On Thursday, August 6, 2015 at 4:19:35 PM UTC+1, RedAcer wrote:
On 06/08/15 15:52, Alastair wrote:
Dawlish,

On Page 576 of University Physics with Modern Physics, Technology
Update, Thirteenth Edition (2010), which continues to set the
benchmark for clarity and rigor combined with effective teaching and
research-based innovation, they write:

"Radiation. Heat transfer by radiation is important in some
surprising places. A premature baby in an incubator can be cooled
dangerously by radiation if the walls of the incubator happened to be
cold, even when the air in the incubator is warm. Some incubators
regulate the temperature measuring the baby's skin ..."

Hot objects radiate heat which warms adjacent objects. Cold objects
radiate cold which cools adjects objects. The latter is difficult to
demonstrate because it is more difficult to maintain a constant cold
temperature than a high temperture. The latter is easy using
electrical heating. However, holding a thermnometer over an object
taken from a freezer will cause the temperature shown to drop.

I hope you will now realise that you are wrong, will apologise and
admit your mistake. Cold radiation does exist.

Cheers, Alastair.


I think you are getting confused with the human feelings of hot and
cold. If you stick with the words 'higher and lower temperatures' and
use well defined objects and laws, viz black-body, black-body radiation
and Stefan-Boltzmann law, then rewrite what you are trying to say and it
may clarify things?

All solid objects emit radiation based on their temperature. An ideal object which emits a Planckian spectrum is called a black body, but the radiation emitted by non-ideal bodies is often called blackbody radiation too. The term cavity radiation is also used since, a cavity produces the thermodynamic equilibrium which is required for true black-body radiation.

If you place two object at different temperatures side by side with a gap between them, then the hotter object (with a higher temperature) will radiate with a greater intensity than the cooler object (with a lower temperature, since the power they emit is determined by the Stefan-Boltzmann Law J = sT^4, where s is the Stefan Boltzmann constant 5.67 E-8 W / sq m K^4. The cooler object will warm (i.e. its temperature will rise) because it is gaining more radiation than it is losing, and the warmer object will cool because it is emitting more radiation than it is absorbing. This is an example of the First Law of Thermodynamics. Eventually both objects will acquire the temperature of their surroundings, since each of their other five faces will be exchanging heat with that.

Now are you convinced? No, I didn't expect you would be. But the clue is in the first sentence of the book that I quoted. "Heat transfer by radiation is important in some SURPRISING places." It may be surprising that cold radiation exists, but that does not stop it being true.