Vocabulary

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Т е к с т 9. Conductors and Insulators

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A free atom or molecule of any substance will exist naturally in an uncharged condition with its normal complement of electrons attracted to and bound to the atomic nucleus or nuclei. Some materials – primarily, metals – have the interesting property that in the bulk state, some of the atomic electrons are not bound to any particular atom, but are free to move around within the material. Such materials are called conductors. If for example, a number of electrons are placed on the surface of a copper sphere, these electrons, because of their mutual repulsion, will almost instantaneously distribute themselves uniformly over the surface of the sphere. Similarly, if the sphere is given a positive charge by removing some of the electrons, the mutual repulsion of the remaining electrons will cause the positions of the missing electrons to be distributed uniformly over the surface. In the first case, there is a uniform distribution of negative charge over the surface of the sphere. In the second case, there is a uniform distribution of positive charge over the surface of the sphere.

Many materials – such as glass, wood, paper, and plastics - do not possess any substantial number of free electrons. These materials are therefore nonconductors or insulators. Because electrons do not move readily in such materials, an electric charge placed on a sphere of glass, for example, will remain localized on the surface for a considerable period of time. No material is a perfect insulator, however, and eventually the charge on a piece of glass will "leak off" (i.e., it will be conducted to the surroundings) and the glass will again become electrically neutral.

Gases are generally good insulators. But if two large charges (of opposite sign) are brought close together in air, a few electrons may be ripped off the air molecules and a path between the two charges will become temporarily conductive. Electrons will then flow from the negatively charged object to the positively charged object and a spark will result. On a large scale in the atmosphere, the same process leads to a lightning stroke.

Pure water is a good insulator. The water from most sources, however, contains impurities that raise the very small conductivity of pure water to the point, that it readily conducts electricity. (It is therefore a poor policy to be in water during a thunderstorm.) On humid days, a thin film of water will collect on most surfaces and will destroy the insulating properties of most materials. Electrostatic experiments can therefore be performed much more easily on dry days than on humid days.

The Earth is a reasonably good conductor and it acts as a giant reservoir for supplying or receiving the electrons that are required to charge or discharge an object. Thus, when we wish to discharge (and render neutral) any charged object, we simply connect a wire between the object and the Earth, (if the amount of charge involved is not too great, the human body - which is a conductor, although a rather poor one - can be used as the connecting link.) Electrons flow through the wire from or to the Earth and the charge is drained from the object. In order to prevent the possibility of electric shocks, the metal shielding, that surrounds most electric devices should be connected by a wire to the Earth (i.e., the shield should be grounded).