The most important type of charge separation involves the contact and friction between solids known as triboelectrification. When two solid materials, A and B (see Figure 3.1), contact and rub against each other, electrons move across the interface.

Metals

        When the two contacting materials are metals, a voltage difference is established across the interface with a magnitude from a couple of tenths to a few volts.

        If the metals are "well-defined" , the contact potential difference can be calculated from the work functions, i.e. the energy it takes to remove a loosely bound electron from the metal. It should be stressed, however, this charge exchange between metals gives only a rise to what we normally understand as static electricity when the two metals are separated extremely quic, as when a metal powder is blown against a metal.

Insulators

        It is obvious that only electrons closely located  to the surface can participate in the charging of highly insulative materials. Like metals, some of these materials is possible to measure the work function for loosely bound electrons. Charging experiments with insulators can show quantitatively predictable results if the surfaces are carefully prepared and the experiments are performed in vacuum only. And such experiments might disclose in little about what one could expect to find under more practical conditions.

Contact Electrification:Triboelectric Series

       One of material parameters that influence the process of the charge between two solid materials is permittivity. The permittivity is defined as a ratio between corresponding values of the dielectric displacement and the electric field strength. The stronger forces are, the higher is the permittivity of the material. 

        So permittivity is in a strong connection with for Coehn's law stating that when two materials are in contact with each other, this one, whose permittivity is the highest, becomes positive. This law was originally based on a comparison of known values of permittivity, which were put down in turboelectric series. 

        Table 1. shows examples of triboelectric. Such series should be used carefully because the order of the materials vary series by series.