DIFFUSION IN A RECTANGULAR ELEMENT

This expression is the equation of continuity for conservation of mass [27], Using eqs. (4.16) and (4.30), with D = constant, yields Pick s second law equation in three dimensions 

one-dimensional diffusion suffices most analysis of isotropic medium. Note that z has been used as a coordinate in eqs. (4.28), (4.30) and (4.31). Hence, for diffusion in the x-direction, the concentration rate in eq. (4.31) becomes the most famihar Pick s second law equation 

If D is not constant, then eq. (4.31) yields the concentration rate in the x-direction as 

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