Simple Electrostatics

(a) Electric Charges:

Electrostatics is the study of electric charges at. In general, when any two different materials are rubbed together they exert electrical forces on each other and each aquires an 'electric charge'. There are two kinds of charge, positive and negative. The fundamental law of charges is that like charges repel and '''unlike charges attract. '''The two kinds cancel one another out and in this respect are opposite.

The allocation of signs to charges was made quite arbitrarily many years ago and materials today are based on the sign convention; that is proton's are positively charged and electrons negatively charged. Atoms are naturally electrically neutral and so contain equal numbers of protons and electrons (the charges cancel out one another). Electrification by rubbing may be explained by supposing that electrons are transferred from one material to the other.

(b) Insulators and conductors:

According to the electron theory, all the electrons in the atoms of electrical insulators. e.g. plastics/ glass, are firmly bound to their nuclei and the removal or addition of electrons at one place does not cause the flow of electrons elsewhere. This means that the charge is confined to the region where it was produced (e.g. by rubbing). Electrical conductors (e.g. metals) have free electrons in their outer valence shells that are free to move from the individual atoms to another part of the material. Charges in conductors evenly distribute throughout the conductor.

We are good conductors so when handling charged objects we need to make sure they are well insulated so that it does not pass through us. The Earth is also a good conductor, so any charge that passes through us will pass through our feet to the earth.The human body and the earth are comparatively good conductors, so if we try to charge a metal rod, it must be well insulated and not held in the hand. Otherwise any charge produced is conducted away through the body to the earth.

(c) Electrostatic induction:

A negatively charged object held close to an insulated, uncharged conductor, attracts it. This may be explained by saying that electrons are on the outside surface of the conductor are repelled by the negatively charged object, leaving the exposed side positively charged. This positively charged side is then attracted to the negatively charged object. The effect is called '''electrostatic induction. '''It accounts for the attraction of scraps of paper by a plastic comb, charged by being drawn through the hair. It is also this effect that enables a party balloon, which has been rubbed on a woollen jumper, to stick to the ceiling.

(d) Electrophorus:

This is a device for producing charges by electrostatic induction. It consists of a circular metal plate with an insulating handle, placed on an insulating sheet (e.g. of polythene) previously charged by rubbing. When the plate is earthed by touching it with the finger and then removed, it has a large charge of opposite sign to that on the insulating sheet. This charge can be transferred to another conductor and the electrophorus plate recharged as before, again and again.

It may seem strange that the metal plate does not become charged by contact with the insulating sheet and have the same sign of charge as it. However, it appears that contact between even plane surfaces occurs at only a few points so that, in fact, the metal plate is charged by induction. This would account for it becoming oppositely charged.

(e) Static and current electricity:

Static charges produced by rubbing insulators (or insulated conductors) give the same effects when they move as do electric currents due to a battery. However, in electrostatics we are usually dealing with a small charge (a few microcoulombs) but a large p.d. (thousands of volts); in current electricity the opposite is usually true.