The
electric field near a sharp end of a conductor is
higher! The
electric field near a sharp end of a conductor is
higher!
There is a simple way to understand why this is so. Consider two conductors at the same potential - one is flat, while the other has a sharp, highly curved, end. A conductor has to be an equipotential, so Electric lines of force always cut conductors normally. Near the flat conductor, the lines of force are equally spaced, hence the field is uniform. But near the curved conductor, the lines of force have to be more closely spaced nearer the conductor, than away from the conductor. Hence the field is higher immediately next to the conductor, than farther away from it.
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Since the field is stronger near a sharp point, the air molecules immediately adjacent to a sharp point feel a strong field, and therefore induced opposite charges develop easily at the nearer end of these molecules. As a result, they get attracted to the conductor, and touch it. Upon touching, they get charged with the same charge as the conductor, and are then repelled away. One then gets a steady stream of molecules, with the same sign of charge as the conductor, moving away from it. This causes a slow leakage of the charge on the conductor. The steady stream of air molecules moving away from the sharply pointed conductor is called the electric wind. Even for a moderate amount of charge on the conductor, it is strong enough to bend the flame of a candle.
Action of a lightning rod:
Clouds, in drifting through the atmosphere develop charges by friction. With low clouds, this can often result in very large electric fields between the cloud and the earth buildings. When the field is larger than about 3·106 V/m, it may rip away electrons from air molecules (ionization). This is called a dielectric breakdown, as the air no longer remains an insulator when this happens - the ions and the ripped out electron flow between the cloud and the earth/ building, giving rise to a large current - what we call lightning. A lightning conductor is a thick metallic rod attached to the side of buildings, that stick out a few feet above the tallest point of the building, ending in a bunch of sharp 'fingers', pointing upward. In the presence of low thunder clouds, opposite charges are induced on the lightning conductor. Due the sharp 'fingers', a steady electric wind is created, which partially neutralizes the opposite charges on the cloud, thus decreasing the chance of a lightning. Even if a lightning does hit the building, the metallic rod conducts it away to the earth, and it does not pass through the building.
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