Relativistic Thermodynamics: A Modern 4-Vector Approach
Figure 6
(a) A gas contained in a cylinder is compressed by a force applied to a piston inside a cavity (thermal resevoir) at temperature (isothermal process). The centre of mass of the gas remains at rest in its initial and final equilibrium states. (b) During the compression process photons with frequency (monochromatic approximation) are emitted with zero linear momentum. (c) Heat emitted during the compression is characterized as the energy associated with the thermal radiation made up of all photons emitted contained into the cavity (with minus sign).
