Surface Potential of a Phase

Consider a conductor a in contact with an insulator (or vacuum) not containing charges (Fig. 9.1). Let x be the distance of a point inside the insulator measured from the interface. The potential will be referred to the conventional point of reference at X— oo within the insulator. 

When the conductor as a whole is charged (i.e., has excess charge of one sign in its surface layer), an electrostatic field and a potential gradient will develop in the insulator region adjacent to it. The name of the outer potential, / f, of the conductor is used for the potential at a point a located in the insulator just outside the conductor. Since point a and the point of reference are located in the same phase, this potential can be measured. 

The concept of just outside must be defined more closely. When the test charge is moved from the point of reference toward the surface, work is performed due to the (primary) electrostatic field being discussed. However, very close to the surface, image forces start to act on the test charge they give rise to an additional 

Fundamentals of Electrochemistry, Second Edition, By V. S. Bagotsky Copyright 2006 John Wiley Sons, Inc. 

---