Naked Science Forum
General Science => General Science => Topic started by: SquarishTriangle on 10/03/2008 06:10:20
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While watching someone fan themselves with sheets of paper earlier on this rather warm day, I came to wonder:
Does the amount of heat you might lose from fanning yourself actually outweigh the extra heat you would generate from constantly moving your arm back and forth in doing the fanning? And if I wanted to fan myself as efficiently as possible, what dimensions would I want my fan to be, at what rate would I move my arm...etc?
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I suspect it also depends on what the ambient temperature really is - are we talking about 40°C, or a sticky, windless 28°C.
The fanning action will generate heat, but if it is mostly done around the wrists, then you have a long thin arm over which to dissipate that heat (this is where the issue of whether the environmental temperature is above, or close to, body temperature will play a part, since you cannot dissipate the heat if the environmental temperature is already above body temperature).
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Sure, let's go with the windless 28°C, but dry. How would humidity play into it, other than the effects you would already experience at rest (without fanning)?
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i think that thermodynamics dictates that you could not possibly cool yourself down more than the heat you are generating by making the movements.
however, You are generating that heat in your wrist/arm to cool your face which is part of your head, which i beleive is wwhere your body senses its core temperature. thus it should be pretty easy to make yourself FEEL cooler that you really are overall. And usually its all about a matter of comfort, so this should to the job. i doubt if you were trapped in the desert, you could keep yourself from dying of heat exhaustion no matter how hard you fanned yourself, but you would be able to make yourself more comfortable for the first while of it.
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Sure, let's go with the windless 28°C, but dry. How would humidity play into it, other than the effects you would already experience at rest (without fanning)?
Humidity might have an effect insofar as forced evaporation (by a slight reduction in pressure caused by the air travelling over a curved surface) could cause a reduction in temperature (even if only slight). On the other hand, allowing water to settle on the skin would reduce any possible reduction of temperature through evaporation of sweat.
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i think that thermodynamics dictates that you could not possibly cool yourself down more than the heat you are generating by making the movements.
This is why I said it depended on the ambient temperature.
As you say, you cannot directly cool your body more than you heat it; but if your body is naturally anyway being cooled (i.e. because the ambient temperature is lower than your body temperature) then you can effect how efficiently this cooling takes place. It makes sense to believe that heat generated in the wrist can be transferred to the environment more quickly than heat generated in your head or torso, simple because of the relative higher surface area of your wrist in relation to its internal volume than is the case in the torso or head. Thus, if you have a mechanism that ensures there is less heat in the head, at the expense of more heat in the wrist, the heat in the wrist is more easily dissipated to the environment anyway.
however, You are generating that heat in your wrist/arm to cool your face which is part of your head, which i beleive is wwhere your body senses its core temperature. thus it should be pretty easy to make yourself FEEL cooler that you really are overall. And usually its all about a matter of comfort, so this should to the job. i doubt if you were trapped in the desert, you could keep yourself from dying of heat exhaustion no matter how hard you fanned yourself, but you would be able to make yourself more comfortable for the first while of it.
Firstly, it is not just comfort. Raising (or dropping) the core temperature of your wrist may reduce the efficiency of the local muscle, but is not directly life threatening. Your brain is extremely temperature sensitive, and having your brain go out of spec on temperature is far more dangerous than having your wrist go out of spec on temperature.
That having been said, if you were in the middle of a desert, in the heat of the day, I would guess you would be in an environment that is already above body temperature (ofcourse, Antarctica is a dessert with a very different temperature profile), and so what I said above about treating an environment of 40°C as something very different to an environment of 28°C is pertinent. Ofcourse, many (maybe most) dessert at night can be extremely cold, so the problem there is very different.
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i think that thermodynamics dictates that you could not possibly cool yourself down more than the heat you are generating by making the movements.
I cant see that as your body is not using very much energy being gently fanned by the hand. Also not all of the energy you are using is remaining in the body as heat as some of the energy is being transfered into heat as the fan flexes and to the air as the air molecules are moved by the fan. Also as the sweat which your body is producing to keep you cool anyway is now being used far much more efficiently as a cooling mechanism i would have though the opposite would be true.
Shame i cant back that up with any scientific evidence [;D] but i think even the best minds out there would be hard pushed to prove it either way [;)]