On 09/08/2015 14:13, Metman2012 wrote:
I've been following this thread with fascination. I'm not a physicist or
even a scientist, but I have a question that perhaps someone can answer.

Let's ask it with an example. There are three bodies, one at -50, one at
0 and one at 50 degrees. It's obvious that the one at 50 degrees is hot
radiating and the one at -50 is cold radiating. What is the one in the
middle doing? Unless I've completely missed the point, it's cold
radiating to the one at 50 and hot radiating to the one at -50. How can
it be both?

No you have pretty much understood it at a handwaving level.

"Cold radiation" is an invention of dodgy double glazing salesmen.
It has as much relevance today as the phlogiston theory of fire.

The question you pose above of -50, 0 and 50 C is what you might expect
if the dominant mechanism of heat transfer was by conduction. The mid
point being half way in temperature between the two ends.

Taking the end set points for temperatures as t=300K and T=400K for
arithmetic convenience and considering only radiative transfer and
taking each surface as a perfect black body radiator.


t=300K u= unknown T=400K
| | |
| -- kt^4 ku^4 --|--ku^4 kT^4--|
| | |

We can say with certainty that at equilibrium without the central
radiation shield the system would have a net flux F of

F = kT^4-kt^4

And with the intermediate radiation shield at equilibrium the net flux f
on either side of the divide must be equal so that

f = kT^4-ku^4 = ku^4 - kt^4

Hence 2u^4 = T^4 + t^4 or u^4 = (T^4+t^4)/2

Hence f = F/2

Putting the numbers in u^4 = (300^4 + 400^4)/2 ~= 360^4

In practice this technique is used in cryogenics where the radiation
shields are mirror finished so that instead of being a perfect black
body they mostly reflect incident photons and behave as a 5% black body.


Now let's add another body, say at 100 degrees. This one is now the hot
radiating one, and the one at 50 degrees now becomes a body which does
both. Now the reality of the universe is that there are many bodies, all
busily radiating. And we can't know which is the hottest and which the
coldest, so everything is radiating both hot and cold.

So am I being simplistic? Am I not understanding what all this is about?

Can someone answer in simple terms (one syllable or less) to explain
this please?

No you have it right. And because radiation loss is such a steep power
of temperature objects more than 10% above ambient on the Kelvin scale
start to radiate at a level which becomes increasingly significant.

Everything at a temperature above absolute zero is radiating energy at
everything else. The balance of an isolated object between incoming and
outgoing energy fluxes determines its final equilibrium temperature.

--
Regards,
Martin Brown