Particle composition and heat capacity of high‐temperature SF 6 present at constant volume: Discussion on formula expressing relationship between constant‐pressure and constant‐volume heat capacities

Until now, the high‐temperature properties of SF 6 have been derived as a function of T for a given constant pressure P . This was based on the adoption of T and P as gas state parameters. In contrast, this paper adopts T and the gas volume V as the gas state parameters and then evaluates the particle composition and gas pressure for high‐temperature SF 6 present at a constant volume state. This evaluation is achieved by the minimization of Helmholtz free energy of the high‐temperature SF 6 . A further derivation for the SF 6 gas at constant volume is performed to reveal the temperature dependence of the molar heat capacity C Vm [J/(mol·K)] and the specific heat C Vg [J/(kg·K)]. Use of the obtained C Vm enabled us to discuss whether the formula C Pm − C Vm = R m holds true for the high‐temperature SF 6 or not, where C Pm and R m are the molar heat capacity at constant pressure and the universal gas constant, respectively. Similar discussion is also performed on the formula indicating the relation between C Pg and C Vg , where C Pg is the specific heat at constant pressure. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.