CBSE Class 12-science Answered

The gold leaf electroscope is a  sensitive instrument. It consists of a draught proof metal case with glass sides housing the gold leaves. The gold leaves 'L' and 'M' are attached at one end of a vertical metal stem S. At the top the stem has a metal place 'C'. The stem is carefully insulated by plastic material from the surrounding metal case. The case is earthed to protect the leaves from electrical disturbances outside.

To detect a charge on a rod 'A' or 'B' bring the rod near to the metallic disc or cap of the electroscope. In either case the leaf diverges as shown in the diagram.

Figure (a) shows what happens when a rod 'A' with a positive charge is brought near the cap. Electrons are attracted and get accumulated on 'C'. This leaves positive charges on 'L' and 'M'. The repulsion caused by the leaves diverges them. When the rod 'A' is taken away the electrons get distributed and hence the leaves close.

When a negatively charged rod 'B' is brought near the cap [Figure (b)], the free electrons in the metal are repelled. Thus all the electrons move away from the cap to 'L' and 'M'. Since both the leaves have negative charges they repel and diverge. The movement of electrons from the cap gives it an equal amount of positive charge. If the charged rod 'B' is taken away the electrons will get redistributed and the leaves collapse.