<P> where W (\ displaystyle W) is intensity of any local source of this quantity (the rate of a chemical reaction, for example). For the diffusion equation, the no - flux boundary conditions can be formulated as (J (x), ν (x)) = 0 (\ displaystyle (\ mathbf (J) (x), \ nu (x)) = 0) on the boundary, where ν (\ displaystyle \ nu) is the normal to the boundary at point x (\ displaystyle x). </P> <P> Fick's first law: the diffusion flux is proportional to the negative of the concentration gradient: </P> <Dl> <Dd> J = − D ∇ n, J i = − D ∂ n ∂ x i . (\ displaystyle \ mathbf (J) = - D \, \ nabla n \, \; \; J_ (i) = - D (\ frac (\ partial n) (\ partial x_ (i))) \ .) </Dd> </Dl> <Dd> J = − D ∇ n, J i = − D ∂ n ∂ x i . (\ displaystyle \ mathbf (J) = - D \, \ nabla n \, \; \; J_ (i) = - D (\ frac (\ partial n) (\ partial x_ (i))) \ .) </Dd>

Molecules move from regions of higher concentration to regions of low concentration