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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.
Why does hydrogen heat up when it expands?
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.pdfand herehttp://home.sou.edu/~chapman/ch444/joulet.htmwhere 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.
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?
Quote from: lancenti on 17/03/2009 03:54:56If 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.
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