now, classical thermodynamics tells us that the average translational energy of a gas goes as follows

Avg E = 3/2 kT.

1/2 m avg(v^2) = 3/2 kT.
avg(v^2) = 3mkT.

So, if you have trouble following the math, a quick summing up. Hotter particle have more energy (thats what it means to be hotter). Because they have more energy, they are able to rise higher into the air. This also explains why air gets colder as it rises, because its kinetic energy is converted to potential as you move up in the atmosphere. Lower energy means lower temperature.
-Will

Will, I've honestly never seen someone put it so eloquently. Thanks for picking up the slack. I meant to finish this but didn't get back to it.

[Vending]

Originally Posted by Erasmus00

So, if you have trouble following the math, a quick summing up. Hotter particle have more energy (thats what it means to be hotter). Because they have more energy, they are able to rise higher into the air. This also explains why air gets colder as it rises, because its kinetic energy is converted to potential as you move up in the atmosphere. Lower energy means lower temperature.
-Will

Perhaps this is because I have not had super high level physics, but I always thought that hot air rises because it is less dense than cold air.

I also thought that air cools as it rises because at higher elevations the air pressure is less, thus the air expands as it rises and cools as a result. This is the same reason why wind comming down a mountain is usually hot, it compresses as it desends, heating due to the compression.


As far as your explination goes, it seems like gas particles bump into eachother rather often, so they will be changing directions quite often. Thus, it seems like their motion would be mostly random and there seems to be no reason why they would prefferentially move upwards unless there was an outside force acting upon them (ie. boyancy?). Am i missing something?

[cwes99_03]

You are only missing the idea that faster moving particles move farther than slower moving particles. Thus they move farther in all directions. Now couple that with your idea of hotter gases being less dense (i.e. less high velocity gas molecules exist in the same volume of air as low velocity molecules.) Edit: Of course, you can't just look at elevation (volume) and temperature when talking about these things. There is the ideal gas law which also has to include pressure. At higher elevation you have lower pressure but lower temperature too yet excess amounts of volume.
With pressure crowding molecules together, the cooler gas molecules can exist more tightly packed at lower elevations (under higher pressure.) Where high velocity molecules are more likely to move farther and find more open space to roam about at higher elevations. Also take into account that the collisions between two particles of air are not perfect, but some energy is transferred from one molecule to another every time they collide to create a bell curve of velocities for the air molecules surrounding you.
As molecules with higher velocities reach higher elevations, gravity takes effect on their velocities (in other words these molecules haven't reached escape velocity so they can't leave the upper atmosphere) and they have to slow down. By slowing down they lose the energy value that we calculate as temperature and they begin to fall back down.

Now you understand the very complex nature of what causes weather patterns. The constant heating and cooling of air mixed with water vapor which holds it's energy longer than regular gas molecules paired with cloud cover, heating of the sun, particulates, etc. and you have widely varying, chaotic weather patterns.

[Zilali]

So how do you think heat would act in a vacuum.

Being primarily interested in chemistry I have little education in physics so I'm finding it a bit hard understand and visulise what heat actually is. I understand it is the degree of vibration a molecule experiences but what form if any does it take in the absense of molecules, is it a wave?