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Description

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:

 

 

with the electrical permittivity ε0, relative electrical permittivity εr, the surface area A of one plate, and the distance d 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.

 

 

Notice the significant difference between the relative 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.