Consider a closed gaseous system in thermal equilibrium. In traditional thermodynamics this means kinematics of the system is in equilibrium.
Based upon the new thermodynamics as presented here it now means the following:
1) The walls absorb as much blackbody/thermal radiation, as they radiate.
2) Polyatomic gas molecules absorb as much blackbody/thermal radiation, as they radiate, hence maintain a constant vibrational energy.
4) The gas molecules equally (give vs take) exchange vibrational energy with the walls. In general net flow of vibrational energy in either direction will be zero.
The above provides us with a new understanding of thermal equilibrium inside of closed system that takes into account a systemís thermal radiation which was never traditionally considered. See thermal equilibrium atmosphere, In the above, the gas is sufficiently dilute that the intermolecular gas collisions are considered as being insignificant, when compared to the gas-wall collisions, hence the gas adheres (or approximately so) to our new Kinetic Theory.
It must be understood that inelastic collisions as described in my peer reviewed kinetic theory (webpage, book, paper) means that intermolecular collisions obey conservation of momentum but not kinetic energy! Hence in order to abide by the principle of energy conservation, radiation is given off during intermolecular collisions. Moreover, that radiation becomes part of the system's blackbody/thermal radiation, which is continually absorbed and re-emitted by both condensed matter (walls) and/or polyatomic molecules.
Even so, it should be stated that the total energy associated with thermal/blackbody radiation is generally considerably less than the total energy associated with matter (condensed matter and/or gas molecules). However, the fact that thermal phtons travels at the speed of light, means that thermal/blackbody radiation can contribute significantly to thermal energy exchanges. Moreover it all becomes part of a systemís temperature, as measured with a thermometer. I.e. A true vacuum still has a temperature associated with it although it has no matter hence no kinematics. This differs from traditional assertions See temperature (traditional).
Next consider that the surroundings are either the atmosphere or a heat bath as illustrated in Fig 6.1. During quai-static expansion, the gas can remains isothermal (Tatm=Tsys ). i.e. sufficient amounts of blackbody/thermal radiation enter the system through its walls. Thus the blackbody/thermal radiation density within the system remains constant i.e. isothermal.
The energy change to the system is the additional blackbody/thermal radiation that entered the expanding system. In terms of thermal radiation density (@), the change in energy within our expanding isothermal system becomes:
Since the energy associated with blackbody/thermal radiation generally is extremely small, when compared to the kinematic energies associated with matter i.e. gas molecules, then the energy change as defined by eqn (1) often approximates zero. Note: For the quasi-static expanding isothermal system, this energy was freely given into the expanding systems from the surrounding heat bath hence the energy transfer went unnoticed. See expanding.
Rather than quasi-static, consider what happens if a system surrounded by our atmosphere experiences a rapid, significant, volume increase? If the blackbody/ thermal radiation cannot be transferred through the walls quickly enough, then the blackbody energy density within the expanding system will not remain constant! Is this a non-equilibrium thermal state? It is impossible to say unless what drove the expansion is fully considered.
If it is energy contained within the gas itself that drives the process, then the gas cools as it does work onto the surrounding atmosphere. If the gas and surroundings start out at the same temperature, then the expanded gas will be at a lower temperature, hence the rapidly expanding gas and its surrounding atmosphere are not in thermal equilibrium. However when the gas stops expanding, and heat is allowed to enter from the surrounding atmosphere then eventually the gas and its surrounding atmosphere will re-attain thermal equilibrium.
If the rapid expansion is due to an external applied force, then again the blackbody/thermal energy density will decrease. Again once the rapid expansion stops, then if the walls are not fully insulated then again freely given energy will enter through the walls until thermal equilbrium between the expanded system and surrounding heat bath (or atmosphere) is re-attained.
Conversly, if an external heat source drives the isothermal expansion then the blackbody/thermal energy density will remain constant and the additional energy into the system will be defined by eqn (1).