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Author Topic: Why can we blow out hot air and cold air?  (Read 24065 times)

Chemistry4me

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Why can we blow out hot air and cold air?
« on: 15/02/2009 03:34:18 »
Why can we blow out hot air and cold air? ???
Say on a cold day, your hands are freezing and you cup them together and you warm them up by blowing into them like trying to fog up a window/glass. But at other times you blow on your hands or arms to try and cool them down and this air is colder. Do I make any sense whatsoever?
Does it have to do with the moisture?  ???

latebind

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Why can we blow out hot air and cold air?
« Reply #1 on: 15/02/2009 14:36:50 »
Hi Chemistry4Me

The human body wants to remain at a set temperature, roughly 37 degrees celcius
http://hypertextbook.com/facts/LenaWong.shtml

If it is too hot outside, then we sweat. When our sweat evaporates off our skin it takes a little bit of heat with it into the atmosphere, this cools us down. Hence if we could not sweat we would overheat very quickly. Also you will notice that if the humidity(moisture level) of the surrounding atmosphere is high then less sweat will evaporate and you will feel as though it is hotter outside.

So on a very hot day, if you were to jump into a pool and then immediately stand by a blowing fan, you will feel like you are cooling down very rapidly(I in fact do this all the time, it gets very hot here in SA). This is because the fan is accelerating the action of all the pool water evaporating off your skin.

So what this means is that when you blow on your skin on a hot day, it feels as though you are cooling your skin down, this is because there is always a tiny layer of sweat on your skin and your blowing is able to accelerate the evaporation of this tiny layer.

Now, On a very cold day, when you blow on your skin, your temperature of your breath has NOT changed, it is still the same, but it feels as though it is warmer because the temperature of your hands have dropped due to more blood being pumped elsewhere to keep the temperature of the core organs at 37C. Also if you were to wet your hands and then blow on them you would feel them getting very cold because the air from your breath is able to evaporate the water on your hands, taking with it some vital body heat into the atmosphere.

Hope that helps: )
Late




« Last Edit: 15/02/2009 14:41:40 by latebind »

Bored chemist

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Why can we blow out hot air and cold air?
« Reply #2 on: 15/02/2009 19:47:59 »
It's slightly more complicated than that.
The air you breathe out is practically saturated with water at about 37C because your lungs are warm and wet.
So, unless your skin is above 37C the air shouldn't evaporate any water from your skin and so it shouldn't cool you down.
But, as you blow, the air in your breath mixes with the ambient air and some of that is dry enough to help evaporate the wseat.

lancenti

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Why can we blow out hot air and cold air?
« Reply #3 on: 16/02/2009 02:55:15 »
From a physics perspective,

The Ideal Gas Law is written as...

pV = nRT

You'll want to notice then that pV/T is always constant. Assuming of course a closed system.

When you blow as if to fog up windows, you exhale slowly and usually your mouth is huge. This is to prevent the air from rapidly expanding. Rapid expansion, or a rapid drop in pressure results in cooling.

When we blow to make ourselves feel cooler, we exhale quickly, and usually pucker your lips such that a high pressure area is built up within our mouth. The air, when it exits, rapidly expands which cools it down.

Chemistry4me

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Why can we blow out hot air and cold air?
« Reply #4 on: 16/02/2009 03:44:21 »
Thanks latebind and Bored Chemist :)
But I also think that is it a bit more complicated than just that.
« Last Edit: 16/02/2009 03:48:51 by Chemistry4me »

Chemistry4me

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Why can we blow out hot air and cold air?
« Reply #5 on: 16/02/2009 03:48:26 »
When you blow as if to fog up windows, you exhale slowly and usually your mouth is huge. This is to prevent the air from rapidly expanding. Rapid expansion, or a rapid drop in pressure results in cooling.

When we blow to make ourselves feel cooler, we exhale quickly, and usually pucker your lips such that a high pressure area is built up within our mouth. The air, when it exits, rapidly expands which cools it down.
Whoa! :o
Your idea certainly seems very plausible lancenti! :)
I like it.

chris

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Why can we blow out hot air and cold air?
« Reply #6 on: 16/02/2009 10:04:27 »
The same breath can also feel hot and cold depending upon how fast you blow! Why's that?

latebind

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Why can we blow out hot air and cold air?
« Reply #7 on: 16/02/2009 11:35:14 »
Hi Chris

My thinking is...

When we pucker up our lips it creates less area for the air to move through and causes greater pressure and faster speed. Similar to when you close off part of a garden hose nozzle with your finger and the pressure increases making the water come out faster and harder.

It is my belief that this increase in pressure and speed creates a more concentrated flow of breath over the skin, and this causes a more noticeable effect.




« Last Edit: 16/02/2009 14:19:57 by latebind »

Madidus_Scientia

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Why can we blow out hot air and cold air?
« Reply #8 on: 16/02/2009 19:02:40 »
Expanding air cools sure, but it was compressed in the first place while it was still in your mouth, which heats it up. So wouldn't the net effect be 0?

Bored chemist

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Why can we blow out hot air and cold air?
« Reply #9 on: 16/02/2009 19:39:25 »
The highest pressure than can be created by blowing is aproximately nothing. The chance in pressure and volume are very small. The effect of these changes on temperature are also small.
Even if you could produce a significant change in pressure the change in temperature isn't big and it depends on the gas. For air it's generally a cooling effect. For hydrogen it heats up when you let it expand ( this has been  considereed as the cause of hydrogen fires, though I'm not convinced). For an ideal gas the temperature change is zero.
Have a look here.
http://www.chemistry.mcmaster.ca/~ayers/chem2PA3/labs/2PA36.pdf
and here
http://home.sou.edu/~chapman/ch444/joulet.htm
where they point out that the temperature difference produced by a drop from 4 bar to 1 bar is 3 degrees or so.
PV=nRT isn't the right equation in this case.




If you blow the air out as a fast stream theough pursed lips  it entrains more of the air round it. That air is generally cold.
Breathing gently onto something means that the air that reaches the object is pretty much the same air as you breathed out which is warm (and wet).
This is also why you can steam up your glassed by breathing on them then blow on them and evaporate off the moisture again.
« Last Edit: 16/02/2009 19:53:27 by Bored chemist »

Madidus_Scientia

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Why can we blow out hot air and cold air?
« Reply #10 on: 16/02/2009 19:42:57 »
Yep, I think that's the best explanation.

Chemistry4me

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Why can we blow out hot air and cold air?
« Reply #11 on: 17/02/2009 04:30:30 »
Thanks everybody :)

Madidus_Scientia

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Why can we blow out hot air and cold air?
« Reply #12 on: 16/03/2009 18:25:43 »
Why does hydrogen heat up when it expands????

Bored chemist

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Why can we blow out hot air and cold air?
« Reply #13 on: 16/03/2009 20:50:54 »
Why does hydrogen heat up when it expands????
The malice of inanimate objects, or possibly
http://en.wikipedia.org/wiki/Joule-Thomson_effect

lancenti

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Why can we blow out hot air and cold air?
« Reply #14 on: 17/03/2009 03:54:56 »
The highest pressure than can be created by blowing is aproximately nothing. The chance in pressure and volume are very small. The effect of these changes on temperature are also small.
Even if you could produce a significant change in pressure the change in temperature isn't big and it depends on the gas. For air it's generally a cooling effect. For hydrogen it heats up when you let it expand ( this has been  considereed as the cause of hydrogen fires, though I'm not convinced). For an ideal gas the temperature change is zero.
Have a look here.
http://www.chemistry.mcmaster.ca/~ayers/chem2PA3/labs/2PA36.pdf
and here
http://home.sou.edu/~chapman/ch444/joulet.htm
where they point out that the temperature difference produced by a drop from 4 bar to 1 bar is 3 degrees or so.
PV=nRT isn't the right equation in this case.

Maybe it's not, but what I understand from the Joule-Thompson Effect is that it is under closed-system conditions with very good insulation. I haven't been able to find any numbers on what the maximum air pressure of blowing is, but I'm pretty sure it's not a closed system, though I think it does sound reasonable to assume that it is.

If you blow the air out as a fast stream theough pursed lips  it entrains more of the air round it. That air is generally cold.
Breathing gently onto something means that the air that reaches the object is pretty much the same air as you breathed out which is warm (and wet).
This is also why you can steam up your glassed by breathing on them then blow on them and evaporate off the moisture again.

If this is true, then if I blow fast really really close to my glasses it should fog up right? I shall go try this as soon as I get the opportunity, but I think it is indeed the case. Someone else try in the mean time?

Chemistry4me

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Why can we blow out hot air and cold air?
« Reply #15 on: 17/03/2009 04:02:07 »
If this is true, then if I blow fast really really close to my glasses it should fog up right? I shall go try this as soon as I get the opportunity, but I think it is indeed the case. Someone else try in the mean time?
It works.

Bored chemist

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Why can we blow out hot air and cold air?
« Reply #16 on: 17/03/2009 20:43:27 »
If this is true, then if I blow fast really really close to my glasses it should fog up right? I shall go try this as soon as I get the opportunity, but I think it is indeed the case. Someone else try in the mean time?
It works.
It works repeatably.
Guess it must be science.

rex789

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Why can we blow out hot air and cold air?
« Reply #17 on: 18/03/2009 17:35:11 »
convection! convection! convection!

along with conduction and radiation are the modes of heat transfer in a vacumless system.

u blow and you convey heat. when its cold u convey heat to ur hands and when its hot u convey it away from your hands. hence in one u feel warm in other cold.

convection is movement of molecules. air blowing past an object will tranfer heat to it or from it depending on the temp differences btw the object and the blowing air. even if they are at the same temperature there will be heat transfer becuase u will be moving molecules past ur hand. moving molecules will have diff energy state than those that are still and there will be heat transfer.

Chemistry4me

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Why can we blow out hot air and cold air?
« Reply #18 on: 19/03/2009 05:31:04 »
Does that have anything to do with it?

rex789

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Why can we blow out hot air and cold air?
« Reply #19 on: 19/03/2009 05:45:32 »
yes. u just pump out the same air, it's moist and heated to body temp. its the convection that results in ur feeling you are blowing out hor or cold air

on the other hand if you breathe in and out rapidly (hyperventilate) you will not move the air all the way into your lungs before expiring it thus maybe blow out cold air... which is not what your question was i think.

Chemistry4me

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Why can we blow out hot air and cold air?
« Reply #20 on: 19/03/2009 13:02:19 »
yes. u just pump out the same air, it's moist and heated to body temp. its the convection that results in ur feeling you are blowing out hor or cold air
HUH? ??????

Bored chemist

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Why can we blow out hot air and cold air?
« Reply #21 on: 19/03/2009 19:29:47 »
Never trust anyone who doesn't understand capital letters.

 

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