Читать книгу Material science. Жаратылыстану факультеттерінің студенттеріне арналған әдістемелік құрал - Шолпан Гумарова - Страница 3

The most important solid conducting materials used in electrical engineering are metals and their alloys.

Among metal conductors, one can distingush high-conductivity metals, whose resistivity at normal temperature is not over 0.1 Mflm, and high-resistivity alloys with a resistivity at normal temperature being no less than 0.3 Mfim. The first type is used for wires, cable conductors, coils of electrical machines and transformers, and the second for resistors, electric heaters, etc.

Of special interest are superconductors and cryoconductors (hyperconductor), which show extremely low resistivity at room temperatures close to absolute zero.

Liquid conductors are molten metals and various electrolytes. The melting points of most metals are high; only mercury and some special alloys can be used as liquid conductors at normal temperature.

As has been mentioned above, metals both in the liquid and solid state have a rather large amount of free electrons, which act as carriers of charge when voltage is applied to a metal conductor. Thus, the mechanism of current flow through liquid or solid metals involves the drift of free electrons under the action of an electric field produced in a metal on applying an external voltage to it. Therefore, metals are called conductors of electronic or metallic conduction, or conductors of the first kind.

Electrolytic conductors or conductors of the second kind are solutions and melts of salt, acids, and alkalis and other substances of ionic structure. The flow of current through electrolytes is associated with the electrolysis phenomenon. In the process of electrolysis, charge-carrying ions migrate in an electrolytic medium, the electrolysis products settle out on the electrodes in accordance with Faraday’s laws, and the electrolyte composition gradually changes as the current passes through it. This is not the case with metals where the current flow does not affect their mass or chemical composition.

In an electric field of rather low strength, all gases and vapors, metal vapors included, display the properties of dielectrics with very high resistivity. But as soon as the field strength exceeds a certain critical value that makes for the onset of ionization, a gas become a conductor featuring electronic and ionic conduction. A strongly ionized gas, in which the number of electrons equals the number of positive ions in a unit volume, makes a specific current-carrying medium known by the name plasma.

Exercises:

I. Memorize the following words and word combinations:

II. Find the Kazakh (Russian) equivalents to the following word combinations:

А) High-conductivity metals; high-resistivity alloys; cable conductors; coils of electrical machines; products settle out on the electrodes; exceeds a critical value.

В) токопроводящий кабель; металлы высокой проводимости; превышать критическое значение; обмоток электрических ма-шин; продукты электролиза выделяются на электродах; сплавы высокого сопротивления.

С) тоқөткізгіш шоғырсым; жоғары өткізгішті металлдар; кри-тикалық мәннен асу; электрлік машиналардың орамы; электро-лиз өнімдері электродтарда ерекшеленеді; жоғапры кедергілі құймалар.

III. Translate into English using the active vocabulary of the lesson:

IV. Translate at sight