Heat and Thermodynamics

4.11 Theory of Self-Diffusion in a Gas

We consider a vertical column of gas in which uniform concentration gradient has been set up in the vertical direction of z axis. Let the concentration of gas in the xy plane through z = 0 (the zero plane) be υ₀. Then, the concentration of gas in the plane a height z is υ where

 

 

dv/dz being the concentration gradient.

Let us consider a small element of volume dv at a height z(= rcosθ ) above the zero plane (Fig. 4.32). The number of molecules having velocity between c and c + dc within the volume dv is B where

 

 

Fig. 4.32 Illustration for evaluation of diffusion coefficient

 

 

Due to collision ...