Triboelectricity


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triboelectricity

[¦trī·bō‚i‚lek′tris·əd·ē]
(electricity)

Triboelectricity

 

(also frictional electricity), electric charges generated by friction. Triboelectricity is observed when friction occurs between two dielectrics, two semiconductors, or two metals; the substances may be of different chemical composition or of identical composition but different density. It is also observed when friction occurs, for example, between a metal and a dielectric, between two identical dielectrics, between liquid dielectrics, or between a liquid dielectric and the surface of a solid. In all cases, both substances are electrified, and their charges are equal in magnitude but opposite in sign.

Triboelectricity is characterized by a number of regularities. When friction occurs between two chemically identical bodies, the denser body is positively charged. When there is friction between a metal and a dielectric, the metal may be electrified with either positive or negative polarity. When friction occurs between two dielectrics, the dielectric with the higher dielectric constant e is positively charged. Substances can be arranged in triboelectric series. Whenever two substances in such a series are rubbed together, the substance that is higher in the series is electrified with positive polarity, and the substance that is lower in the series receives a negative charge. An example is Faraday’s series, which is as follows: ( + ) fur, flannel, ivory, feathers, rock crystal, flintglass, cotton cloth, silk, wood, metals, sulfur (–). Dielectrics arranged in a triboelectric series exhibit a decrease in hardness. This fact is illustrated by Gezekhus’ series, which is as follows: ( + ) diamond (hardness 10), topaz (8), rock crystal (7), polished glass (5), mica (3), calcite (3), sulfur (2), wax (1) (–). An increase in hardness is characteristic of metals. In the case of liquid dielectrics, the positive charge is acquired by the substance with the higher e or with the higher surface tension.

The larger the surfaces of the bodies that are rubbed together, the greater the electrification of the bodies. Dust that slides across the surface of the body from which it was formed—for example, marble, glass, or snow dust—receives a negative charge. Powders sifted through a sieve are also charged. For example, powdered sulfur and red lead are negatively charged if they are sifted separately, but are oppositely charged (sulfur negatively and red lead positively) if they are sifted together; the charging occurs as a result of friction between particles. When liquids are spattered, as in the impact of a liquid on a solid or liquid surface, electrification of both the liquid and the ambient gas is observed; the polarity of the charges depends on the type of gas. Electrification is also observed when a gas passes through a liquid. The phenomenon of triboelectricity is made more complicated by the presence of a moisture film on the surface and by surface contamination.

Triboelectricity in solids is attributed to the transfer of charge carriers from one body to the other. In the cases of two metals, two semiconductors, or a metal and a semiconductor, triboelectricity is caused by the transfer of electrons from the substance with the lower work function to the substance with the higher work function (seeCONTACT POTENTIAL). When a metal and a dielectric are in contact, triboelectricity arises as a result of the transfer of electrons from the metal to the dielectric and the transfer of ions of either polarity from the dielectric to the surface of the metal. When friction occurs between two dielectrics, triboelectricity is caused by the diffusion of electrons and ions. The different heating of bodies by friction may also play an important role. Such heating causes charge carriers to pass from local inho-mogeneities of the more intensely heated surface; this effect is called true triboelectricity. When friction occurs between two dielectrics that differ only in density, more electrons diffuse from the denser substance, which becomes positively charged. When two different dielectrics are in contact, more electrons pass from the substance with the higher €. Triboelectricity can also be caused by mechanical separation of individual sections of the surface of a piezoelectric (seePIEZOELECTRICITY).

The triboelectricity of liquids is associated with the occurrence of an electric double layer at an interface between two liquid media or at a liquid-solid interface. When friction occurs between a liquid and a metal in flow processes or during spattering upon impact, triboelectricity results from electrolytic charge separation at the metal-liquid interface. Electrification by friction between two liquid dielectrics is a consequence of the existence of a double layer at the interface between liquids with differing e. According to Coehn’s law, the liquid with the higher e is positively charged, and that with the lower e is negatively charged. The triboelectricity generated during the spattering of liquids as a.result of impact on the surface of a solid dielectric or the surface of a liquid (for example, electrification in waterfalls) is attributed to the breakup of the double layer at the interface between the liquid and the gas.

Triboelectricity causes an undesirable accumulation of electric charges in dielectrics, such as synthetic fabrics or the paper used in printing. The charges are removed by grounding metal parts, ionizing the air, using electrical dischargers, or increasing the conductivity of the dielectrics.

REFERENCES

Chwolson, O. D. Kurs fiziki, 5th ed., vol. 4. Berlin, 1923.
Loeb, L. Staticheskaia elektrizatsia. Moscow-Leningrad, 1963. (Translated from English.)

A. N. GUBKIN

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