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Markg2

Hot and Cold air

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Hot air rises due largely to fewer molecules = lighter = rising.
Cold air sinks since more molecules = heavier = sinking.

Fine. But that regards the two conditions as separate instances. In reality hot and cold air are both present.

My question: Isn't a certain amount of hot air 'pushed' up by sinking cold air?


Mark

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19 minutes ago, Markg2 said:

Hot air rises due largely to fewer molecules = lighter = rising.
Cold air sinks since more molecules = heavier = sinking.

Fine. But that regards the two conditions as separate instances. In reality hot and cold air are both present.

My question: Isn't a certain amount of hot air 'pushed' up by sinking cold air?


Mark

The analysis doesn't work if you look at this on a molecular basis. One molecule doesn't "know" if it's part of a group that has fewer molecules in it. 

Hot vs cold is a matter of temperature, which relates to the average KE of the molecules, and thus to their speed. Faster means more collisions per unit time, which increases the pressure, which tends toward expansion (if that's possible)

PV = nRT for an ideal gas. If the temperature goes up, the volume has to increase (at the same pressure). There's only one direction it can go. Up. Colder, more dense air tends to collect lower.

 

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If the 'test' sample was a theoretical column of air of a certain height and within that column were various strata of cold and hot air then wouldn't the two 'separate' conditions of hot and cold be then forced to interact and one strata to 'win out' over the other (move up or down)? Or am I insisting on making this more complicated than it is?

Mark

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1 hour ago, Markg2 said:

Hot air rises due largely to fewer molecules = lighter = rising.
Cold air sinks since more molecules = heavier = sinking.

Hot air has faster moving particles, cold air has slower moving particles.

If they are put to balloon, separating them, you need to compare their densities. Density is mass divided by volume. Denser object sinks in less dense medium. It does not matter, if object or medium are gas, liquid or solid. e.g. solid Lithium floats on the surface of water, as it's less dense than water. Lithium has density 0.534 g/cm3, water has density 1 g/cm3 (forget ongoing reaction between them).

Edited by Sensei

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1 hour ago, Markg2 said:

My question: Isn't a certain amount of hot air 'pushed' up by sinking cold air?

The only force present is gravity, which acts downwards. So you are right that the reason the relatively less dense air goes up is because the denser cold air is pulled down. This is basically the same mechanism for hot air balloons going up or for bubbles rising through water.

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On 9/17/2019 at 8:14 PM, Strange said:

The only force present is gravity, which acts downwards

I'm not sure that's right, or a gas would just collapse down in a puddle. You have pressure, which is the combined effect of collisions of the gas molecules. 

I think, if you picture yourself as a molecule, getting hit from side to side, and above and below, you will be scoring more hits from below than above, because it's denser below.  So the extra hits from below force you upwards. Would that be a fair picture?

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6 minutes ago, mistermack said:

I'm not sure that's right, or a gas would just collapse down in a puddle. You have pressure, which is the combined effect of collisions of the gas molecules. 

I think, if you picture yourself as a molecule, getting hit from side to side, and above and below, you will be scoring more hits from below than above, because it's denser below.  So the extra hits from below force you upwards. Would that be a fair picture?

 

Yup that is a fair picture +1

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15 minutes ago, mistermack said:

I'm not sure that's right, or a gas would just collapse down in a puddle. You have pressure, which is the combined effect of collisions of the gas molecules. 

I think, if you picture yourself as a molecule, getting hit from side to side, and above and below, you will be scoring more hits from below than above, because it's denser below.  So the extra hits from below force you upwards. Would that be a fair picture?

I think that is reasonably accurate. But it doesn't contradict the fact that only force acting on the gas is gravity. The reason the gas below is denser is because of gravity.

 

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39 minutes ago, Strange said:

I think that is reasonably accurate. But it doesn't contradict the fact that only force acting on the gas is gravity. The reason the gas below is denser is because of gravity.

 

 

56 minutes ago, mistermack said:

Would that be a fair picture?

 

Let me pour some oil here.

Strange is talking about a body force by an external agent (gravity)

Such body forces due to an external agent act on the body as a whole (ie directly on each and every particle of the body) and their effect can be described by a single concentrated force acting at the centre of mass of the body, which is the sum of all the individual actions on each body particle.

 

mistermack is talking about a surface force (due to pressure) which acts only on the surface of a body and is transmitted (wholly or partially) to other parts of the body by internal forces.
In this case the body needs to be subdivided into adhjacent control volumes to model the effect.

So yes the internal pressure forces are caused by gravity, but at any section the pressure force from below balances the gravitational force from above.

 

Weve been through this one before, quite recently.

 

 

Edited by studiot

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10 hours ago, Strange said:

I think that is reasonably accurate. But it doesn't contradict the fact that only force acting on the gas is gravity. The reason the gas below is denser is because of gravity.

 

This is true if the packet of air is accelerating downward at g. Otherwise it is not.

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On 9/17/2019 at 3:14 PM, Strange said:

The only force present is gravity, which acts downwards. So you are right that the reason the relatively less dense air goes up is because the denser cold air is pulled down. This is basically the same mechanism for hot air balloons going up or for bubbles rising through water.

Isn’t all hot air pushed up? What is buoyancy? Isn’t the mechanism the same?

On 9/24/2019 at 3:20 PM, studiot said:

Strange is talking about a body force by an external agent (gravity)

Such body forces due to an external agent act on the body as a whole (ie directly on each and every particle of the body) and their effect can be described by a single concentrated force acting at the centre of mass of the body, which is the sum of all the individual actions on each body particle.

Maybe I’m starting to understand here. I don’t sink to the earths core because gravity is the grand master of the mechanism that supports my weight?

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Posted (edited)
1 hour ago, jajrussel said:

Isn’t all hot air pushed up? What is buoyancy? Isn’t the mechanism the same?

Maybe I’m starting to understand here. I don’t sink to the earths core because gravity is the grand master of the mechanism that supports my weight?

 

Bouyancy is the result of the balance of foces on a body totally immersed in a fluid that is subject to a body force field such as gravity.

You don't fall to the core because you are standing on the deck of a ship in your sou'wester

Edited by studiot

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2 minutes ago, studiot said:

 

Bouyancy is the result of the balance of foces on a body totally immersed in a fluid that is subject to a body force field such as gravity.

You don't fall to the core because you are standing on the deck of a ship in your sou'wester

Actually, I used to be quite proud of the fact that I could stand on the deck of the fishing boat in a storm. Learning the skill and a decent pair of deck shoes helped to cut down on the number of bruises. Now a days I would just as soon  stand on the deck of a Pier and reminisce  from afar.🙂

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1 hour ago, jajrussel said:

Actually, I used to be quite proud of the fact that I could stand on the deck of the fishing boat in a storm. Learning the skill and a decent pair of deck shoes helped to cut down on the number of bruises. Now a days I would just as soon  stand on the deck of a Pier and reminisce  from afar.🙂

My older brother is 76 and has two boats. I dread the times I have to go and help him with one of them.

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