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Electrical Properties of Solids |
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A. Electrical Properties of Solids  The first artificial electrical phenomenon to be observed was the property displayed by certain resinous substances such as amber, which become negatively charged when rubbed with a piece of fur or woolen cloth and then attract small objects. Such a body has an excess of electrons. A glass rod rubbed with silk has a similar power to attract uncharged objects and attracts negatively charged objects even more strongly. The glass has a positive charge, which can be described either as a deficiency of electrons or an excess of protons. When some atoms combine to form solids, one or more electrons are often liberated and can move with ease through the material. Electrons are easily liberated in some materials, which are known as conductors. Metals, particularly copper and silver, are good conductors. Materials in which the electrons are tightly bound to the atoms are known as insulators, nonconductors, or dielectrics . Glass, rubber, and dry wood are examples of these materials. A third kind of material is a solid in which a relatively small number of electrons can be freed from their atoms in such a manner as to leave a "hole" where each electron had been. The hole, representing the absence of a negative electron, behaves as though it were positively charged. An electric field will cause both negative electrons and positive holes to move through the material, thus producing a current of electricity. Such a solid, called a semiconductor, generally has a higher resistance to the flow of current than a conductor such as copper but a lower resistance than an insulator such as glass. If most of the current is carried by the negative electrons, the semiconductor is called n-type. If most of the current is carried by the positive holes, the semiconductor is said to be p-type. If a material were a perfect conductor, a charge would pass through it without resistance, and a perfect insulator would allow no charge to be forced through it. No substance of either type is known at room temperature. The best conductors at room temperature offer a low resistance (but not zero) to the flow of current. The best insulators offer a high resistance (but not infinite) at room temperature. Most metals, however, lose all their resistance at temperatures near absolute zero; this phenomenon is called superconductivity. B. Electric Charges One quantitative tool used to demonstrate the presence of electric charges is the electroscope. This device also indicates whether the charge is negative or positive, and it determines and measures the intensity of radiation. As first used by the British physicist and chemist Michael Faraday, the device is shown in Fig. 1. The electroscope consists of two leaves of thin metal foil (a,a-) suspended from a metal support (b) inside a glass or other nonconducting container (c). A knob (d) collects the electric charges; charges, either positive or negative, are conducted along the metal support and travel to both leaves. The like charges repel one another, and the leaves fly apart, the distance between them depending roughly on the quantity of charges. Three methods may be used to charge an object electrically: (1) by contact with another object of dissimilar substance (such as contact between amber and fur), followed by separation; (2) by contact with another charged body; and (3) by induction. The effect of electrical charges on conductors and nonconductors is shown in Fig. 2. A negatively charged body, A, is between a neutral conductor, B, and a neutral nonconductor, C. The free electrons in the conductor are repelled to the side of the conductor away from A, whereas the positive charges are attracted to the nearer side. The entire body B is attracted toward A, because the attraction of the unlike charges that are close together is greater than the repulsion of the like charges that are farther apart. The forces between electrical charges vary inversely according to the square of the distance between the charges. In the nonconductor, C, the electrons are not free to move, but the atoms or molecules of the nonconductor reorient themselves so that their constituent electrons are as far as possible from A; the nonconductor is also attracted to A, but to less of an extent than the conductor. The movement of electrons in the conductor B of Fig. 2 and the reorientation of the atoms of the nonconductor C give these bodies positive charges on the sides nearest A and negative charges on the sides away from A. Charges produced in this manner are called induced charges. |
