Coating Capacitance

A capacitor is formed when two conducting plates are separated by a non-conducting medium, called the dielectric. The value of the capacitance depends on the size of the plates, the distance between the plates, and properties of the dielectric. The relationship is:


ε0 = electrical permittivity

εr = relative electrical permittivity

A = surface area of one plate

d = distance between two plates

Whereas the electrical permittivity is a physical constant, the relative electrical permittivity depends on the material. The following table gives you a few useful εr values.

Typical Relative Electrical Permittivities

Notice the large difference between the electrical permittivity of water and that of an organic coating. The capacitance of a coated substrate changes as it absorbs water. EIS can be used to measure that change in capacitance.