Thermal velocity

The thermal velocity or thermal speed is a typical velocity of the thermal motion of particles which make up a gas, liquid, etc. Thus, indirectly, thermal velocity is a measure of temperature. Technically speaking it is a measure of the width of the peak in the Maxwell–Boltzmann particle velocity distribution. Note that in the strictest sense thermal velocity is not a velocity, since velocity usually describes a vector rather than simply a scalar speed.

Since the thermal velocity is only a "typical" velocity, a number of different definitions can be and are used.

Taking $k_{B}$ to be the Boltzmann constant, $T$ is the temperature, and $m$ is the mass of a particle, then we can write the different thermal velocities:

In one dimension

If $v_{th}$ is defined as the root mean square of the velocity in any one dimension (i.e. any single direction), then

$v_{th}={\sqrt {\frac {k_{B}T}{m}}}$.

If $v_{th}$ is defined as the mean of the magnitude of the velocity in any one dimension (i.e. any single direction), then

$v_{th}={\sqrt {\frac {2k_{B}T}{\pi m}}}$.

In three dimensions

If $v_{th}$ is defined as the most probable speed, then

$v_{th}={\sqrt {\frac {2k_{B}T}{m}}}$.

If $v_{th}$ is defined as the root mean square of the total velocity (in three dimensions), then

$v_{th}={\sqrt {\frac {3k_{B}T}{m}}}$.

If $v_{th}$ is defined as the mean of the magnitude of the velocity of the atoms or molecules, then

$v_{th}={\sqrt {\frac {8k_{B}T}{m\pi }}}$.

By all of these definitions $v_{th}$ falls in the range of

$v_{th}=(1.6\pm 0.2){\sqrt {\frac {k_{B}T}{m}}}$.

Thermal Velocity at Room Temperature

At 20 °C (293.15 Kelvin), the mean thermal velocity of common gasses is:

Gas	Thermal velocity
Hydrogen	1,754 m/s (5,750 ft/s)
Helium	1,245 m/s (4,080 ft/s)
Water vapor	585 m/s (1,920 ft/s)
Nitrogen	470 m/s (1,500 ft/s)
Air	464 m/s (1,520 ft/s)
Argon	394 m/s (1,290 ft/s)
Carbon dioxide	375 m/s (1,230 ft/s)