"Wind chill" below absolute zero
Oct. 24th, 2006 02:21 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
jack1. What is wind chill temperature? A human in cold air in a wind feels as cold as a human in colder but still air. We have fitted some approximations to this and come up with an official scale, but it was originally based on subjective judgements, and depends on other conditions as well, eg. humidity, so isn't definitive, but is a useful measure.
2. How a human feels doesn't really have meaning when you get near *that* cold. Instead death feels much like instant death :)
3. However, you should be able to create a standardised measure, right? Have object X at temperature T K in medium Y at temperature 0.x K and pressure Z, flowing at speed v. Establish the rate of heat loss at that moment (which is at least theoretically calculable). Define "wind chill temperature" to be the temperature in still medium at which the rate of heat loss is the same.
4. Could X lose heat faster under some speed than in still near absolute zero medium? I don't see why not. Physics is weird down there, but it can still heat up the surroundings, etc.
5. Does that make a negative Kelvin wind chill? On the one hand, it implies a wind chill colder than absolute zero. On the other hand, it doesn't actually define a wind chill at all because there is no temperature to compare it to. If you have a nice non-asymptotic graph you could extend it, but does that have any meaning?
2. How a human feels doesn't really have meaning when you get near *that* cold. Instead death feels much like instant death :)
3. However, you should be able to create a standardised measure, right? Have object X at temperature T K in medium Y at temperature 0.x K and pressure Z, flowing at speed v. Establish the rate of heat loss at that moment (which is at least theoretically calculable). Define "wind chill temperature" to be the temperature in still medium at which the rate of heat loss is the same.
4. Could X lose heat faster under some speed than in still near absolute zero medium? I don't see why not. Physics is weird down there, but it can still heat up the surroundings, etc.
5. Does that make a negative Kelvin wind chill? On the one hand, it implies a wind chill colder than absolute zero. On the other hand, it doesn't actually define a wind chill at all because there is no temperature to compare it to. If you have a nice non-asymptotic graph you could extend it, but does that have any meaning?
no subject
Date: 2006-10-24 09:09 am (UTC)Consider (for simplicity) a gas of molecules of some sort. At temperatures above absolute zero, these molecules whizz about and exert pressure on their surroundings. If you give it more heat (energy), they speed up and whizz about faster. Conversely, if you remove heat (energy) from the system, they slow down. At absolute zero, all atoms and molecules are in their ground state. Where do you think this energy is going to come from to give to the surroundings at this point? (which, incidentally are going to have substantially more energy, it's actually really hard to move heat from a colder body to a hotter, just listen to Flanders and Swann singing about it)
no subject
Date: 2006-10-24 10:22 am (UTC)Up to a point. I have a qualitative idea of bouncing molecules and absolute zero is when they would stop (uh, for our purposes). But the connection with individual molecules and entropy and the connection of different sorts of entropy and the physics of low temperatures seemed very interesting but I wasn't really explained and I've still not gone back to, unfortunately.
Where do you think this energy is going to come from to give to the surroundings at this point?
Well explained, thank you, but I didn't think I *was* doing that: what part worried you? X is at a positive temperature, Y is marginally above absolute zero, the "wind chill" is entirely theoretical...