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pm1

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Thermo is still a mystery to me..
Its a simple question but can someone explain this statement, please?

"Air rises spontaneously and does work on the surroundings."

Thank you!
 

chiddler

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I'm not sure if this is thermo as it is buoyancy! I know warm air is less dense than cold air so it rises.

I can't think of a thermodynamic reason for the warm air to rise. What is the thermodynamic reason for a light object rising in water?

i forgot second part:

The work on surrounding is because energy is transferred from the warm air to surrounding. Energy diffuses from hot to cold, yes? If energy is transferred to something, then work is done on the object.

I rub a brush against the wall to heat it up. I am doing work on it. Increased outside pressure causes a piston to reduce its volume. Work is done on the piston system. When the the gas inside the piston is heated up and it expands, then the piston pushes up and does work on the surrounding.
 
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milski

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Treating it as buoyancy force is simpler - a parcel of less dense air displaces more dense air and that creates a buoyancy force. As the air moves up, you have a displacement and consequently work.

At the end of the day, everything happens because of thermodynamics. Depending exactly on system you are considering there are different things to be said.
 

chiddler

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Treating it as buoyancy force is simpler - a parcel of less dense air displaces more dense air and that creates a buoyancy force. As the air moves up, you have a displacement and consequently work.

At the end of the day, everything happens because of thermodynamics. Depending exactly on system you are considering there are different things to be said.

cool. i didn't think of it as doing work in that sense.
 
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milski

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I'm not sure if this is thermo as it is buoyancy! I know warm air is less dense than cold air so it rises.

I can't think of a thermodynamic reason for the warm air to rise. What is the thermodynamic reason for a light object rising in water?

i forgot second part:

The work on surrounding is because energy is transferred from the warm air to surrounding. Energy diffuses from hot to cold, yes? If energy is transferred to something, then work is done on the object.

I rub a brush against the wall to heat it up. I am doing work on it. Pressure causes a piston to reduce its volume. Work is done on the piston system. When the the gas is heated up and it expands, then the piston pushes up and does work on the surrounding.

There are serious issues with the second part of your answer. Energy can be transferred as heat or work - both are two separate and different things. Work in general will involve displacement of objects/change of volume. Heat transfer will be a change in the internal energy without either of those.

In your example of brush against a wall, you are not doing any work unless you eventually move the wall - anything else is just heat transfer.

The gas expansion is work - heating the gas and making it expand and do work is one way to convert heat to work.
 

chiddler

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There are serious issues with the second part of your answer. Energy can be transferred as heat or work - both are two separate and different things. Work in general will involve displacement of objects/change of volume. Heat transfer will be a change in the internal energy without either of those.

In your example of brush against a wall, you are not doing any work unless you eventually move the wall - anything else is just heat transfer.

The gas expansion is work - heating the gas and making it expand and do work is one way to convert heat to work.

thanks. makes a lot of sense.

i'll go review now.

sorry OP i'll try to be more careful with the information i write in case it is incorrect :<
 
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pm1

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Treating it as buoyancy force is simpler - a parcel of less dense air displaces more dense air and that creates a buoyancy force. As the air moves up, you have a displacement and consequently work.

At the end of the day, everything happens because of thermodynamics. Depending exactly on system you are considering there are different things to be said.

I still have a question that haunts me about colder air being denser. If cold air is denser, how come it is colder at higher altitudes? I know that the colder temperature at higher altitudes has to deal with lower pressure, but still... How come are they not conflicting principles?
Thanks!
 

milski

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I still have a question that haunts me about colder air being denser. If cold air is denser, how come it is colder at higher altitudes? I know that the colder temperature at higher altitudes has to deal with lower pressure, but still... How come are they not conflicting principles?
Thanks!

Colder air is denser only when you are comparing it at the same temperature. It's fairly easy to see that from n=PV/(RT). If you increase the temperature, the amount of gas per volume decreases and it is less dense. As you go higher, the pressure decreases but so does temperature. The pressure decreases faster than the temperature which leads to air at higher altitudes being colder and less dense.

---end of MCAT material--- (I think)

The pressure decrease rate is driven by simple mgh formule, at least for the first few tens of thousand feet. The temperature rate drop is a bit more complicated, it's driven by all that air being in thermal equilibrium, meaning that there is no heat exchange going on between the lower and the higher layers. That gives you something called adiabatic lapse rate, which is about 2 K/1000 ft altitude. Sometimes that equilibrium is disrupted by huge masses of cold air moving around - that results in exchange of spectacular amounts of heat and fun stuff like thunderstorms!
 
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