Материалы представленные на сайте исключительно с целью ознакомления пользователям Интернета и не преследуют коммерческих целей или нарушение авторских прав.
© 2018 - 2024
Вариант 1
Read the text:
THE NATURE OF ELECTRICITY
The ancient Greeks knew that when a piece of amber is rubbed with wool or fur, it achieves the power of attracting light objects. Later on, the phenomenon was studied and the word electric, after the Greek word "electron" meaning amber, was used. Many scientists investigated electric phenomena, and during the nineteenth century many discoveries about the nature of electricity, and of magnetism, which is closely related to electricity, were made. It was found that if a sealing-wax rod is rubbed with a woolen cloth, and a rod of glass is rubbed with a silken cloth, an electric spark will pass between the sealing-wax rod and the glass rod when they are brought near one another. Moreover, it was found that a force of attraction operates between them. An electrified sealing-wax rod is repelled, however, by a wax rod, and also an electrified glass rod is repelled, by a similar glass rod. The ideas were developed that there are two kinds of electricity, which were called resinous electricity, and that opposite kinds of electricity attract one another, whereas similar kinds repel one another.
Memorize the pronunciation of the following words:
amber— янтарь
after the Greek word — от греческого слова
sealing-wax rod — палочка из сургуча
a rod of glass — стеклянная палочка
to attract — притягивать
Exercise 1.1.Answer the questions:
1. What does a piece of amber achieve when it is rubbed with wool or fur?
2. When was the word electric used?
3. Is the word electric after the Greek word "electron"?
4. When were many discoveries about the nature of electricity and of magnetism made?
5. When does an electric spark pass between the sealing-wax rod and the glass rod?
6. What operates between two rods?
7. What are two kinds of electricity?
Exercise 1.2. Find the following words and word combinations in the text:
1. соприкасается с шерстью или мехом
2. приобретает способность притягивать легкие предметы
3. феномен был изучен
4. слово «электрический» от греческого слова «электрон»
5. тесно связаны с электричеством
6. электрическая искра возникнет между палочкой из сургуча и стеклянной палочкой
7. возникает сила притяжения
8. наэлектризованная стеклянная палочка отталкивается от
9. противоположные виды электричества притягивают друг друга
10. одинаковые виды отталкивают
Exercise 1.3. Find the wrong statements and correct them:
1. When a piece of amber is rubbed with wool or fur, it achieves the power of repelling light objects.
2. An electric spark will pass between the glass rod and the sealing wax-rod because a force of attraction operates between them.
3. An electrified glass rod is repelled by a wax rod.
4. Similar kinds of electricity attract one another.
5. A force of attraction operates between a sealing-wax rod and a woolen cloth.
Exercise 1.4. Translate into English:
1. В 19 веке были сделаны многие открытия о природе электричества и магнетизме.
2. Обнаружили, что палочка из сургуча отталкивается от шерстяной одежды и стеклянная палочка отталкивается от одежды из шелка.
3. Наэлектризованная палочка из сургуча отталкивается от такой же палочки.
4. Существует два вида электричества, которые были названы смоляным электричеством.
Exercise 1.5. Translate the text
Вариант 2
Read the text:
NATURE OF ELECTRIC CURRENT
In the modern conception of the constitution of matter it is composed of atoms. The atom is made up of a positive nucleus surrounded by negative charges of electricity, called electrons, which revolve about the nucleus at tremendous speed. 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. The atom is neutral when it contains equal numbers of electrons and protons. A negatively charged body contains more electrons than protons. A positively charged body is one which contains fewer electrons than its normal number. When the two ends of a conductor are connected to two points at different potentials, such as the terminals of a battery, we say that there is an electric current in the conductor. What actually happens? The conductor has equal numbers of positive and negative charges in its atoms, and we want to know how the charges can be made to produce a current. The atoms in metals are packed so closely that overlap to some extent, so that it is comparatively easy for the outer electrons to pass from one atom to another if a small force is applied to them. The battery causes a potential difference between the ends of the wire, and thus provides forces that make the negative electrons in the wire move toward the point of higher potential electrons.
This electron flow toward the positive electrode is the electric current. Naturally materials differ considerably in the ease with which electrons can be made to migrate from atom to atom. The current will not flow unless there is an electric circuit. The magnitude of the current depends simply on the rate of flow of electrons along the conductor.
Вариант 3
Read the text:
WHAT IS AN ELECTRON?
1. What is an electron? We can think of the electron as a very small, indivisible, fundamental particle — a major constituent of all matter. All electrons appear to be identical and to have properties that do not change with time. Two essential characteristics of the electron are its mass and its charge. Qualitatively, we can think of an electron as a "piece of matter" that has weight and is affected by gravity. Just as the mass of any object is defined, we can define the mass of the electron by applying a force and measuring the resulting rate of change in the velocity of the electron, that is, the rapidity with which its velocity changes.
2. This rate of change is called acceleration, and the electron mass is then defined as the ratio of the applied force to the resulting acceleration. The mass of the electron is found to be about 9.11х10-28 grams.1 Not only the electron but all matter appears to have positive mass, which is equivalent to saying that a force applied to any object results in an acceleration2 in the same direction as the force. How does the other aspect, the charge of the electron, arise? If we investigate further, we find that all electrons have an electric charge, and the amount of charge, like the mass, is identical for all electrons. No one has ever succeeded in isolating an amount of charge smaller than that of the electron. The sign of the electron charge is conventionally defined as negative; the electron thus represents the fundamental unit of a negative charge.
3. No experiment has yet succeeded in removing the charge from the electron, leaving only its mass. Therefore, instead of considering the electron a "massive" body that has somehow acquired a charge, it seems more realistic to think that the charge and the mass are two inseparable aspects of a single unity. The motion of an electron, like that of any other body, results from a force acting on it. How can force be applied to an electron? One way is by gravity. Another is by bringing a second charge near the electron, thus exerting an attractive or repulsive force on it. In this case we may say that the second charge sets up electric field which applies a force to the first charge. Finally, we find that an electric current flow will affect the motion of a nearby charge, but only if that charge is already in motion. In this case, we say that the current sets up a magnetic field which applies a force to the moving charge. These three are the only known ways of applying force to an electron. The relationship between these fields, the charges producing them and resulting effects on other charges are the laws of electron motion.
Вариант 4
Read the text:
ELECTRON EMISSION
1. The electron tube depends for its action on a stream of electrons that act as current
carriers. To produce this stream of electrons, a special metal electrode (cathode) is present in every tube. But at ordinary room temperatures the free electrons in the cathode cannot leave its surface because of certain restraining forces that act as a barrier. These attractive surface forces tend to keep the electrons within the cathode substance, except for a small portion that happens to have sufficient kinetic energy (energy of motion) to break through the barrier. The majority of electrons move too slowly for this to happen.
2. To escape from the surface of the material, the electrons must perform a certain
amount of work to overcome the restraining surface forces. To do this work, the electrons must have sufficient energy imparted to them from some external source of energy, since their own kinetic energy is inadequate. There are four principal methods of obtaining electron emission from the surface of the material: thermionic emission, photoe-electric emission, field emission, and secondary emission.
3. Thermionic emission. It is the most important and one most commonly used in
electron tubes. In this method the metal is heated, resulting in increased thermal or kinetic energy of the unbound electrons. Thus, a greater number of electrons will attain sufficient speed and energy to escape from the surface of the emitter. The number of electrons released per unit area of an emitting surface is related to the absolute temperature of the cathode and quantity of the work an electron must perform when escaping from the emitting surface.
4. The thermionic emission is obtained by heating the cathode electrically. This may be produced in two ways: (1) by using the electrons emitted from the heating spiral for the conduction of current (direct heating) or (2) by arranging the heating spiral in a nickel cylinder coated with barium oxide which emits the electrons (indirect heating). Normally, the method of indirect heating is used.
5. Photoelectric emission. In this process the energy of the light radiation falling upon the metal surface is transferred to the free electrons within the metal and speeds them up sufficiently to enable them to leave the surface.
6. Field or cold-cathode emission. The application of a strong electric field (i.e. a high positive voltage outside the cathode surface) will literally pull the electrons out of the material surface, because of the attraction of the positive field. The stronger the field is, the greater the field emission from the cold emitter surface is.
7. Secondary emission. When high-speed electrons suddenly strike a metallic surface,they give up their kinetic energy to the electrons and atoms which they strike. Some of the bombarding electrons collide directly with free electrons on the metal surface and may knock them out from the surface. The electrons freed in this way are known as secondary emission electrons, since the primary electrons from some other source must be available to bombard the secondary electron-emitting surface.
Вариант 5
Read the text:
ELECTRIC CURRENT
1. Ever since Volta first produced a source of continuous current, men of science have been forming theories on this subject. For some time they could see no real difference between the newly-discovered phenomenon and the former understanding of static charges. Then the famous French scientist Ampere (after whom the unit of current was named) determined the difference between the current and the static charges. In addition to it, Ampere gave the current direction: he supposed the current to flow from the positive pole of the source round the circuit and back again to the negative pole. We consider Ampere to be right in his first statement that he was certainly wrong in the second, as to the direction of the current. The student is certain to remember that the flow of current is in a direction opposite to what he thought. Let us turn our attention now to the electric current itself. The current which flows along wires consists of moving electrons. What can we say about the electron? We know the electron to be a minute particle having an electric charge. We also know that that charge is negative. As these minute charges travel along a wire, that wire is said to carry an electric current.
2. In addition to travelling through solids, however, the electric current can flow through liquids and even through gases. In both cases it produces some most important effects to meet industrial requirements. Some liquids, such as melted metals, for example, conduct current without any change to themselves. Others, called electrolytes, are found to change greatly when the current passes through them. When the electrons flow in one direction only, the current is known to be d.c., that is, direct current. The simplest source of power for the direct current is a battery, as a battery pushes the electrons in the same direction all the time (i.e., from the negatively charged terminal to the positively charged terminal).
3. The letters a.c. stand for alternating current. The current under consideration flows first in one direction and then in the opposite one. The a.c. used for power and lighting purposes is assumed to go through 50 cycles in one second. One of the great advantages of a.c. is the ease with which power at low voltage can be changed into an almost similar amount of power at high voltage and vice versa. Hence, on the one hand, alternating voltage is increased when it is necessary for long-distance transmission and, on the other hand, one can decrease it to meet industrial requirements as well as to operate various devices at home. Although there are numerous cases when d.c. is required, at least 90 per cent of electrical energy to be generated at present is a.c. In fact, it finds wide application for lighting, heating, industrial, and some other purposes. One cannot help mentioning here that Yablochkov, Russian scientist and inventor, was the first to apply a.c. in practice.
Вариант 6
Read the text:
ELECTRIC CIRCUIT
1.The electric circuit is the subject to be dealt with in the present article. But what does the above term really mean? We know the circuit to be a complete path, which carries the current from the source of supply to the load and then carries it again from the load back to the source. The purpose of the electrical source is to produce the necessary electromotive force required for the flow of current through the circuit.
2.The path along which the electrons travel must be complete otherwise no electric power can be supplied from the source to the load. Thus we close the circuit when we switch on our electric lamp. If the circuit is broken or, as we generally say, "opened" any-where, the current is known to stop everywhere. Hence, we break the circuit when we switch off our electrical devices. Generally speaking, the current may pass through solid conductors, liquids, gases, vacuum, or any combination of these. It may flow in turn over transmission lines from the power-stations through transformers, cables and switches, through lamps, heaters, motors and so on. There are various kinds of electric circuits such as open circuits, closed circuits, series circuits, parallel circuits and short circuits.
3. To understand the difference between the following circuit connections is not difficult at all. When electrical devices are connected so that the current flows from one device to another, they are said to be connected in series. Under such conditions the current flow is the same in all parts of the circuit, as there is only a single path along which it may flow. The electrical bell circuit is considered to be a typical example of a series circuit. The parallel circuit provides two or more paths for the passage of current. The circuit is divided in such a way that part of the current flows through one path, and part through another. The lamps in your room and your house are generally connected in parallel.
4. Now we shall turn our attention to the short circuit some-times called "the short". The short circuit is produced when the current is allowed to return to the source of supply without control and without doing the work that we want it to do. The short circuit often results from cable fault or wire fault. Under certain conditions, the short may cause fire because the current flows where it was no supposed to flow. If the current flow is too great, a fuse is to be used as a safety device to stop the current flow. The fuse must be placed in every circuit where there is a danger of overloading the line. Then all the current to be sent will pass through the fuse. When a short circuit or an overload causes more current to flow than the carrying capacity of the wire, the wire becomes hot and sets fire to the insulation. If the flow of current is great than the carrying capacity of the fuse, the fuse melts and opens the circuit. A simple electric circuit is illustrated in Figure. In this figure a 4-cell battery has been used, the switch being in an open position. If the switch is in closed position, the current will flow around the circuit in the direction shown by the arrows.
Вариант 7
Read the text:
MEASURING DEVICES
Ammeters and Voltmeters. Ammeters measure the current flowing in a circuit and normally have scales, which are graduated or calibrated in amperes, milliamperes or microamperes. Voltmeters are used to measure the potential difference between two points in a circuit. The calibration of voltmeters is usually in volts, millivolts, or microvolts. The main difference between the two instruments of the same type or design is in the resistance of the operating coil; identical moving units may be used for either meter. An ammeter is connected in the positive or negative lead in series with a circuit and, therefore, must have a low resistance coil; otherwise the readings would be incorrect, as the coil would absorb an appreciable amount of power. A voltmeter is connected in parallel across the points of a circuit where the difference of potential is to be measured. The resistance of the operating coil must, in this instance, be as high as possible, to limit the amount of current consumed by it, or else a drop in potential due to the meter would occur and the pointer indication would not represent the true potential difference across the circuit. Wattmeters. The measurement of the power in a D. С circuit at any instant can be achieved by means of an ammeter and voltmeter, as the power in watts is the product of the current and the voltage. With A. С circuits, however, the instantaneous values are always changing. To measure A. C. power correctly,therefore, it is necessary to use the third instrument to measure the phase difference. The normal practice, however, is to combine these three instruments in one which will give a direct reading of power in watts. The most commonly used apparatus for measuring insulation resistance is the megohmmeter or "megger". The device is easy to handle. It consists of a hand-driven generator in a permanent magnet frame, which causes a moving coil to register the insulation resistance in ohms or megohms, the amount of which is indicated by a pointer. The "megger" is also used for continuity, ground, and shot-circuit testing in general electrical power work.
Вариант 8
Read the text:
Вариант 9
Read the text:
Вариант 10
Read the text:
CONDUCTORS AND INSULATORS
1. 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.
2. Let us first turn our attention to conductivity that is the conductor's ability of passing electric charges. The four factors, conductivity 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. Hence, the longer the wire, the greater is its opposition, that is resistance, to the passage of current. As mentioned above, there is a great difference in the conducting ability of various substances.
3. 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 me metal wire out of the socket, the light will go out at once .The electricity has not been turned off but it has no path to travel from the socket to your electric lamp. 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 metal wire, we shall also find that current stops flowing.
4. A material like string which resists the flow of the electric current is called insulator. There are many kinds of insulation used to cover the wires. The kind used depends upon the purposes the wire or a 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. Glass insulator strings are usually suspended from the towers of high voltage transmission lines. One of the most important insulators of all, however, is air. That is why power transmission line wires are bare wires depending on air to keep the current from leaking off. Conducting materials are by no means the only materials to play an important part in electrical engineering. There must certainly be a conductor that is a path, along which electricity is to travel and there must be insulators keeping it from leaking on the conductor.
Вариант 1
Read the text:
THE NATURE OF ELECTRICITY
The ancient Greeks knew that when a piece of amber is rubbed with wool or fur, it achieves the power of attracting light objects. Later on, the phenomenon was studied and the word electric, after the Greek word "electron" meaning amber, was used. Many scientists investigated electric phenomena, and during the nineteenth century many discoveries about the nature of electricity, and of magnetism, which is closely related to electricity, were made. It was found that if a sealing-wax rod is rubbed with a woolen cloth, and a rod of glass is rubbed with a silken cloth, an electric spark will pass between the sealing-wax rod and the glass rod when they are brought near one another. Moreover, it was found that a force of attraction operates between them. An electrified sealing-wax rod is repelled, however, by a wax rod, and also an electrified glass rod is repelled, by a similar glass rod. The ideas were developed that there are two kinds of electricity, which were called resinous electricity, and that opposite kinds of electricity attract one another, whereas similar kinds repel one another.
Memorize the pronunciation of the following words:
amber— янтарь
after the Greek word — от греческого слова
sealing-wax rod — палочка из сургуча
a rod of glass — стеклянная палочка
to attract — притягивать
Дата: 2019-11-01, просмотров: 370.