Steady state obstructed diffusion

We will place ourselves on the assumption of steady state condition, and for that we use relation [5.3] again, that says 

If Cj and Cx indicate the two concentrations in diffusing particles at the two interfaces tliat limit the zone of diffusion (C =C j), concentrations that are fixed by external conditions, for exanqile, of inteifacial equilibriums, we can write fiom what precedes the whole of relations giving the differences between the concentrations in two successive equilibrium positions of balances in a file  

Adding up member to member the preceding equalities and neglecting 1/Co according to 1, we get 

Indicate by p q,X) the probability of encountering q abnormal barriers on a diffusion file whose length is X. All files containing q barriers, whatever be their places, have the same flux such as 

The number of these files on a total of 0 files is 0p(q, X). The average flux is given by the average of all fluxes on the 0 files when q varies  

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