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Charge Distribution on Conductors’ Surface
The electric charges on the surface of a charged object do not necessarily spread out evenly. If the surface is flat or spherical, the charge shall be evenly distributed on all points on the surface.If the curvature is not regular, for instance, the charges tend to accumulate more at the sharper places than flat areas.
Charges on or action at sharp points
The density of charge is greatest where curvature is greatest.As a result, the charge density at sharp points becomes large. Since like charges repel, the electrons are emitted rapidly from sharper points. This is called action at points and it works on the principle of corona discharge, based on ‘action at points’
Thunder clouds carry a vast amount of negative charges at the bottom and an equal amount of corresponding positive charges at the top. When they loom over us in the gloomy sky, positive charges are induced on the objects on the ground, as a result of electrostatic induction. The induced charges accumulate at sharper points due to action at points. Then, suddenly a massive electric flow takes place between such a point and the cloud in question. This is called a lightning strike or thunderbolt. You are therefore not supposed to hold sharp metal objects during thunderstorms, as the contribution of the phenomenon, action at points, is well known in the process
The lightning arresters
The action of losing charged conductor due to its pointed ends is called action of points. The action of points is used in lightning in conductors. Lightning conductors with sharp pointed ends are fitted at the top of a building in order to have safe discharge of electricity generated in the atmosphere Lightning arrestor is a very simple device in its structure, despite doing a marvellous job in saving lives and properties around the world.
It consists of a three or two metal spikes attached to a vertical rod, which in turn is connected to the Earth by a thick copper wire. When a thunder cloud is around, it induces positive charges on the objects on the ground. Most of the induced charges prefer to reach sharp edges to flat edges. Therefore, the induced charges accumulate at the metal spikes.
As a result, a vast flow of electrons takes place between the cloud and the ground through the lightning conductor, thanks to the high conductivity of the copper cable. This arrangement saves both the property and the occupants from a potentially lethal damage.