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Distribution of Electricity over a Conductor.

المؤلف:  GEORGE A. HOADLEY

المصدر:  ESSENTIALS OF PHYSICS

الجزء والصفحة:  p-324

2025-11-27

479

If a metal sphere is placed upon an insulating support, such as a glass rod, and charged with a certain quantity of electricity, a proof plane placed upon any part of its surface will carry away the same charge, as may be found by tests with an electroscope (the leaves diverging as much from one charge as from another). But if an equal charge is given to an insulated metal cylinder with rounded ends the proof plane will carry a much greater charge away from the ends than from any other part of the surface. The comparative density of charge, or quantity of charge per unit area, is represented by the distance of the dotted line from the surface in Fig. 1. From this it appears that the density of charge is greater at the projecting parts of an insulated conductor.

Fig. 1

Demonstration. - Support a short metal cylinder upon an insulating stand. Fasten a wire 3 in. long vertically into the top of the cylinder. From the top of this wire suspend two pith balls by linen threads. Suspend two others from the inside of the cylinder. Charge the cylinder with the charge from a glass rod; the outer pair of balls will fly apart, while the inner ones will remain undisturbed.

FIG.2

From this we learn that on an insulated conductor the electrical charge is located on the outside. This location is caused by the mutual repulsion of like charges. It follows that, with the same charge, as the outside surface increases the surface density diminishes.