Induced charge and the Electroscope

Suppose a positively charged metal object is brought close to an uncharged metal object. If the two touch, the free electrons in the neutral one are attracted to the positively charged object and some will pass over to it. Since the second object, originally neutral, is now missing some of its negative electrons, it will have a net positive charge. This process is called “charging by conduction,” or “by contact,” and the two objects end up with the same sign of charge.

Now suppose a positively charged object is brought close to a neutral metal rod, but does not touch it. Although the free electrons of the metal rod do not leave the rod, they still move within the metal toward the external positive charge, leaving a positive charge at the opposite end of the rod.

A charge is said to have been induced at the two ends of the metal rod. No net charge has been created in the rod: charges have merely been separated. The net charge on the metal rod is still zero. However, if the metal is separated into two pieces, we would have two charged objects: one charged positively and one charged negatively.

Another way to induce a net charge on a metal object is to first connect it with a conducting wire to the ground. The object is then said to be “grounded” or “earthed.” The Earth, because it is so large and can conduct, easily accepts or gives up electrons; hence it acts like a reservoir for charge. If a charged object—say negative this time—is brought up close to the metal object, free electrons in the metal are repelled and many of them move down the wire into the Earth. This leaves the metal positively charged.

If the wire is now cut, the metal object will have a positive induced charge on it. If the wire were cut after the negative object was moved away, the electrons would all have moved back into the metal object and it would be neutral.

Charge separation can also be done in nonconductors. If you bring a positively charged object close to a neutral nonconductor, almost no electrons can move about freely within the nonconductor. But they can move slightly within their own atoms and molecules. The negatively charged electrons, attracted to the external positive charge, tend to move in its direction within their molecules. Because the negative charges in the nonconductor are nearer to the external positive charge, the nonconductor as a whole is attracted to the external positive charge.

electroscopeAn electroscope is a device that can be used for detecting charge. As shown in Fig. inside of a case are two movable metal leaves, often made of gold. If a positively charged object is brought close to the knob, a separation of charge is induced: electrons are attracted up into the knob, leaving the leaves positively charged. The two leaves repel each other, because they are both positively charged. If, instead, the knob is charged by conduction, the whole apparatus acquires a net charge. In either case, the greater the amount of charge, the greater the separation of the leaves.

Note that you cannot tell the sign of the charge in this way, since negative charge will cause the leaves to separate just as much as an equal amount of positive charge; in either case, the two leaves repel each other. An electroscope can, however, be used to determine the sign of the charge if it is first charged by conduction, say, negatively. Now if a negative object is brought close, more electrons are induced to move down into the leaves and they separate further. If a positive charge is brought close instead, the electrons are induced to flow upward, leaving the leaves less negative and their separation is reduced.

The electroscope was used in the early studies of electricity. The same principle, aided by some electronics, is used in much more sensitive modern electrometers.