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In the beginning of the 17th century Sir William Gilbert discovered that many substances could be electrified by friction. Gilbert named this effect "electric" after the word "electron" — the Greek name for amber. In 1756 the great Russian scientist M. V. Lomonosov was the first to make theoretical analysis of electrical phenomena.

At present the nature of electrification is explained by the electron theory. According to the modern theory all matter is composed of atoms or tiny particles. There are many kinds of atoms. Each atom consists of a nucleus, a small positi­vely charged mass and a number of lighter negatively charged particles called electrons, which revolve around the nucleus. Nor­mally each atom of a substance is electrically neutral, or it has equal amounts of negative and positive charges, i.e. produces no electrical effects. If the number of negative charges is not equal to the number of positive charges, the matter will produce electrical effects.

When an electric charge is at rest it is spoken of as static electricity, but when it is in motion it is referred to as an electric current. In most cases, an electric current is described as a flow of electric charges along a conductor.

Not all substances are good conductors of electricity, as a general rule metals are good conductors of electricity, whereas nonmetals are poor conductors. The poorest of conductors are commonly called insulators or noncon­ductors. There are a large number of substances that are neither good conductors of electricity nor good insulators. These substances are called semi-conductors.

An electric current which flows in the same direction through a conductor or a current which does not change its polarity is called a direct current or a continuous current. Its abbreviation is D. C. An alternating current (A. C.) flows first in one direction and then in the other.

An electric circuit is a path through which an electric current flows. This is a complete path along which electrons can transmit their charges. An electric circuit includes a battery, generator, or magnetic means for producing current flow. Some portion of the circuit is made to do useful work.

The circuit is said to be open when no charges can move due to a break in the path. The circuit is said to be closed when no break exists — when switches are closed and all connections are properly made.

Special symbols are used to show electrical systems. There is a wide range of these symbols. There are some of them which are used when we draw circuits. Look at Fig. 5. And now look at the diagram of series and parallel arrangements (Fig. 6).


Fig. 6. The diagram of series and parallel arrangements


Date: 2015-10-21; view: 1040; Нарушение авторских прав

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