Contact electricity

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As contact electricity , contact stress , contact potential , contact electricity or contact voltage (not to be confused with the contact voltage in electrical devices) are referred to collectively electrical phenomena at interfaces between different substances or between one substance and the environment (eg. For example vacuum ) occur . The term "contact electricity" was coined by Alessandro Volta and at that time stood in contrast to the older thesis of "animal (animal) electricity", which goes back to Luigi Galvani and his experiments with frogs' legs. As the understanding of electrical phenomena increased during the 19th and early 20th centuries, it became clear that the phenomena known as "contact electricity" are very different in nature, and the term is no longer used in modern physics.

Phenomena of "contact electricity"

If a metal takes a (resting) electron from the vacuum, it ends up in it at the Fermi level . The released binding energy is work function . If two conductors with different work functions, which are initially uncharged (number of electrons matching the nuclear charges), come into electrical contact with each other, electrons briefly flow in the direction of the conductor with the higher work function (to the lower Fermi level) until they are in equilibrium the Fermi levels are aligned. Explanation for the equalization: The net charge is distributed 'in' the surface of the conductor, due to influence preferentially where the surfaces are close. Both in the space charge zone (very thin in the case of metals) and in the outer space, an electric field is created which counteracts the further flow of current - quantitatively: the path integral of the field strength from the inside of one conductor to the inside of the other, the so-called "contact potential", is the same Difference in workloads. (Actually, the "contact potential" is a potential difference , ie a voltage, but the designation contact voltage also has a different meaning, see contact resistance ).

The electric field in the outer space makes z. B. noticeable in electron tubes whose grid is short-circuited with the cathode: electrons thermally emitted by the cathode are driven back to the cathode by this field. If the grid is made more positive by an external voltage, the anode current increases until this voltage just balances the contact potential and the external field disappears. The external field in electron optics for low-energy electrons and in precision experiments such as Gravity Probe B is more disruptive . The variation in the work function is caused by contamination of the surfaces.

If the electrical conductors are separated, the majority of the charges flow back as the contact area decreases. The remaining amount of charge depends on how closely large areas of the conductor are opposite each other at the exact moment when the last conductive contact breaks off, because until then the voltage remains constant, while the field strength decreases with increasing distance. Dielectric layers, e.g. B. oxidized metal, over a large part of the surface can increase the amount of charge, not only through their relative permittivity , but above all by allowing close mechanical contact without simultaneous electrical contact. After the final separation, the charge and field strength remain constant (except for redistributions), so that the voltage increases proportionally to the distance (until the distance becomes greater than the expansion of the bodies).

The amounts of charge that can be separated by means of insulators, see static electricity , are much greater, and so are the voltages that crackle dangerously when taking off 'suitable' sweaters. However, there is no well-defined charge equalization between insulators when they are touched and therefore the concept of contact potential is not defined. The electrical charge cannot be recorded exactly, there is only a qualitative triboelectric voltage series which indicates which of two materials is positively or negatively charged when they come into contact with one another.

Normally no current will flow in a circuit due to the contact potential described above. In particular, you do not measure voltage if you connect the two different conductors to one of the two test probes of a measuring device, because the two additional contact points differ by the same work function difference as the original contact. However, the Fermi level and thus the work function is temperature-dependent, and in some materials it is even strong due to an asymmetry of the density of states . As a result, in an ABA arrangement of two materials A and B with contact points AB and BA at different temperatures, a thermal voltage ( Seebeck effect ) or, in the event of a short circuit, a thermal current is created.

Another cause of a sustained flow of current and thus easily measurable effects are electrochemical reactions. The complex interplay of electrical, electrochemical and membrane potentials was explored by Galvani, Volta and Humboldt and interpreted in different ways.

literature

  • Wilhelm von Zahn : Investigations into contact electricity. Teubner, Leipzig 1882.

Web links

Individual evidence

  1. ↑ H. Barkhausen: Textbook of electron tubes. 1. Volume General Basics. 11th edition. S. Hirzel Verlag, 1965, p. 41 (chapter 4 starting current).
  2. ↑ Oliver Lodge: On the Controversy Concerning Volta's Contact Force , Kessinger Publishing, 2005, ISBN 978-1-4179-7464-1 , limited preview in the Google book search.