Use the formula P = -(∂U/∂V) s,n,to show that the pressure of a photon gas is 1/3 times the energy density (UIV). Compute the pressure exerted by the radiation inside a kiln at 1500 K, and compare to the ordinary gas pressure exerted by the air. Then compute the pressure of the radiation at the center of the sun, where the temperature is 15 million K. Compare to the gas pressure of the ionized hydrogen, whose density is approximately 105 kg/m3 .