Re: Heat vs IR

[Wayde Allen <allen@xxxxxxxxxxxxxxxx>]

On Tue, 26 Nov 1996, Tom Benedict wrote:

> The definition of "heat" is pretty fuzzy, so if you disagree with my
> numbers, I'd probably agree with you (go figure).

No, heat is just another term for energy!  The definition of a calorie
(measure of heat energy) is:

   1 calorie = 4.184 Joules (measure of energy)

Radiation of any kind (IR, visible light, radio) can therefore constitute
radiative heat tranfer.

What tends to be confusing is the concept of temperature, since different
materials require different amounts of energy to change temperature.  This
is the concept of heat capacity (The amount of energy needed to raise the
temperature of a body by 1 degree.). 

When discussing radiation, one uses the purely theoretical construct of a
fictitious body that absorbs all radiation incident on it (Black Body). 
If this body is at thermal equilibrium, meaning it isn't heating up or
cooling off, then all of the incident radiation is being re-radiated.
Using Planck's law you can compute the radiation spectrum of the black
body if you know its equilibrium temperature.  This is how you get the
term color temperature.  The color temperature of a light source is the
temperature that a black body radiator would need to be in order to
radiate the same spectrum of energy.

So...the radiation spectrum of something at a known temperature can be
computed.  This will tell you how much IR is being radiated at that
temperature.  However, the IR is not the temperature, but rather a
transfer of heat energy.  This transfer depends on the absorbtivity
(measure of how close to a black body it is)  of the object as well as its
temperature!

I think I'd better quit!

- Wayde Allen
  (allen@xxxxxxxxxxxxxxxx) 






------------------------------

Topic No. 6