As shown in Fig. , the gas and outer systems can be regarded as two particles; the gas system is an exothermic particle (A), i.e., energy donner, while the outer system is an endothermal particle (B), i.e., energy acceptor. B is passive. It records how much A donated. Hence, the exothermic force of Eq. 8 is focused on in the following discussion, using the endothermal force of Eq. 9 as a comparison.