Radiation pressure vs air pressure

1. Dec 3, 2013

S_Flaherty

1. The problem statement, all variables and given/known data
Consider the electromagnetic radiation inside a kiln, with a volume of 1 m3 and a temperature of 1500 K.

Calculate the radiation pressure inside the kiln. How does it compare with the air pressure?

2. Relevant equations
PV = NkT

3. The attempt at a solution
I got the total energy using U/V = 8π5(kT)4/15(hc)3 and got U = 3.83 kJ. To find the radiation pressure would I just divide the energy by c? Also, how do I find the air pressure if I don't have a value for N?

2. Dec 4, 2013

clamtrox

Radiation pressure is related to the energy density, not total energy. If you don't have a formula at hand, you can calculate the relation pretty easily by using for instance $$C_v = \frac{\partial U}{\partial T} = T\frac{\partial S}{\partial T}$$ to find entropy. Use that to find the free energy $F = U - TS$ and pressure $p = -\frac{\partial F}{\partial V}$.

3. Dec 4, 2013

S_Flaherty

Thanks!, this helps a lot