In this post, Asimovian makes reference to a Futurama joke about the temperature, with wind chill, being below absolute zero.
At first, I laughed, because it’s silly to consider a temperature below absolute zero, but when I thought about it a little bit, I thought it might be theoretically possible.
As I understand it, wind chill is a (somewhat fuzzy) factor that gets added in to more accurately determine how quickly a warm body will lose heat to the surrounding environment when that surrounding environment is a moving fluid. And the rate at which heat will flow is directly proportional to the temperature differential, modified by the insulation properties of the barrier between, right? So might it make sense for wind chill to actually be below absolute zero?
I don’t really know how to do the calculation, but assuming either (a) a rapidly flowing supercooled liquid or (b) an even more rapidly flowing very cold gaseous atmosphere, is it possible, or would the heat energy present in such a quickly moving fluid always keep the “wind chill modified” temperature above 0 K?
Yup. Or, at those low levels of temperature, the percieved lowering of temperature would be a percentage. There are a variety of ways to say no this question, but it makes a good nerd joke.
I’m guessing not. By the second law of thermodynamics, you can’t move the fluid without heating it, and the heat gain from the fluid movement probably offsets whatever the wind chill would do. That’s my intuition, anyway. But how do we prove it?
I found a theoretically-dubious calculation that we can use to calculate wind chill. Converting from Celsius to Kelvins in the equation, we get:
…which is as far as I can get. I’m not sure what the theoretical maximum efficiency of a supercooled-fluid-mover is, so I can’t calculate the other half. We could solve for it, given reasonable values for fluid volume and density.
On preview: oh, right, friction. That wouldn’t help.
Even if the wind were blowing 100 mph and with a temperature of 0 K I don’t think you can make the energy of the particles in any object lower than the energy they have at 0 K.
But the object (usually a person) wouldn’t be at that sort of temperature. I mean if we were cold-blooded and our bodies dropped to 40 degrees in 40 degree air, what would it matter if the wind was blowing? On the other hand, I could perceive flowing 5K liquid nitrogen pulling heat from our 98.6 degree bodies faster than a stationary liquid at 0K. (But no liquids exist at 0K, right? We’ll ignore that fact though, if possible.)
If you are thinking hypotheticaly if at all any thing can be considered possible, impossible or not at all except if you are in a black hole or passed thru a worm hole or one of another kind or two.
Since the whole problem is in a universe of absolute zero K or R there is NO Motion of any kind. Hence No Wind Chill & No Wind Chill
I think it certainly could. I think wind chill estimates what temperature would feel as cold with no wind speed, as does whatever weather you’re having. This means, causing the same heat flux at the skin’s surface.
A heat transfer coefficient of 50 W/(m^2 K) is certainly possible with a brisk breeze, and if you were in the arctic at -70 C, this would be 5000 W/m^2. With a heat transfer coefficient of 5 in still air, you’d have to have a temperature differential of 1000 to get this flux, which suggests a static temperature of -770 C or almost -500 K (that is, 500 Celsius degrees lower than absolute zero).
It’s a whole separate debate whether wind chill is a very useful concept. I think it’s a bit daffy - it’s rare to sense very cold air with no breeze, and you’d then be surprised to hear how cold it really was.
It’s a joke kid, just a joke.
In a universe or enviornment of absolute zero K or R there is NO WIND and therefore there is NO WIND CHILL.
If anything in that universe is at zero K or R everything is at the same temperature else the temperature is a bit shy of a full deck… er absolute zero but headed in that direction.
[QUOTE=iamthewalrus(:3=As I understand it, wind chill is a (somewhat fuzzy) factor that gets added in to more accurately determine how quickly a warm body will lose heat to the surrounding environment when that surrounding environment is a moving fluid.[/QUOTE]
I think your understanding is a little off.
Wind chill is the apparent temperature felt by exposed human skin. It is lower than the actual air temp, because the wind velocity increases the speed with which sweat evaporates from the human skin, thus hastening up the heat transfer, so it is perceived by the human as colder.
Hardly any of this applies near absolute zero.
There wouldn’t be exposed human skin, not living human skin.
It wouldn’t be sweating at that temp.
There wouldn’t be any wind, the gasses in the air would have frozen into solids.
And since wind chill is ‘apparent’ and ‘perceived’, it wouldn’t apply; the human who would have perceived it would be dead at those temps.
You’re trying to apply rules outside the range they are defined for. Rather like trying to explain atomic level physics with Newtonian laws – you can’t; while Newtonian physics laws work fine in the ‘normal’ human world, at certain limits you need Einsteinian rules.
Like that, wind chill is an effect that only has meaning in a specific range of temps, roughly from human body temp down to the limit of human habitable temps. (So about 98ºF/37ºC down to -80ºF/-63C.) As it gets very cold, the difference between actual temp and wind chill temp decreases. Basically, once it’s extremely cold, the heat is removed from exposed human skin so fast that wind velocity hardly matters. (Growing up in Minnesota winters, wind chill is something we all understand!)
Er, ah, if there is not motion then the velocity of the particles is zero and we can determine their position and doesn’t this violate the ucertainty principle?
I read somewhere that there is a small residual energy in the particles but the available energy is zero.
I’d say the answer is yes. “Wind chill” measures how fast an object at 36C (body temperature) loses heat. A flow of 0K air would remove heat more efficiently than a stagnant 0K air (forced convection vs. natural convection).
Helium is the only substance still liquid at ~0 K. (It will freeze if you increase the pressure, though.)
As has been noted above, any “air” present would not be in gas form, so it wouldn’t be flowing at all. It would be solid, unless you are talking about pure helium, which would be liquid.
And “wind chill” would seem to be a poor expression to use in such a circumstance, there not actually being any wind to speak of.
But we wouldn’t be removing energy from the liquid helium (thank you robby), we’d be removing energy from the exposed body faster. Mind you, I still doubt that it would work…
You can’t have a wind because as was stated a wind is moving gas. All gases solidify before absolute zero is reached. You can’t have a wind chill effect after all gases solidify.
Suppose the warm body were at 2 K. The wind chill would exist but I don’t believe it could be more than 2K. That is, the warm body can’t be cooled by more than from 2 K down to 0 K.
Helium is a liquid at ~0 K and 1 atm pressure, not a solid.
And it would seem logical that flowing liquid helium at ~0 K would remove heat faster from a warmer object than stationary liquid helium at ~0 K. This effect might be described as being analogous to “wind chill.”
David Simmons, when you talk about “wind chill,” you are describing a situation where you are at body temperature, and a 30 deg F wind (for instance) is perceived by you as removing heat from you as fast as, for example at 10 deg F stationary air mass. This does not mean that the person’s temperature is 30 deg F or 10 deg F. Similarly, our object experiencing “superfluid chill” from flowing liquid helium is not (initially, at least) anywhere near absolute zero. It is much warmer. What we are trying to describe here is how fast the object will cool down.
By the way, some people in this thread seem to think that you can actually achieve at temperature of absolute zero. You can’t. The approach to absolute zero is asymptotic. One way to think of this is to realize that it takes as much effort to get from 100 K to 50 K, as it does to get from 10 K to 5 K, as it takes to get from 2 K to 1 K, as it takes to get from 0.0002 K to 0.0001 K, and so on. I think the record low temperature reached is in the nanokelvin range, but absolute zero itself cannot be reached. That’s why I’ve been writing ~0 K (i.e. approximately zero kelvins).
Actually I’m still not convinced. One could extrapolate and calculate how fast “air” at 0K would remove heat from a 300K object, if air were a gas at that temperature. One could then ask whether 78K air flowing very fast would remove heat faster than that rate. If the answer is yes, you’ve got a sub-0K wind chill. I haven’t done the math but I suspect the answer is yes.