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 to be the Boltzmann constant, is the temperature, and is the mass of a particle, then we can write the different thermal velocities:
In one dimension
If is defined as the root mean square of the velocity in any one dimension (i.e. any single direction), then
- .
If is defined as the mean of the magnitude of the velocity in any one dimension (i.e. any single direction), then
- .
In three dimensions
If is defined as the most probable speed, then
- .
If is defined as the root mean square of the total velocity (in three dimensions), then
- .
If is defined as the mean of the magnitude of the velocity of the atoms or molecules, then
- .
By all of these definitions falls in the range of
- .
Thermal Velocity at Room Temperature
At 20 °C (293 Kelvin), the mean thermal velocity of common gasses is:[1]
Hydrogen 1,754 m/s Helium 1,245 m/s Water vapor 585 m/s Nitrogen 470 m/s Air 464 m/s Argon 394 m/s Carbon dioxide 375 m/s