"Metman2012" wrote in message
...
On 09/08/2015 18:24, Alastair McDonald wrote:
"Metman2012" wrote in message


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.

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.

The second paragraph does make sense, but I notice that no mention is made
of hot or cold radiation.

In general, each of the several sources will be emitting either cold or hot
radiation and you need to sum them. However, an object in a room will only
have one source, the walls of the room. If the object has been there long
enough, it will be at room temperature and so the input radiation will be
neither hotter nor colder (compared with the radiation being emitted.)

But your first paragraph is less clear (partly because I have to reread it
to see what's cooling what). My understanding is that a body will cool by
itself unless it is subject to heating from another body hotter than
itself. So without the sun, the earth would be considerably cooler.

It is certainly true that without the sun the earth would be considerably
colder :-)

Howerver, if you put an object in a room its temperature will change to that
of the room. If you put it in a freezer it will cool, because the walls of
the freezer are emiting cold radiation. There is also some conduction. But
in neither case will the object cool to a temperature lower than its
surrounding.

The surroundings of the Earth is space, and that radiates at a temperature
of about -270 deg. C, - pretty cold! During the day we are warmed by the
Sun, emitting at 5,000 deg . C, but it is far away, and the radiation
ariving at Earth is about 1kW per square metre. At night the heat from the
background radiation is practically zero, so the surface of the Earth cools
pretty quickly. What prevents it from freezing over at night are the
greenhouse gases which absorb radiation from the surface during the day and
reemit it back to the surface at night.

The sun itself is cooling, or at least it would be if there were no
processes maintaining it's heat. However the fuel for these will
eventually run out and it will cool of its own accord without any nearby
cool bodies making a ha'p'orth of difference.

The Sun is actually warming and expanding. One day it will envelope the
Earth (in four billion years time) and then collapse and cool due to lack
of fuel and the cold background radiation.

If you have two bodies, one hotter than the other and they are touching,
then heat flows from the warmer to the cooler until they are the same
temperature. The cooler body doesn't reduce the temperature of the warmer
body. I believe this is because of increasing entropy. My (limited)
understanding of entropy is that everything is cooling and that eventually
(next week?) we will have the heat death of the universe. Everything will
be cool (pretty cold).

In fact the cooler body does reduce the temperature of the hotter, but only
to the average temperature of the two. The warmer body will also cause the
temperature of the cooler body to rise to the average temperature. When that
happens the entropy has increased and the two objects cannot spontaneously
become hot and cold again as that would cause the entropy to decrease, which
is forbidden by the second law of thermodynamics.

If the two objects are not touching they will exchange radiation until they
are at the same temperature, with the cold object radiating cold radiation
and the hot object radiating hot radiation.

I suspect that we might be in a war about semantics in this thread but I
don't think it's worth arguing over as nobody will be changing their
position on it (unless it gets very hot, or cold).

It not just about semantics. It is also about my reputation. It has been
destroyed by Dawlish ridiculing me for maintaining that bodies absorb
cold radiation. I have just realised that his problem is he is
misinterppreting the Second Law of Thermodynamics. He think it means
that heat only travels from hot to cold, but of course as you have pointed
out it, it states entropy increases. Hot becomes colder and cold becomes
hotter. Heat travels in both directions, just as does radiation. Thanks!

HTH,

Cheers, Alastair.