WHAT IS DIFFUSION

In earlier publications, heat Q- is defined as an interaction that is entirely distinguishable from work W-. The energy exchanged Q- is T(Q) times the entropy exchanged S-, where T(Q) is the almost common tempe rature of the interacting systems. Here, we define diffusion as anothe r interaction that is entirely distinguishable from both work and heat , and that involves exchanges of energy, entropy, and amount of a cons tituent. It is an interaction between two systems A and B that pass th rough stable equilibrium states while their respective parameters rema in fixed, and that have almost equal temperatures T(A) almost-equal-to T(B) almost-equal-to T(D) and almost equal total potentials mu(A) alm ost-equal-to mu(B) almost-equal-to mu(D) of the diffusing constituent. The exchanges of entropy S , energy E-, and amount of constituent n- out of one system satisfy the relation S- = (E-- mu(D)n-)/T(D). In the limit of n- = 0, a diffusion interaction becomes heat.