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

[rhino1000]

N_A = J_A^* + C_A*(N_A+N_B)/C for a binary mixture.

N_A is the flux of the fluid relative to a stationary point.
J_A^* is Fick's law, the flux relative to the bulk motion of the fluid?
(N_A+N_B)/C is the bulk velocity of the fluid.
​

But I don't see why this is the case. I don't see why N_B should affect N_A 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 N_A. An explanation or clarification would be appreciated!

 

[Chestermiller]

N_A = J_A^* + C_A*(N_A+N_B)/C for a binary mixture.

N_A is the flux of the fluid relative to a stationary point.
J_A^* is Fick's law, the flux relative to the bulk motion of the fluid?
(N_A+N_B)/C is the bulk velocity of the fluid.
​

But I don't see why this is the case. I don't see why N_B should affect N_A 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 N_A. An explanation or clarification would be appreciated!

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?