However it is useful to make the connection with temperature and pressure, and that is the next topic. In fact this is true for even extremely loud sound levels, as will be verified later . The mean value of the kinetic energy per molecule, for each degree of freedom is then As noted, there are 5 degrees of freedom for a diatomic molecule, so the total energy per molecule is 5 times the above equation. Before developing the equation for pressure, we will first consider the way that molecular motion carries mass, momentum, and kinetic energy through space. We will quantify the flux of these three quantities through one face of our small cube, a face parallel to the y-z plane.
All fluxes can be positive or negative, except for the x-directed momentum flux.
The 2 s 2 term in the kinetic energy flux is due to the rotational energy for each of the two spin axes. This term would be absent for monatomic molecules. The total net fluxes across the face are obtained by integrating with respect to v x , which is equivalent to calculating the mean values of the fluxes. The pressure exerted by a gas is due to molecular collisions against a surface. Pressure is equal to the change in momentum caused by the collisions, per unit time and per unit area.