"Metman2012" wrote in message
...
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?

The term cold radiation really only applies to the radiation when it arrives
at a body. If it has originated from a cooler body then it is cold radiation
and the other body will cool. If it has originated from a warmer body then
it is hot radiation and the other body will warm.

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?

The temperature of a body depends on the net (a word of one syllable)
radiation it absorbs and emits. There is only one source for the
emissions, the body itself, but there can be lots of sources of the
radiation it is absorbing. Obviously it is the sum of the radiation from
all those sources that will determine how the temperature of the body
itself changes. And its final temperature will be reached when that sum
equals the radiation it emits as a blackbody.

Does that make sense?

Cheers, Alastair.