(B) ELECTRICITY

13. Nature of Electricity.- As stated in Chapter I, all matter is composed of small particles called molecules. These molecules themselves are composed of smaller particles which have positive and negative charges of electricity. Some of the negative charges, called "electrons," are more or less free to move. Normally a molecule consists of an equal number of positive and negative charges. If one electron is taken away, the molecule is said to be positively charged; or, if a molecule has one more electron than normal, it is said to be negatively charged. Likewise, if a neutral body has one or more electrons removed from it, the entire body is said to be positively charged or, if electrons have been added, it is said to be negatively charged. If either a positively charged or a negatively charged body is brought near a neutral body (one without charge), there will be an attractive force between them; or, if a negatively charged body is brought near a positively charged body, there will be an attractive force between them. On the other hand, if a positively charged body is brought near another which is also positively charged, or, if a negatively charged body is brought near another negatively charged body, there will be a repelling force between them. In other words, bodies having like electrical charges repel each other, and bodies having unlike electrical charges attract each other. The reason that a neutral body is attracted by a charged body is that, although the neutral body is neutral within itself, it is not neutral with respect to the charged body, and the two bodies act as if oppositely charged when brought near each other.

If an electrically charged body, such as a metal sphere, is connected by a wire to a neutral body, such as the earth, a charge will flow between the neutral body and the sphere so as to equalize the charge on the two bodies. This flow of electric charges is known as an electric current. The moving charges themselves are called "electrons" and are always negative.

The flow of an electric current can be understood most readily by considering the molecules of which all matter is composed. As stated before these molecules are made up of equal positive and negative charges (protons and electrons). Some of the negative particles (electrons) are more or less free to move. The number of free electrons and the freedom with which they can move depends upon the substance. Good conductors of electricity, such as copper, have large numbers of free electrons which move with comparative ease, and the application of energy (chemical, magnetic, etc.) in the proper form will cause some of the electrons to move from one molecule to another in a direction from a point of low to a point of high "potential"-as electrical pressure is called. This flow of electrons constitutes an electric current. New electrons to take the place of those which have traveled away from the point of low potential are supplied from the source of potential (battery, generator, etc.). In other words, the source of potential acts as a pump driving electrons into the point of low potential, and pumping them out of the point of high potential.

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