# Fick's Law and the Diffusion/Convection combined equation.

1. Dec 26, 2015

### rhino1000

NA = JA* + CA*(NA+NB)/C for a binary mixture.
NA is the flux of the fluid relative to a stationary point.
JA* is Fick's law, the flux relative to the bulk motion of the fluid?
(NA+NB)/C is the bulk velocity of the fluid.
But I don't see why this is the case. I don't see why NB should affect NA at all. For example, for an ideal gas, the molecules of component B should have no impact on the motion of the molecules of component A. I (think I) understand the need for accounting for a moving reference frame, but it doesn't make sense to me that if we had a hypothetical situation where A is concentrated initially at a small volume, and somehow we were able to make all of the "B" molecules move in some direction, that this would affect NA. An explanation or clarification would be appreciated!

Last edited: Dec 26, 2015
2. Dec 27, 2015

### Staff: Mentor

Suppose no diffusion were occurring, and the molar average velocity (averaged over A and B) were v. Then the total molar flux would be $$N_A+N_B=vC$$. What would the molar flux of A be?

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