19 replies and not a single link to the Master’s column? It talks about blowing on coffee, not your hand, but the answer is the same.
The air pressure difference between your mouth and outside is extremely small, and the temperature drop due to expansion is negligible. You can check this by blowing on a thermometer.
I just did a quick experiment. It involved a five pound sack of rice and a plastic bag with a hole in it. With these, I get that when we pucker, we’re adding about 0.5 psi above atmospheric pressure. The expansion is adiabatic, at least for most of the way, so the change in internal energy of the gas is exactly the work done in expanding the gas, or integral(p(V)dV,V[sub]0[/sub],V[sub]1[/sub]). Since delta_p/p << 1, I will just use p=1 atm. The change in internal energy, then, is (1 atm)(V[sub]1[/sub]-V[sub]0[/sub]). Any temperature change there is will be small compared to 300 K, so a good guess for the volume change comes from the pressure change: about 7%. Thus, the change in internal energy is:
deltaU = (10[sup]5[/sup] N/m[sup]2[/sup])(.07)(V[sub]0[/sub])
The internal energy for this diatomic gas was originally:
U = (5/2)(k[sub]B[/sub])(T[sub]0[/sub])(number of moles)
U = (5/2)(8.3 J/K)(310 K)(V[sub]0[/sub]/.0224 m[sup]3[/sup]) <-- close enough to STP to use 22.4 L/mol
The fractional change in temperature, then, is:
delta_T/T = delta_U/U = 0.024
==> near 300 K, delta_T is around 13 deg F.
I don’t know who was on the Straight Dope Science Advisory Board back in May '94, but I wouldn’t consider the idea as preposterous as they did.
What’s your margin of error on this? I am skeptical because no matter how hard I try to pressurize the air in my mouth (keeping my lips tightly closed) I can’t feel an increase in temperature.
Okay, I just performed a quick experiment, with two disposable Vicks thermometers, marked in increments of 0.2F°. First oral reading was at normal pressure, second oral reading was when I was pressurizing my mouth as much as I could (turning quite red in the process). Both were for 60 seconds.
First reading: 97.4°F
Second reading: 97.4°F
So, to within 0.2F°, I observe no change. This is a factor of 65 different from your calculation of 13F°.
Most mention of cooling in this post refers to evaporation of water from the skin, which is wrong.
Most of the wind chill comes from breaking down the layers of warm air close to the surface of the skin, unless you are sweating. Notice how dry skin also cools. Breaking the boundary layers of air at the skin surface increases the rate of heat transfer.
The only thing the calculation really depends heavily on is delta_V, so maybe you could swing by a factor of 3 or 4 either way there. My estimate came from the pressure change, which I would believe to within a factor of 2, but then there is the question of how adiabatic the expansion is (the first 50% of the expansion is pretty darn adiabatic I would think)…
Regarding your experiment:
Take a typical industrial cylinder of compressed gas. The gas might be at 2000 psi, but it is at room temperature. (However, when it comes screaming out the nozzle, it gets a lot cooler; you can even get frost on the nozzle.) Of course, the expelled gas reaches thermal equilibrium with the room air in mere seconds.
-P
Okay I didn’t realize how fast the air would come into thermal equilibrium, but your explanation of my experiment makes sense. However, I still think I should be able to detect a several-degree temperature change during the first few seconds, hot when exhale into my closed mouth, cold when I inhale from it. I can even buy the order-few-percent-atmospheric pressure difference, thinking about how easy it is to maintain pressure in your mouth underwater.
OK, I read this thread last night so I did an experiment at work today to see if what I said yesterday was correct. Turns out I was wrong, as is every other post is this thread about the OP. I’m not addressing all that pressure, condensor stuff.
Here is what is happening:
Air coming from mouth is same temperature (within the accuracy of my test setup ~.1 degreeC) no matter what you are doing with your lips or how hard you blow.
The air hitting your hand is drastically different temperature depending on what you do with your lips.
When blowing had through a small hole in your lips, the high speed air creates lots of trubulence, mixes with room air, and the mixure hits your hand about 1 degree C higher than room temp.
When blowing with your mounth wide open, there is little mixing going on so the warm air makes it to your hand relatively pure.
It’s a dimension of opening vs distance to detector thing.
Do this test as an illustration: Take a deep breath, hold your hand 15 cm from your mouth, open your mouth wide and blow out just hard enough so you can feel the air on your hand - time this and see how long takes before you run out of air. Now do the same test with your lips pursed - you can blow like this much longer than with your mouth open wide. This should help convince you that when you purse your lips, very little of the air that hits your hand actually comes from your mouth.
For those of you wanting test setup stuff - done with 24AWG thermocouple read by a National Instruments Fieldpoint FP-TC-120 module. Room temp was about 25 C, my breath about 34.5 C. All measurements were made after holding breath for 20
seconds to make sure I didn’t influence test by not giving fair warmup time to air in lungs.
When you purse your lips to blow, you force the same amount of air through a smaller opening, so the air moves faster, which generates ebbies that draw in the cooler ambient air.
HEY!!! I already said that…except I mis-spelled turbulence when I said.
Aesop tells us:
A man and a satyr met in the woods, and decided to travel together. The satyr had never met a human before, and asked the man why he was blowing on his hands. “To warm them,” the man replied. The two stopped at an inn for lunch, where the satyr asked the man why he was blowing on his soup. “To cool it,” the man said. The satyr decided that he had better part ways with the man, for he couldn’t trust a creature that blew hot and cold with the same breath.
I know that this has nothing to do with the thread, really, but I like it.
Okay, so, antechinus, billy, Urban Ranger, you’re all saying that if you’re in a room which is at 97.4°F, provided you haven’t started sweating yet, blowing on your hands will not feel cool?
Which, of course, raises the question as to why moving air isn’t, by definition, hotter than still air. After all, isn’t heat defined in terms of the motion of molecules?
Barry
Entering a room at 97.4 F you would be sweating immediately even if it wasn’t visible yet.
No.
What I am saying is that moving air evaporating water is not the explanation to the OP. The air hitting your hand when you purse your lips, IS COOLER, at normal room temp, than when blowing lightly with your mouth open. What the OP wanted to know was why it is cooler?
It’s really cooler. I measured it. With a thermocouple. Then I explained why it was cooler.
You are trying to argue that the air FEELs cooler due to evaporation. We all understand the concept of evaporation and wind chill and all that but it doesn’t effect the process in the OP. You can do this same test with a latex glove on and it still feels cooler when you purse your lips. In this case, no water is evaporating, proving that your answer is irrelevent to the OP…even though, of course, the physics you describe are real. What you really should focus on is the boundry layer that is broken down by the fast moving air. This air, at near room temp, draws heat away from the skin faster than still air. No evaporation required to explain it.
No. Not sure how you came to that conclusion.
Billy’s last post explains things adequately. Blowing quickly increases the rate of heat transfer from skin to air.
In addition, cool surrounding air is drawn along with the air from pursed lips. I think this is what a couple of others were getting at when eddies were mentioned. This is wrong, it is more accurate that the cooler surrounding air is drawn along, via momentum transfer - like the principle behind a diffusion pump.
Right, but by your explanation, it would not be cooler if room temperature is body temperature, right? I’m not looking for holes in your argument, mind you; I’m just looking for ways to test it myself. I’ll try the latex glove thing next chance I get, thanks.
Related article from Cecil may be found here:
http://www.straightdope.com/classics/a4_178.html
Yep. Then the only cooling effect would be due to evaporation, which is normally not that significant for dry skin.
btw UncleBeer - I think Cec’s article attributes ‘coffee cooling’ too much on evaporation and not enough on forced convection. Although of course the evaporative cooling effect would be much more significant with a hot liquid than with dry skin.
You could test the contribution from both effects by putting a membrane over the surface of the hot drink - like billy’s glove experiment. Anyone want to do this?