I'm fully aware of Gay-Lussacs Law, but when I was reading Feynman Lectures on Physics volume 1, sir said that the temperature and the speed of the gas (Ideal Gas) are proportional to each other. But if that's the case, shouldn't a fan never server its purpose if all that's happening is that its blades are imparting momentum to the adjacent air molecules, in turn increasing their speed and also increasing the temperature of the gas.

A.If we proceed with our last development so is it something like an AC, where the coolant is subjected to series of processes leading it to adsorb heat from the air from the place that it's serving?

B.Does it also have something to do with the moisture content of the skin (A person felling better under the fan.) And the moisture content of the air.

Please answer if you know the answers to anyone of the sections if not all.

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    $\begingroup$ what-if.xkcd.com/71 I was absentmindedly stirring a cup of hot tea, when I got to thinking, "aren't I actually adding kinetic energy into this cup?" $\endgroup$
    – user191954
    Jul 22 '18 at 17:08
  • $\begingroup$ Nice thinking, Heres what I think,sitrring would decrease the temperature of an object if its not insulated if its isulated that its temperature would definitely Increase("The Joules Experiment").Here is what I think the Possible explanation would be, When the cup is open to atmosphere the air right above the cup is hot and assuming there is no wind, The heat is spreading to the adjecent air molecule but the temperature would go on decreasing as we go away from the cup.And hence there would be heat transfer but not a very efficient one.But when you are stirring... $\endgroup$ Jul 22 '18 at 17:25
  • $\begingroup$ you are actually increasing the surface exposed to the atmosphere with all the ripples(Yes we are considering All of it). But the amout of surface area exposed to the atmosphere is of no use, If there is no proper circulation, so where you stir you are also creating a vortex of air right above it, and hence the air rises up, allowing fresh air to come right above the cup, and hence efficient heat transfer. $\endgroup$ Jul 22 '18 at 17:30
  • $\begingroup$ so the faster you stir better is the circulation of air and greater is the heat transfer, But then agian we come back to my question , If you stir faster the the air passing over the cup has a higher velocity, And should have been hotter therefore hampering the heat transfer(Lesser temperature Difference). $\endgroup$ Jul 22 '18 at 17:34
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    $\begingroup$ Duplicate of physics.stackexchange.com/questions/96327/… and links there in (especially physics.stackexchange.com/questions/90343/…) $\endgroup$ Jul 22 '18 at 18:45

I will deal here with the sensations you would feel if you were sitting in front of a desk fan on a hot day.

It is true that the kinetic energy added to a parcel of air by that fan which accelerates it into a moving stream will eventually become heat via viscous dissipation, but for small fans that move air relatively slowly (as, for example, a fan sitting on a desk top in front of you) this effect is small.

A much larger effect is that of evaporative cooling, which occurs when that stream of air is blown over your skin, enhancing the evaporation of your sweat and thereby carrying heat away from your body. Note that for evaporative cooling to occur, the moisture content of the moving air must be significantly below the saturation point at the temperature of your room. The drier the air, the more cooling effect you will experience, and this effect will overwhelm the other.


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