4 ms·
Thermal radiative coupling is bidirectional. Sucking away is appropriate and important as you do actually have to displace the energy somewhere. It works the sa
by labawi 6y ago
Thermal radiative coupling is bidirectional. Sucking away is appropriate and important as you do actually have to displace the energy somewhere. It works the same way when sucking air.
A simpler non-radiating equivalent would be a reflective foil, or a polished metal - you could enclose yourself within it, get no radiating heat from environment, yet it wouldn't help cool you, because your radiation has no place to go.
- ladberg 6y agoI still feel like "sucking" is misleading because it doesn't change how much heat you radiate, only how much heat is radiated on to you. A better analogy would be like a curtains for heat.
- ants_a 6y agoI think that the analogy is used because people underestimate how much heat is lost to radiation. Human body radiates energy away at about 800W. It's not noticeable, because normally it is balanced by a similar flux of thermal radiation coming in from the environment. If the apparent radiative temperature of the environment is reduced it can indeed feel like it is sucking heat out.
- CorrectHorseBat 6y agoYou're off by about an order of magnitude there. The human body uses ~10MJ per day which is ~100W. This not only includes radiation losses but also convection and evaporation.
- ants_a 6y agoI am not. As I said this outgoing radiation is mostly balanced by incoming radiation. But it does illustrate the theoretical maximal capacity for radiative cooling, when stuck naked in interstellar space.
- julienfr112 6y agoThat's a bit misleading. It's like saying i can blow at a pressure of 16 psi in the void, but when I blow into a balloon whith the atmospheric pressure of the environnement, I can only reach 2 psi. The 16 psi is purely theorical.
- froj 6y agoThat seems like a false analogy. In the case of thermal radiation you actually are emitting photons with a total power of 800W independent of your environment.
- CorrectHorseBat 6y agoMy bad, you're right.
- froj 6y ago> normally it is balanced by a similar flux of thermal radiation coming in from the environment The parent comment wasn't talking about net power dissipation. 800W radiation would be the case if the environment was at 0K. Usually, the environment radiates about 800W back at you.
- taneq 6y agoWow, I'd never realised the total radiative transfer was that high! I guess this explains why those silver foil emergency blankets work so well...
- taneq 6y ago> It works the same way when sucking air. This is a good analogy because (being pedantic) you can't suck air either. You can only make a region of lower pressure, into which the higher-pressure air pushes itself.
- labawi 6y agoMay I then ask what can you suck, by your definition?
- taneq 6y agoGood question! 'Sucking' here is a bit like 'centrifugal force' - it's a real thing, but only in the right frame of reference. In the case of centrifugal force, it's a real thing in a rotating reference frame, but doesn't exist in an inertial reference frame (because in that frame the acceleration is towards the center, ie. centripetal). In the case of suction, it's a real thing when your zero-pressure reference point is above 'absolute zero' pressure, such as when you're measuring gauge pressure. In this case you can have an area of 'negative pressure' which is 'sucking' the surrounding fluid into it.