Peak discharge

Discharge at a metal tip in the high-voltage laboratory

As a point discharge are spark discharges or corona discharges referred applied to electrostatically with high-voltage charged or under a high voltage conductors preferably take place at protruding tips, because there the density of the field lines and thus also the electric field strength is greatest.

root cause

Fig. 1: Dependence of the field line density on the distance

Fig. 2: Dependence of the field line density on the radius

Peak discharges were recognized as an electrical phenomenon as early as the 18th century, but their physical cause was not yet fully understood. It was not until the late 19th century, with the establishment of field theories about electric fields , that the cause of the peak discharge became clear.

As can be seen in Figure 1, the field lines are perpendicular to an electrically charged conductive surface. The strength of the electric field is greater, the closer the field lines are to one another. The field strength is therefore greatest near a convexly curved surface. In Figure 2 it can be seen that the field lines emerge more closely from the surface at the reduced radius than at the larger radius shown in dashed lines.

The electric field strength E on the surface of a sphere with the charge Q with the radius r is, for example, according to Coulomb's law :

${\displaystyle E={\frac {Q}{4\pi \varepsilon _{0}r^{2}}}}$

( is the electric field constant ) ${\displaystyle {\varepsilon _{0}}}$

Each point can be roughly imagined as a spherical surface with a small radius r . If the tip is part of a conductor that is under a constant, externally maintained voltage U , it becomes

${\displaystyle E={\frac {U}{r}}}$

If the field strength is sufficiently high, field ionization of the air occurs on the surface of the tip . Therefore, spark and corona discharges take place almost exclusively at the tip.

System with typical rounded, voluminous parts to avoid peak discharges ( Cockcroft-Walton accelerator , exhibit at the National Museums of Scotland )

To avoid undesirable peak discharges, metallic surfaces that carry high voltages must be designed with the largest possible radii while avoiding sharp edges and points. This gives high-voltage system parts the typical round, voluminous-looking shape (see example photo on the right). For the same reason, the ends of high-voltage insulators are provided with corona rings .

application

The knowledge about the cause of the peak discharge is widely used in our everyday life. One of the reasons why a lightning rod is so effective in the house is because its electrically conductive tip has a much higher field line density than the rest of the house. As a result, the surrounding air is pre-ionized and weakly conductive; therefore, lightning tends to strike the lightning rod rather than the house.

Peak discharges can be observed as "spraying" from high-voltage lines or as " Elmsfeuer " z. B. in the masts of sailing ships during thunderstorms, since the discharge occurs at sharp points in air under atmospheric pressure at 1000 to 1500 volts. In the experiment, therefore, the charge in an electroscope equipped with a sharp point drops quickly to these values ​​of the voltage. In the case of the Elmsfeuer (at field strengths close to thunderstorms of around 100,000 V / m compared to around 100 V / m in the normal state on the earth's surface), positively charged tips are observed near-by cluster discharges (light clusters) and with negatively charged tips by glow discharges (points of light).

In electrostatic precipitators and ionizers , dust particles are electrically charged by peak discharge and then drawn to the oppositely charged electrode.

Another application of the peak discharge is the demonstration of corona discharges in experiments. By using a conductor with a pronounced tip, they can be generated with considerably less voltage and in a more targeted manner. In experiments with Tesla transformers , a wire is attached to the upper toroid , which serves as the electrode of a capacitor (the other electrode is earth). Even at lower voltages, this leads to peak discharges at the end of the wire, instead of the uncontrolled discharges otherwise observed at the toroid.

In experiments with electrostatics , the peak discharge in the so-called spark inductor is used. The use of tips leads to more evenly spaced discharges than with spherical spark inductors.

As smooth and round surfaces as possible are used in experiments in which a large amount of electrical charge is to be accumulated before the discharge, because the density of the electrical field lines is more evenly distributed on them.

See also

• Field emission

literature

• Karl Küpfmüller: Theoretical electrical engineering and electronics . 14th edition. Springer, 1993, ISBN 3-540-56500-0 , pp. 95 .

Web links

Wiktionary: tip discharge  - explanations of meanings, word origins, synonyms, translations