ABSTRACT
This chapter discusses internal energy of gas, enthalpy, work performed by gases, gas heat capacities, and their relationships at low pressures. The laws of thermodynamics can be applied to phase transformation, inherent saturated vapor pressures of solid and liquids, and evaporation processes at vacuum conditions in thin-film technology. The heat capacity can be determined under the conditions at which either volume or pressure of a gas system is maintained constant. The assumption of constant heat capacity in a considered range of temperatures allows us to separate variables and integrate equations for heat capacities. In the isothermal process, an ideal gas accepts the heat, but it neither heats up nor alters its internal energy. The process at which the transfer of energy between the walls and molecules is negligible is the adiabatic process. Simple analyses of polytropic process can demonstrate that the isochoric, isobaric, isothermal, and adiabatic processes are boundary cases of the polytrophic process.
