Perfect gas

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In physics, a perfect gas is a theoretical gas that differs from real gases in a way that makes certain calculations easier to handle. Its behavior is more simplified compared to an ideal gas (also a theoretical gas). In particular, intermolecular forces are neglected, which means that one can use the ideal gas law without restriction and neglect many complications that may arise from the Van der Waals forces.

Perfect gas nomenclature[edit]

The terms perfect gas and ideal gas are sometimes used interchangeably, depending on the particular field of physics and engineering.[1] Sometimes, other distinctions are made, such as between thermally perfect gas and calorically perfect gas, or between imperfect, semi-perfect, perfect, and ideal gases. The assumptions are summarized in the following table.

Nomenclature 1
Nomenclature 2
Specific heat capacity at
constant V C_V
or constant P C_P
Ideal-gas law
pV=nRT and C_p-C_V=R
Calorically perfect Perfect Constant Yes
Thermally perfect Semi-perfect T-dependent Yes
Ideal May be T and/or p-dependent Yes
Imperfect T and p-dependent No

Thermally and calorically perfect gas[edit]

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.

A thermally perfect gas

This type of approximation is 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. Heat capacity is still allowed to vary, though only with temperature, and molecules are not permitted to dissociate. The latter implies temperature limited to 1500 K.[2]

Even more restricted is the calorically perfect gas for which, in addition, the specific heat is assumed to be constant: e = C_vT and h = C_pT.

Although this may be the most restrictive model from a temperature perspective, it is accurate enough to make reasonable predictions within the limits specified. A comparison of calculations for one compression stage of an axial compressor (one with variable Cp, and one with constant Cp) produces a deviation small enough to support this approach. As it turns out, other factors come into play and dominate during this compression cycle. These other effects would have a greater impact on the final calculated result than whether or not Cp was held constant. (examples of these real gas effects include compressor tip-clearance, separation, and boundary layer/frictional losses, etc.)


  1. ^ J.B. Young, Thermodynamics, Engineering lecture. Cambridge University.
  2. ^ John, James (1984). Gas Dynamics. Allyn and Bacon. p. 256. ISBN 0-205-08014-6. 

See also[edit]