In earlier publications, heat Q is defined as an interaction that is entirely distinguishable from work W. The energy exchanged Q is TQ times the entropy exchanged S, where TQ is the almost common temperature of the interacting systems. Here, we define diffusion as another interaction that is entirely distinguishable from both work and heat, and that involves exchanges of energy, entropy, and amount of a constituent. It is an interaction between two systems A and B that pass through stable equilibrium states while their respective parameters remain fixed, and that have almost equal temperatures TA ≈ TB ≈ TD and almost equal total potentials μA ≈ μB ≈ μD 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 −μDn)/TD. In the limit of n = 0, a diffusion interaction becomes heat.

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