Points as collectors of charge

Points as collectors of charge

A sharp point not only enables a conductor to lose charge but it can also act as a collector of charge. This aspect of point action can be shown by the following experiment. A gold-leaf electroscope is provided with a sharp point by placing a needle or a
piece of wire on the cap. When a positively charged rod is brought near to the point a divergence of the leaf rapidly builds up and the leaf will stay diverged when the rod is removed. On testing by the usual method the electroscope is found to be positively
charged. Fig. 32.19 explains what happens in this case. The positive charge on the rod acts inductively on the electroscope so that the point is negatively charged and the leaf positively charged. Point action then takes place, with the result that positive ions are attracted towards the point, receive electrons from it and become neutralized. The electroscope thus loses negative charge, i.e., it acquires a net positive charge.

At the same time negative ions are attracted towards the positively charged rod, where they give up electrons to it and neutralize its positive charge. The resultant effect, therefore, is a gain of positive charge by the electroscope and a loss of positive charge by the rod. It is usual to describe this process by saying that charge has been “drawn off” the rod by the point, but once again this is merely a figure of speech, since positive charge does not actually pass into the electroscope any more than positive charge
leaves the rod.

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