Top effect

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The peak effect is a term used in physics to refer to high gradients on pointed shapes as the cause of effects. Well-known examples are the increased mechanical stress in front of a crack in a workpiece ( called the notch effect on surfaces ) and the increased electrical field strength in front of a point or sharp edge.

Mechanical peak effect

Since a crack cannot transfer tension, the material next to the crack remains relaxed when the workpiece is loaded, while the crack surfaces move against each other (apart or parallel to each other, depending on the direction of the load). In front of the crack, this movement leads to a strongly localized deformation, elastic or plastic depending on the material, and possibly to crack growth .

Electric peak effect

Field lines have a tendency to shorten and repel each other (in order to minimize the field energy). If the field lines in the far field run in the direction of the tip, they can become shorter if their foot points on the surface migrate towards the tip (charge carriers actually migrate). They only have to move very closely together in the immediate area in front of the tip, so that the high field strength there does not contribute much to the total field energy. This field strength can cause a voltage breakdown in gas (see peak discharge ), in a vacuum field emission . In order for the tip effect to be effective in alternating fields, especially when electromagnetic waves are incident, the tip (more precisely: its radius of curvature) must be small in relation to the wavelength. This is used when performing spatially resolved spectroscopy with a scanning tunneling microscope . For a long time this effect was also held responsible for the shape of the end of lightning rods . Here, the stronger ionization of the ambient air due to the high field strength speaks in favor of finer tips in order to prepare a lightning bolt, as it were, an ion channel. In contrast to this model, however, calculations have shown that this effect only dominates locally, and that rounded ends in the distance result in a greater field strength gradient.

literature

  • Johann Christian Poggendorff : About electrical peak effect . In: Annals of Physics and Chemistry. Volume 215, Joh. Ambr. Barth, Leipzig 1870, pp. 341-349.

Web links

Individual evidence

  1. ^ CB Moore, William Rison, James Mathis, and Graydon Aulich, Lightning Rod Improvement Studies . Journal of Applied Meteorology: Vol. 39, No. 5, pp. 593-609. Langmuir Laboratory for Atmospheric Research, New Mexico Institute of Mining and Technology, Socorro, New Mexico. April 10, 1999.