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ELECTRICITY
What is electricity? First of all we must get acquainted with electricity itself. In an electrical dictionary written fifty years ago there is such a definition: Electricity. - The unknown thing, matter or force, or both, which is the cause of electric phenomena. Electricity is no longer an unknown thing. We know that electricity consists of particles, some of which may be made to move about and do work. We know the size of these particles, we know their mass, and we know how many of them must be moved to do any given amount of work. There is electricity in every gas, every liquid, and every solid substance which we know. All these substances are made up of different kinds of atoms in various combinations. Each atom consists of a central part, called the nucleus, around which, as in Fig. 1 whirl from one to 92 electrons. Electrons whirl around their nucleus as the Moon rotates around the Earth, and as the Earth and the other planets rotate around the Sun.
Fig. 1. Shelles in which electrons rotate.
Fig. 1. On the left is represented an atom of carbon with its central nucleus, around which rotate two electrons in an inner shell, and four more in an outer shell. Here is a simplified diagram of an aluminium atom having three shells in which are respectively two, eight and three electrons, making 13 electrons in all.
An atom is small. 250,000,000 atoms laid side by side in a row extend one inch. Yet the central nucleus of the inconceivably small atom occupies only one ten-thousandth of the diameter of the atom, and an electron is only about one-fifth the diameter of the nucleus. The only parts of the nucleus that interest us now are called protons, which are particles of positive electricity. We are much more interested in the electrons, which are particles of negative electricity. Our greater interest in the electrons arises from the fact that they can be separated from their atoms, and can be moved about and controlled. Electrons are the electricity with which we do things.
Positive and negative electricity, that is, protons and electrons have a strong attraction for each other. This attraction between the positive protons and the negative electrons holds the atom together.
ATOMS AND ELECTRONS
Now let's talk about a familiar substance - aluminium. In each atom of aluminium there are 13 negative electrons whirling around the positive nucleus. The attraction between these 13 particles of negative electricity and the positive protons in the nucleus is sufficient to hold the negative electrons in the atom under normal conditions. The positive force in the nucleus is equal to the negative forces of the 13 electrons as has been indicated in Fig. 2.
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Fig. 2. The positive forces in the nucleus, represented by plus (+) signs, exactly balance the negative forces of all the electrons, represented by minus (-) signs.
In the atoms of aluminium and of many other substances one of the negative electrons farthest from the nucleus frequently breaks away from the atom to become a free electron. Ordinarily these free electrons almost immediately enter other atoms which have lost an electron because of the attraction between the positive electricity in the atoms and the negative electricity of the free electrons.
Assume that an aluminium atom has lost one of its negative electrons, as in Fig. 3. That free electron now is negative, it is a wandering particle of negative electricity. The nucleus of the atom still has as much positive electricity as ever, it still has enough positive electricity to attract and hold 13 negative electrons, but actually there are only 12 electrons remaining in the atom.
Fig. 3. Positive Atom. The aluminium atom has lost one electron from its outer shell, leaving the atom positive while the electron becomes a free particle of negative electricity.
This atom which has lost an electron contains more positive electricity than negative electricity. So we may call it a positive atom. This positive atom has strong attraction for any free negative electrons in its vicinity and that is the reason that remaining free electrons are continually re-entering the atoms.
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