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Section 28.5 Electroscope and Electrostatic Induction

The repulsion of like charges is demonstrated by an elegant instrument called electroscope invented by the French physicist Jean Antoine Nollet in 1748 (Figure 28.5.1). To make an electroscope you will need a metallic strip, so thin that it is limp and flexible. You can also cut two thin strips of aluminum or some other metal foil and put a pin through them at one end so that the pair can rotate away from each other at the other end. The limp metallic strip or metal foil is then hung over a metal support. The structure so constructed is then attached to a metallic rod and placed inside a clear (for viewing) glass jar using a cork stopper. Finally place a metallic knob at the top.

Figure 28.5.1. An electroscope - the thin aluminum leaves are not visible in the picture. The sketches illustrate the positive and negative charging of the electroscope by induction.

If you bring a charged material, such as a glass rod rubbed with silk, near the metallic knob of the electroscope, but not touching the electroscope, you will see that the leaves of the electroscope separate awayfrom each other. This happens because of charges induced in the electroscope as shown in panel Figure 28.5.1(b). Electrons in the leaves are attracted towards the positive charges on the glass rod and migrate towards the knob of the electroscope, leaving excess of protons over electrons in the atoms of the leaves, which makes them both positively charged. Therefore, they repel. We find that, as long as the charged rod is held in place, the leaves stay separated due to the repulsion between the induced charges on the leaves. When the charged rod is removed, the extra electrons from the ball in the electroscope move towards the leaves and the electroscope becomes neutral again.

Induced effect always creates a separation of equal amount of negative and positive charges. Metals contain a large number of readily movable electrons bound to fairly fixed centers of positively charged atomic nuclei. These electrons in a metal are called conduction electrons. When you bring a charge near a metal, the electrons move, creating regions that have excess of electrons and others that have deficiency of electrons. The regions that have excess of electrons are negatively charged while the regions deficient in electrons are positively charged.

In non-metals, such a piece of glass or plastic, electrons are not free to move to large distances. When an external charge is brought near these materials, electrons in their molecules tend to move within the molecules and rearrange so that there is a polarization of molecules with one end more negative than the other end, creating induced dipoles in the material. Induced dipoles are attracted to the external charges as easily seen in the attraction of papers to charges on a brushed comb.

Although the charge separation by an induced effect disappears when the external charge is removed, a conductor can be permanently charged if the charges at the remote part of the conductor can be somehow removed or neutralized. Consider bringing negatively charged ebonite rod near one end of a metal rod (Figure 28.5.2).

Figure 28.5.2. Electrification by induction. (a) Equal amout of positive and negative induced charges in metal rod. (b) The far-end of the metal connected to a large conductor by a conducting wire makes it possible for charges to move further out in the large metal piece. (c) After the wire is disconnected from the metal, metal rod is left positively charged even when at no point the positively charged ebonite rod touched the metal.

This causes extra negative charges on the far end, and extra positive charges on the near end. Now, if we touch the far-end of the rod with a metallic wire connected to a large conductor, such as earth, then the induced negative charges on the rod migrate to the large body. Removing the wire leaves the metal rod positively charged since it has excess positive charge. Now, when you remove the charged ebonite rod, the metal does not become neutral now. Basically, by severing a conductor, which is under the influence of an external charge, into two parts will leave each part with a net charge.

An instrument called the electrophorus was invented by Allesandro Volta in 1777 that makes use of induction to accumulate large quantities of charge on a conductor. Figure on the right illustrates an electrophorus. In the simplest form, it consists of an insulated plate, here a hard rubber plate of diameter about 30 cm, and a metal plate with a handle.

First, the insulator plate at the base is charged by rubbing it with a rabbit fur giving the plate a net negative charge. Then the metal plate is placed on the charged insulator plate. The negative charge on the insulator plate induces a charge separation in the metal plate with the bottom of the metal plate becoming positive and the top becoming negative. The top part is then touched with hand that drains the negative charge to the body leaving a net positive charge on the metal plate. The positively charged metal plate is then lifted with its plastic handle and transported for static electricity demonstrations.