3.2.2 Read, translate and retell. The Electron Theory

According to modern theory whole matter is composed of atoms, tiny particles that are the building blocks of the universe. There are many kinds of atoms, one or more for each chemical element. Each atom consists of a nucleus, a small, tightly packed positively charged mass, and a number of larger, lighter, negatively charged particles called electrons, which revolve about the nucleus at tremendous speed. The centripetal force necessary to draw these electrons into their circular or elliptical paths is supplied by the electrical attraction between them and the nucleus. The nucleus consists of a number of protons, each with a single positive charge, and (except for hydrogen) one or more neutrons which have no charge. Thus the positive charge on the nucleus depends upon the number of protons that it contains. This number is called the atomic number of the atom. A neutral atom contains equal numbers of electrons and protons. Each electron carries a single nega­tive charge of the same magnitude as the positive charge of a proton, so that the attraction between the nucleus of an atom and one of the electrons will depend on the number of protons in the nucleus.

An electron has a mass of 9.105X10^-28 gm. Since the mass of a proton or a neutron is about 1.840 times that of an electron, the mass of the atom is concentrated in the nucleus. The chemical properties of the atom are determined by the number of protons in the nucleus.

A solid piece of material consists of unconceivably large number of atoms clinging together. Though these atoms may be vibrating about their normal positions as a result of thermal agitation, their arrange­ment is not permanently altered by this motion. Also present in solids are numbers of free electrons, so called because they are temporarily detached from atoms. The number and freedom of motion of these electrons determine the properties of the material as a conductor of electricity. A good conductor is a material containing many free elec­trons whose motion is not greatly impeded by the atoms of which the material is composed.

As a result of the repulsive forces between them, free electrons spread throughout the material, and any concentration of them in any one region of the material will tend to be relieved by a motion of the electrons in all directions away from that region until an equilibrium distribution is again reached.

In the best conductors the outer electrons of the atoms can easily be removed, so that a free electron with an atom often causes an outer electron to leave the atom. When this happens the ejected elec­tron becomes a free electron, moving on, while its place in the atom is taken by another free electron that encounters the atom. An insula­tor or poor conductor is a substance that contains very few free elec­trons and whose atoms have no loosely held orbital electrons.

Electrification. - If a piece of sealing wax, hard rubber, or one of many other substances is brought into intimate contact with wool or cat's fur, it acquires the ability to attract light objects such as bits of cork or paper. The process of producing this condition in an object is called electrification, and the object itself is said to be electrified or charged with electricity.

There are two kinds of electrification. If two rubber rods, electri­fied by being brushed against fur, are brought near each other, they will be found to repel each other. But a glass rod rubbed with silk will attract either of the rubber rods, although two such glass rods will repel each other. The charge on the glass is evidently unlike that on the rubber. These facts suggest that objects that are similarly charged repel each other; bodies with unlike charges attract each other.

The electrification produced in a glass rod by stroking it with silk is arbitrarily called positive electrification, while that produced in the rubber rod by contact with wool is called negative electrification. It is ordinarily assumed that uncharged objects contain equal amounts of positive and negative electricity. When glass and silk are rubbed to­gether, some negative electricity is transferred from the glass to the silk, leaving the glass rod with a net positive charge, and the silk with an equal net negative charge. Similarly, hard rubber receives negative electricity from the wool with which it is in contact, causing the rod to be negatively charged and leaving the wool positive.

Though a similar explanation could be made by assuming a transfer of positive electricity, it has been shown that in solids only negative electricity is transferred.