In physics, engineering, and physical chemistry, a perfect gas is a theoretical gas model that differs from real gases in specific ways that makes certain calculations easier to handle.
In all perfect gas models, intermolecular forces are neglected.
This means that one can neglect many complications that may arise from the Van der Waals forces.
However, the idea of a perfect gas model is often invoked as a combination of the ideal gas equation of state with specific additional assumptions regarding the variation (or nonvariation) of the heat capacity with temperature.
Two of the common sets of nomenclatures are summarized in the following table.
Along with the definition of a perfect gas, there are also two more simplifications that can be made although various textbooks either omit or combine the following simplifications into a general "perfect gas" definition.
This is because the internal energy of an ideal gas is at most a function of temperature, as shown by the thermodynamic equation[1]
are at most functions of only temperature for the ideal gas equation of state.
From both statistical mechanics and the simpler kinetic theory of gases, we expect the heat capacity of a monatomic ideal gas to be constant, since for such a gas only kinetic energy contributes to the internal energy and to within an arbitrary additive constant
Moreover, the classical equipartition theorem predicts that all ideal gases (even polyatomic) have constant heat capacities at all temperatures.
However, it is now known from the modern theory of quantum statistical mechanics as well as from experimental data that a polyatomic ideal gas will generally have thermal contributions to its internal energy which are not linear functions of temperature.
[4] But even if the heat capacity is strictly a function of temperature for a given gas, it might be assumed constant for purposes of calculation if the temperature and heat capacity variations are not too large, which would lead to the assumption of a calorically perfect gas (see below).
These types of approximations are useful for modeling, for example, an axial compressor where temperature fluctuations are usually not large enough to cause any significant deviations from the thermally perfect gas model.
In this model the heat capacity is still allowed to vary, though only with temperature, and molecules are not permitted to dissociate.
The latter generally implies that the temperature should be limited to < 2500 K.[5] This temperature limit depends on the chemical composition of the gas and how accurate the calculations need to be, since molecular dissociation may be important at a higher or lower temperature which is intrinsically dependent on the molecular nature of the gas.
Even more restricted is the calorically perfect gas for which, in addition, the heat capacity is assumed to be constant.
Although this may be the most restrictive model from a temperature perspective, it may be accurate enough to make reasonable predictions within the limits specified.
For example, a comparison of calculations for one compression stage of an axial compressor (one with variable
In addition, other factors come into play and dominate during a compression cycle if they have a greater impact on the final calculated result than whether or not