as the equation describing the conservation of mass. by the empirical relationship (4-2) which is called fick's law of difusion. in (4-2), u-un® uj) represents the velocity of the fluid and is assumed known. the quantity d 0 is called the diffusion coefficient. in general it depends on c but we shall assume that it is constant here. inserting (4-2) into (4-1) yields the vector q is related to c is insulated ge edium perature di -0? suppo the bounda (4-3) as the basic equation governing diffusion-dispersion phenomena. if the velocity while if d is negligibly small, (4-3) becomes the first-order partial differential equation dan of the medium is negligible, the diffusion equation is the same as (3-8) with f-o, d heat flo condition (4-4) boundary a limiting proble m s se that a substance is dissolving into a liquid which is at rest in a container. assume further that no mass enters or escapes the container. on the basis of (4-2) with 1. suppos 0, give a physical argument showing that lim c(x, t)-co se of that the where co is a constant. argue that co- c(x, 0) dx, is its volume, is the set making up the container and where the lines of fourier's law 3. what form does (4-3) have when the diffusion coefficient d depends on c and internal 2. give a physical motivation for the validity of fick's law. model your argument along