Two Identical Phases Not at Equilibrium

We turn our attention now to another special case of Eq. (2.4), in which the electrochemical potential of a species (an electron this time), p., is considered in two identical phases that are not at equilibrium. In this case we have 

The quantity A c ) in Eq. (2.17) is the actual potential measured between two copper wires attached to the terminals of a battery. We note that potentials are measured, as a rule, with a device that has a very high input resistance, which prevents the establishment of equilibrium between the electrons in its two terminals. 

The important physical understanding that can be gained from Eq. (2.17) is that the potential difference measured is nothing but the difference in the electrochemical potentials of the electrons in the two terminals of the measuring instrument (divided by Faraday s constant, F, for consistency of units). 

It is important to understand clearly the difference between the two special cases of Eq. (2.4). First we discussed equilibrium between dissimilar phases. Then we discussed non-equilibrium between identical phases. The former led to  

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