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If heat is motion, why do fans cool you off?

  1. May 20, 2014 #1

    Nathanael

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    If heat is motion, why do fans cool you off?

    My guess is that if you had a closed room (so no air goes in or out, but obviously energy would be coming in from the fan) then the fan actually would heat up the room on average.

    My guess is that fans only create some kind of apparent coolness, or maybe the motion of air somehow "collects" and "takes away" the heat from your body, so you feel cooler, even though the room (as a whole) is warmer.


    Any explanations?
     
  2. jcsd
  3. May 20, 2014 #2

    Doc Al

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    You've got it right. The fan does heat up the room, but cools you off via evaporation.
     
  4. May 20, 2014 #3

    DrClaude

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    Careful here. "Heat is motion" is a nice soundbite for popular science, but not that good a picture of what heat actually is. That question is like asking why it's cold inside an airplane while it's going at 800 km/h.

    Correct.

    What your skin feels is not the actual temperature, but the rate at which heat is being dissipated. The fan helps increase the transfer of heat from your body to the air (including through sweating).
     
  5. May 20, 2014 #4
    You already gave an answer:

    If air is cooler than the skin than the air stream increases the temperature gradient and therefore the heat flow from the skin to the air. Therefor you will not only feel cooler but actually be cooled.

    The other part is transpiration. If humidity is below 100 % the air stream will also increase the humidity gradient and therefore increase the evaporation rate. The loss of evaporation heat cools the body even more effective.
     
  6. May 20, 2014 #5

    Nathanael

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    Well I was talking specifically about atomic motion (perhaps I should have said that). Wouldn't "heat is atomic motion" be a valid way of thinking about it? Or maybe you have to specify that it's irregular, or average, or internal, or what not?
    I don't exactly know but if you could provide a better definition, that would be appreciated.
     
  7. May 20, 2014 #6

    Nathanael

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    So if you're in a room with air already warmer than your skin (or body or whatever) and you turned on a fan, the opposite effect would occur? Namely, there would be a stream of air which would increase the transfer of heat from the air to your body/skin, and you would be heated up?
    (By heated up I mean more quickly than if you were just standing in the same room without the fan on.)
     
  8. May 20, 2014 #7

    WannabeNewton

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    If heat simply corresponded to "atomic motion" then it would be an equilibrium property (or state function) of a given system, such as a dilute gas of molecules in a cylinder, but it certainly isn't. One cannot talk about the heat of a dilute gas of randomly moving molecules in a cylinder-rather it is temperature which fills this role. Heat on the other hand is a special type of energy flow i.e. it is a property of processes between equilibrium states (specifically that which is not accounted for by work done on the system).
     
  9. May 20, 2014 #8
    Yes, without transpiration or with a relative humidity near 100 % you would heat up accelerated.
     
  10. May 20, 2014 #9

    Nathanael

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    So you're saying there's a difference between "temperature" and "heat"?

    I never knew this. All the times I said "heat" in my previous posts, what I was actually talking about is "temperature."

    So, "heat" can be considered as "the flow of temperature"? Or am I misunderstanding?
     
  11. May 20, 2014 #10

    WannabeNewton

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    Yes, although the common usage of the terms in the English language might obscure that.

    Very roughly speaking, yes, in the sense that if you consider two nearby bodies of different temperatures with no work being done on this system whatsoever, heat will flow from the hotter body to the colder body until the two bodies are at equal temperature i.e. heat is a spontaneous flow of energy from one object to another due to some temperature gradient.
     
  12. May 20, 2014 #11

    D H

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    "Heat" is not motion.

    Example: Consider an ice-cold comet falling sunward from beyond Pluto's orbit. Even though the comet is moving rather quickly, it's still ice cold. It's the random motions of the atoms and molecules in the comet that determine it's temperature. You have to subtract the average motion of the comet as a whole to see those random motions.

    Internal energy and temperature are attributable to random atomic motion. At least in an ideal gas. What about a non-ideal gas, or a solid such as that comet? Heat, temperature, and internal energy are distinct concepts, and there's a hidden elephant in the room called "work". And another called "entropy".

    A number of factors come into play in determining the internal energy of some object. One is that random motion. Adding heat to the object and the atoms and molecules that comprise it increases those random motions. It can also induce phase changes such as making the solid melt into liquid, the liquid boil off into a gas. Those phase changes also are a part of the overall internal energy. If one ignores those details, you can think of internal energy as being a measure of those random motions.

    Temperature is also related to those random motions. In general, understanding entropy is crucial to understanding the connection between internal energy and temperature. It's much simpler with an ideal gas. In an ideal monatomic gas such as helium, the relationship is ##\frac 3 2 kT^2 = K.E. = \frac 1 2 m\bar v^2## where ##\bar v## is the mean random velocity of the atoms that comprise the gas.

    Heat and work are related to change in internal energy by the first law of thermodynamics. The change in internal energy is equal to the heat added to the system less the work done by the system. There's a big problem with looking at "heat" as a property of a system. Suppose a system starts at one temperature/energy/volume state and ends at another. The amount of heat flow and the amount of work done depend on the path between those start and end states. One path might involve more work and less heat transfer than another.

    This is a very important concept. It is how heat engines operate. Suppose instead of taking a system from point A to point B we take it from point A to point B by one path and then from point B back to point A by another path. That's a heat engine. Even though the engine has come right back to where it started from, that work and heat are path-dependent means the engine can be used to produce a net amount of work on the external environment. A heat engine converts heat into work. Heat engines can also convert work into heat transfer.
     
  13. May 20, 2014 #12

    A.T.

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    Think about the fans in ovens.
     
  14. May 20, 2014 #13
    Actually, this is a little confusing. Your skin temperature really does get cooler as a result of transfer of heat from your body to the air.

    Chet
     
  15. May 20, 2014 #14

    Pythagorean

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    I think DrClaude's point is that we sense the speed at which it leaves, not the temperature itself (well, predominantly anyway... we actually have more than one kind of thermosensors and absolute temperature is detected to some degree). If you have a piece of metal and a rag that are the same temperature, the metal will feel colder than the cloth to a human because it conducts heat faster.
     
  16. May 20, 2014 #15
    Lets get into reality. We have a room with fan. When fans starts to rotate, it starts disturbing the air molecules, mainly water molecules, oxygen molecules, nitrogen molecules, etc. Though, the motion of the other molecules increases the temperature, the water molecules when they hit to you often might be giving you a feeling of coolness.

    You can see that by a google search: https://www.google.co.in/search?q=humidity&rlz=1C1DFOC_enIN544IN544&oq=humidity&aqs=chrome..69i57j0l5.3592j0j4&sourceid=chrome&es_sm=122&ie=UTF-8, the humidity will be as high as 84% or more.

    Initially your room and the outside environment will be having the same temperature. If you stay in the room for more hours with your fan on, and come out later to the environment. You can observe that your room was hot.
     
    Last edited: May 20, 2014
  17. May 20, 2014 #16
    I could make a case for saying that the temperature at the interface between the object and your body (your skin) will be colder when it is contact with the metal than with the rag because of the difference in thermal properties of the metal and rag. The new information to me is that ones feeling of being cold depends more on the heat flux at the skin surface than on the skin temperature. Is this really correct? Literature reference?

    Chet
     
  18. May 20, 2014 #17

    Pythagorean

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    Sure, and the relevant quote for convenience (no metadata so I hope an image will suffice):

    8oUw9ck.jpg

    the paper should be free:
    http://www.nature.com/jid/journal/v69/n1/abs/5616722a.html

    There are actually two separate thermoreceptors: one for sensing cooling, one for sensing warming. Scholarpedia has an article that reviews the general literature:

    http://www.scholarpedia.org/article/Thermal_touch
     
    Last edited: May 20, 2014
  19. May 20, 2014 #18
    I am not familiar with all the terminology, but what I got out of these references is that our sense of skin hotness or coldness depends both on the actual skin temperature and on its time rate of change. Is this a correct interpretation?

    Chet
     
  20. May 20, 2014 #19

    D H

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    You have the order wrong. Water is a trace component. The air is mainly nitrogen and oxygen, with a small amount of water and other molecules.

    Secondly, a fan does not contribute to humidity. It will raise the temperature of a room, but not by much.

    What's raising the temperature? It's you. And your companion. And your TV. And your stereo. And your laptop. And your light bulbs. The fan? That's 5 watts for a small fan, 25 watts for a rather large one. You alone, resting, are generating 70 to 100 watts. Quadruple that if you have a companion and the two of you are participating in physical activities. Your TV, that's using 100 to 1000 watts, depending on size, type, and age. When you turn off the sound on your TV, crank up your stereo, keep a couple of lights on, participate in physical activities, and then brag about it on Facebook, your fan represents but a tiny blip in the amount of heat transferred to the room.
     
  21. May 20, 2014 #20

    Pythagorean

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    More specifically, the relevant quote is "intensity discrimination [...] depends on the rate of change of temperature". The absolute temperature does factor in as a modulator, but the primary sensation is a direct relationship to the rate of change itself. Of course, we're limiting the discussion to feeling a fan in a room. When you are an extreme temperatures, the signal is more prominent (because of the danger, we might argue from evolution) as another set of receptors are activated, but it's still cooling and warming that acts on the sensing mechanism of the receptors, the absolute temperature plays more of a role in inactivating and inactivating the different receptor groups:

    ("Nocireceptors" is jargon for pain signaling doohicky).
     
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