сОДЕРЖАНИЕ:

I.English text

ELECTRIC Current………………………………..…………………………. 2

CONDUCTORS AND INSULATORS …………………………………………2-3

SEMICONDUCTORS …………………………………………………………..3-4

ELECTROMOTIVE FORCE AND RESISTANCE ……...……………………….4

Energy Conversion……………………………………………………….4-5

  Dynamos……………………………………………………………………   5-6

GENERATORS…………………………………………………………………….6

 

     II.Русский текст

ЭЛЕКТРИЧЕСКИЙ ТОК………...……………………………………………… 7

ПРОВОДНИКИ  И ИЗОЛЯТОРЫ ……………………………………………..7-8

ПОЛУПРОВОДНИКИ  …………………………………………………………8-9

ЭЛЕКТРОДВИЖУЩАЯ СИЛА И СОПРОТИВЛЕНИЕ …...………………9-10

Преобразование  Энергии……………………………………………….10

Динамо……………………………………………………………………..10-11

ГЕНЕРАТОРЫ…………………………………………………………………..11

СЛОВАРЬ………………………..………………………………………..… 12-13

СПИСОК ИСПОЛЬЗОВАННОЙ ЛИТЕРАТУРЫ……………………………..14

                                                

                           

                                       ELECTRIC Current.

The electric current is a quantity of electrons flowing in a circuit per second of time. The unit of measure for current is ampere. If one coulomb passes a point in a circuit per second then the current strength is 1 ampere.

      The electrons move along the circuit because the e .m. f. drives them. The current is directly proportional to the e. m. f.

            A stream of electrons in a circuit will develop a magnetic field around the conductor along which the electrons are moving. The strength of the magnetic field depends upon the current strength along the conductor. The direction of the field is dependent upon the direction of the current.

           If the force causing the electron flow is unidirectional, the current is called direct. If the force changes its direction of effort periodically, the current is called as alternation.

CONDUCTORS AND INSULATORS

     All substances have some ability of conducting the electric current, however, they differ greatly in the ease with which the current can pass through them. Metals, for example, conduct electricity with ease while rubber does not allow it to flow freely. Thus, we have conductors and insulators.

    What do the terms "conductors" and "insulators" mean?

Substances through which electricity is easily transmitted are called conductors. Any material that strongly resists the electric current flow is known as an insulator.

     Let us first turn our attention to conductance. The four factors conductance depends on are: the size of the wire used, its length and temperature as well as the kind of material to be employed.

    It is not difficult to understand that a large water pipe can pass more water than a small one. In the same manner, a large conductor will carry the current more readily than a thinner one. It is quite understandable, too, that to flow through a short conductor is certainly easier for the current than through a long one in spite of their being made of similar material.As mentioned above, there is a great difference in the conducting ability of various substances. For example, almost all metals are good electric current conductors. Nevertheless, copper carries the current more freely than iron; and silver, in its turn, is a better conductor than copper.

     Generally speaking, copper is the most widely used conductor. That is why the electrically operated devices in your home are connected to the wall socket by copper wires. Indeed, if you are reading this book by an electric lamp light and somebody pulls the metal wire out of the socket, the light will go out at once. The flowing electrons cannot travel through space and get into an electrically operated device when the circuit is broken. If we use a piece of string instead of a metal wire, we shall also find that the current stops flowing.

     A material like string which resists the flow of the electric current is called an insulator.

     There are many kinds of insulation used to cover the wires. The kind used depends upon the purposes the wire or cord is meant for. The insulating materials we generally use to cover the wires are rubber, asbestos, glass, plastics and others.

     Rubber covered with cotton, or rubber alone is the insulating material usually used to cover desk lamp cords and radio cords.

     Glass is the insulator to be often seen on the poles that carry the telephone wires in city streets. One of the most important insulators of all, however, is air. That is why power transmission line wires are bare wires.

     Conducting materials are by no means the only materials to play an important part in electrical engineering.

SEMICONDUCTORS

     The periodic law of elements discovered by Mendeleyev had a number of important scientific and industrial results, one of them being the discovery of germanium. Germanium is the semiconductor used in most transistors available at present.

     But what are semiconductors? They include almost all minerals, many chemical elements, a great variety of chemical compounds, alloys of metals, and a number of organic compounds. Like metals, they conduct electricity but they do it less effectively. In metals all electrons are free and in insulators they are fixed. In semiconductors electrons are fixed, too, but the connection is so weak that the heat motion of the atoms of a body easily pulls them away and sets them free.

     Minerals and crystals appear to possess some unexpected properties. For instance, it is well known that their conductivity increases with heating and falls with cooling. As a semiconductor is heated, free electrons in it increase in number, hence, its conductivity increases as well. However, heat is by no means the only phenomenon influencing semiconductors. They are sensitive to light, too. Take germanium as an example. Its electrical properties may greatly change when it is exposed to light. With the help of a ray light directed at a semiconductor, we can start or stop various machines, effect remote control, and perform lots of other useful things. Just as they are influenced by falling light, semiconductors are also influenced by all radiation.

     As previously mentioned, such dependence of conductivity on heat and light has opened up great possibilities for various uses of semiconductors. The semiconductor devices are applied for transmission of signals, for automatic control of a variety of processes, for switching on engines, for the reproduction of sound, protection of high-voltage transmission lines, speeding up of some chemical reactions, and so on

     Russian engineers and scientists turned their attention to semiconductors more than thirty years ago. They saw in them a means of solving an old engineering problem, namely, that of direct conversion of heat into electricity without boilers or machines.

 

ELECTROMOTIVE FORCE AND RESISTANCE

     As was previously stated, there is always a disorderly movement of free electrons within all substances, especially metals.

     Let us assume that there is a movement of electrons through the wire, say, from point A to point B. What does it mean? It means that there is an excess of electrons at point A. Unless there were a flow of electric current between A and B in any direction, it would mean that both the former and the latter were at the same potential. Of course, the greater the potential difference, the greater is the electron flow.

     The electromotive force (e.m.f.) is the very force that moves the electrons from one point in an electric circuit towards another. In case this e.m.f. is direct, the current is direct. On the other hand, were the electromotive force alternating, the current would be alternating, too. The e.m.f. is measurable and it is the volt that is the unit.

     In addition to the electromotive force and the potential difference reference should be made here to another important factor that greatly influences electrical flow, namely, resistance. So, to resistance shall we turn our attention now. The student probably remembers that all substances is to say resistance, to the passage of current. This resistance may be high or low depending on the type of circuit and the material employed.

     Imagine two oppositely charged balls suspended far apart in the air. In spite of our having a difference of potential, no current flows. How can we explain this strange behavior? The simple reason is that the air between the balls offers too great a resistance to current flow. However, the electrons could certainly flow from the negatively charged ball towards the positively charged one provided we connected them by a metal wire. All that we have to do is to increase the charges. If the potential difference becomes great enough, the electrons will jump through the air forming an electric spark.

     One should mention in this connection that certain factors can greatly influence the resistance of an electric circuit. Among them we find the size of the wire, its length, and type. In short, the thinner or longer the wire, the greater is the resistance offered. Besides, could we use a silver wire, it would offer less resistance than an iron one.

Energy Conversion

Since energy cannot neither be created nor destroyed, any process of producing voltage must be a conversion from one from of energy to another. There are several names for the machines that convert mechanical energy into electrical energy. The dynamo is the source of huge amounts of power. All of his work at the principle demonstrated by Faraday when he discovered that relative motion between a magnetic field and a conductor in that field would induce a current in the conductor. It makes no real difference whether the conductor is stationary and the field moving or the field is stationary and the conductor moving. The important factor is the relative motion in a manner that will cause flux to cut across the conductor.

  Dynamos

 

The term „dynamo" is applied to machines which convert either mechanical energy into electrical energy or electrical energy into mechanical energy by utilizing the principle of electromagnetic induction. A dynamo is called a generator when mechanical energy supplied in the form of rotation is converted into electrical energy. "When the energy conversion takes place in the reverse order the dynamo is called a motor.

Thus a dynamo is a reversible machine capable of operation as a generator or motor as desired.

A generator does not create electricity, but generates or produces an induced electromotive force, which causes a current to flow through a properly insulated system of electrical conductors. The amount of electricity obtainable from such a generator is dependent upon the mechanical energy supplied. In the circuit external to a generator the e.m.f. causes the electricity to flow from a higher or positive potential to a lower or negative potential, just as water flows from a higher to a lower level In the internal circuit of a generator the e.m.f. causes the current to flow from a lower potential to a higher potential just as water is pumped or forced from a lower to a higher level The action of a generator is based upon the principles of electromagnetic induction.

The dynamo consists essentially of two parts: a magnetic field, produced by electromagnets, and a number of loops or coils of wire wound upon an iron core, forming the armature. These parts аrе so arranged that the number of the magnetic lines of force of the field threading through the armature coils will be constantly varied, thereby producing a steady e.m.f. in the generator or a constant torque in the motor.

GENERATORS

     The dynamo invented by Faraday in 1831 is certainly a primitive apparatus compared with the powerful, highly efficient generators and alternators that are in use today. Nevertheless, these machines operate on the same principle as the one invented by the great English scientist. When asked what use his new invention had, Faraday asked in his turn: "What is the use of a new-born child?" As a matter of fact, "the new-born child soon became an irreplaceable device we cannot do without.

     Although used to operate certain devices requiring small currents for their operation, batteries and cells are unlikely to supply light, heat and power on a large scale. Indeed, we need electricity to light up millions of lamps, to run trains, to lift things, and to drive the machines. Batteries could not supply electricity enough to do all this work.

     That dynamo-electric machines are used for this purpose is a well-known fact. These are the machines by means of which mechanical energy is turned directly into electrical energy with a loss of only a few per cent. It is calculated that they produce more than 99.99 per cent of all the world's electric power. 

     There are two types of dynamos, namely, the generator and the alternator. To generate electricity both of them must be continuously provided with energy from some outside source of mechanical energy such as steam engines, steam turbines or water turbines, for example.

Both generators and alternators consist of the following principal parts: an armature and an electromagnet. The electromagnet of a d.c. generator is usually called a stator for it is in a static condition while the armature (the rotor) is rotating. Alternators may be divided into two types: 1. alternators that have a stationary armature and a rotating electromagnet; a 2. alternator whose armature serves as a rotor but this is seldom done. In order to get a strong e.m.f., the rotors in large machines rotate at a speed of thousands of revolutions per minute (r.p.m.). The faster they rotate, the greater the output voltage the machine will produce. In order to produce electricity under the most economical conditions, the generators must be as large as possible. 

                                 

ЭЛЕКТРИЧЕСКИЙ ТОК.

 Электрический  ток - количество электронов, текущих  в кругообороте в секунду времени.  Единица измерения для потока - ампер. Если один кулон пропускает  точку в кругообороте в секунду,  тогда текущая сила - 1 ампер.

        Электроны проходят кругооборот, потому что e .m. f. ведет их. Поток непосредственно пропорционален e. м. f.

             Поток электронов  в кругообороте разовьет магнитную  поле вокруг проводника, вперед  которому электроны двигаются.  Сила магнитной поля зависит  от текущей силы по проводнику. Направление поля зависит на направление потока.

            Если сила, причиняющая  электронный поток однонаправлена, поток называют прямым. Если сила  изменяет ее направленные усилия  периодически, поток называют как  чередование.

                                    ПРОВОДНИКИ И ИЗОЛЯТОРЫ

     Все вещества имеют немного способности  провести электрический ток, однако, они отличаются очень по непринужденности, с которой поток может пройти через них. Металлы, например, проводят электричество с непринужденностью, в то время как каучук не позволяет этому течь свободно. Таким образом, мы имеем проводников и изоляторы.

    Что такое термины, "проводники" и "изоляторы"?

Вещества  через которые электричество  легко передано, называют проводниками. Любой материал, который настоятельно сопротивляется потоку электрического тока, известен как изолятор.

     Позвольте нам сначала поворачивать наше внимание к проводимости. Эти четыре проводимости факторов зависят от: размер используемого  провода, его длины и температуры  так же как вида материала, который используется.

    Не  трудно понять, что большая водная труба может передать больше воды чем маленький. В той же самой  манере, большой проводник будет  нести поток с большей готовностью  чем более тонкий. Это весьма понятно, также, что течь через короткого проводника является конечно легче для потока чем через длинное несмотря на то, что они были сделанными из подобного материала. Как упомянуто выше, есть большое различие в способности проведения различных веществ. Например, почти все металлы - хорошие проводники электрического тока. Однако, медь несет поток более свободно чем железо; и серебро, в свою очередь, является лучшим проводником чем медь.

     Вообще  говоря, медь - наиболее широко используемый проводник. Именно поэтому электрически используемые устройства в вашем доме связаны со стенным гнездом медными проводами. Действительно, если Вы читаете эту книгу электрическим светом лампы, и кто - то тянет металлический провод из гнезда, свет выйдет сразу. Плавные электроны не могут путешествовать через место и входить в электрически используемое устройство, когда кругооборот нарушен. Если мы используем часть нитивместо металлического провода, мы также найдем, что поток прекращает течь.