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Rick Ape

Blowing hot and cold

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Hello everybody.

Is there anyone who can help me with a couple of questions about pressure, heat and the conservation of energy that have been niggling me for years?

It all stems from when I first read Aesop's Fable about a man who confounds a Satyr by blowing on his hands to warm them up, and then paradoxically blows on his soup to cool it down.

Both of the air flows begin their journey in the lungs with the same temperature and they travel the same route and distance, so the only variable appears to be the shape and size of the aperture; ie his lips.

Reducing the size of the aperture increases the pressure and my understanding is that an increased pressure on a gas increases the kinetic energy of its molecules which increases its temperature.

So why is the air blown through pursed lips significantly cooler than that blown through an open mouth?  It is also significantly cooler than its originating body temperature, so where does the heat go to?

In a similar vein (but I don't know if it's actually related or not) a fan creates a stream of air molecules that are moving faster than the surrounding air yet they are significantly cooler.  I thought that faster moving molecules are warmer than slower ones so why does increasing the speed decrease the temperature, and again, where does the heat go to?

I hope this all makes sense, and please be gentle - I am not a scientist.

Thank you

 

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Your breath is cooler than the soup and warmer than the air. There is no inherent contradiction, if that’s all there was to it.

Blowing on the soup promotes evaporation, which is a cooling process. 

Blowing through an aperture means there is expansion afterwards. If the pressure and composition of a gas remains constant and it expands, the temperature goes down. (an ideal gas follows PV = nRT, so they would be proportional under that condition)

edit: see below

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9 hours ago, swansont said:

Blowing on the soup promotes evaporation, which is a cooling process. 

There is also the effect of moving the hot moist air above the soup away, allowing more water to evaporate from the soup. I'm not sure how much that contributes.

 

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13 hours ago, Rick Ape said:

Reducing the size of the aperture increases the pressure and my understanding is that an increased pressure on a gas increases the kinetic energy of its molecules which increases its temperature.

So why is the air blown through pursed lips significantly cooler than that blown through an open mouth?  It is also significantly cooler than its originating body temperature, so where does the heat go to?

If you measure the temperature of your breath coming out of an open mouth vs pursed lips right at your mouth the temperature will be the same.  When you blow out of pursed lips the higher velocity breath entrains the surrounding air so it is much cooler even a couple inches from your mouth.

14 hours ago, Rick Ape said:

In a similar vein (but I don't know if it's actually related or not) a fan creates a stream of air molecules that are moving faster than the surrounding air yet they are significantly cooler.  I thought that faster moving molecules are warmer than slower ones so why does increasing the speed decrease the temperature, and again, where does the heat go to?

The temperature of the air from a fan is the same temperature as the surrounding air.  The reason that moving air cools you is 2 fold.  First the moving air will increase evaporation of any sweat on your skin, cooling you.  Secondly, if the ambient air temperature is less than your skin temperature the high mass flow of air will transfer more heat away from your skin due to more cool air molecules per second hitting your skin than stagnant air, cooling you.  

If the ambient air temperature is above your skin temperature and there is no sweat or moisture on your skin, then a fan will heat your skin making you hotter.  This is because the higher mass flow of air means more hot air molecules per second are hitting your skin than stagnant air, transferring more heat to you.

Heat transfer is always from hot to cold.

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23 hours ago, swansont said:

Your breath is cooler than the soup and warmer than the air. There is no inherent contradiction, if that’s all there was to it.

Blowing on the soup promotes evaporation, which is a cooling process. 

Blowing through an aperture means there is expansion afterwards. If the pressure and composition of a gas remains constant and it expands, the temperature goes down. (an ideal gas follows PV = nRT, so they would be proportional under that condition)

Doesn't that particular isobaric process require an addition of a certain amount of heat, enough to to maintain the pressure as well as enough to compensate for the work done?

 

(so the temperature would rise)

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Sorry, I mucked that up that (not sure what I was thinking) Pressure isn’t constant going through a nozzle (and V is proportional to T for the case I gave) 

As the gas moves, the pressure tends to drop (Bernoulli’s equation).

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Feynman Lectures on Physics, volume 1, chapter 1, Section 1-3. Atomic Motion. Go to your local library and start reading Feynman now.

Then start mimicking Feynman in whatever way you can without giving up your principles, and maybe buy a pair of bongos.

Just joking. But the key word in all of this is: Feynman.

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Posted (edited)
On 5/6/2020 at 4:01 PM, joigus said:

Feynman Lectures on Physics, volume 1, chapter 1, Section 1-3. Atomic Motion. Go to your local library and start reading Feynman now.

Then start mimicking Feynman in whatever way you can without giving up your principles, and maybe buy a pair of bongos.

Just joking. But the key word in all of this is: Feynman.

Surely you're joking...

...Mr. Joigus

image.png.7cfd9a4994100acba56e4402f94672ab.png

Edited by J.C.MacSwell

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30 minutes ago, J.C.MacSwell said:

Surely you're joking...

...Mr. Joigus

It never crossed my mind. Well, it did, but it was a virtual process. ;)

 

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