### Heat Capacity of Ideal Gases

In statistical thermodynamics [175,138], it is derived that each molecular degree of freedom contributes to the molar heat capacity of an ideal gas, where again is the ideal gas constant.

An ideal *monatomic* gas molecule (negligible spin) has only
three degrees of freedom: its kinetic energy in the three spatial
dimensions. Therefore,
. This means we expect

For an ideal *diatomic* gas molecule such as air, which can be
pictured as a ``bar bell'' configuration of two rubber balls, two
additional degrees of freedom are added, both associated with spinning
the molecule about an axis orthogonal to the line connecting the
atoms, and piercing its center of mass. There are two such
axes. Spinning about the connecting axis is neglected because the
moment of inertia is so much smaller in that case. Thus, for diatomic
gases such as dry air, we expect

^{B.32}

**Next Section:**

Speed of Sound in Air

**Previous Section:**

Adiabatic Gas Constant