## tisdag 22 november 2011

### Heat Capacity of Blackbody 2?

To add to the mystery of the concept of a blackbody as a "cavity absorbing all incident radiation", we now ask about the heat capacity/specific heat of such a thing?

We know that the heat energy/internal energy per unit time (radiance) radiated from a blackbody scales with temperature T like T^4, but what is the connection between T and absorbed/radiated heat energy Q(T), that is what is the heat capacity dQ/dT of a blackbody?

1. If the cavity is filled with a perfect gas, then Q scales like T.

2. If the cavity is filled with a solid, then Q should scale like T^2 according to Mathematical Physics of Blackbody Radiation.

3. If the cavity filled with a photon gas, then Q scales like T^4.

We thus get a heat capacity dQ/dT scaling with temperature in a wide range from T^0 = constant to T^3.

This is yet another indication that the concept of a blackbody is confusing since it is not at all clear what physics it models, and thus is unfortunate as a scientific concept.

#### 4 kommentarer:

1. Are you this dense by nature, or did you have to practice hard? A cavity with a very narrow entrance is just something that behaves like a black body. The definition of a black body makes no reference to geometry, and is not at all confusing.

2. I asked about the heat capacity of a blackbody, not it's geometry.

3. Claes wrote:
I asked about the heat capacity of a blackbody, not it's geometry.

I have a fun exercise for you.

Try to calculate the heat capacity for the blackbody of blackbodies, a black hole.

Sincerely,
Dole

4. What is it you consider tor be strange?

That a perfect gas, a solid body and a photon gas (you must realy love the behaviour ofta the black hole ;-) ) behaves physicaly different?

Sincerely,
Dol