Архитектура Аудит Военная наука Иностранные языки Медицина Металлургия Метрология
Образование Политология Производство Психология Стандартизация Технологии 


VII. Talk on the types of electric circuits.




UNIT 5

I. Read the text.

Alternating Current

Current is defined as increment of electrons. The unit for measuring current was named in honor of A.M. Ampere, the French physicist. Thus it is called ampere. The symbol for current is I. The electric current is a quantity of electrons flowing in a circuit per second of time. The electrons move along the circuit because the e.m.f. drives them. The current is directly proportional to the e.m.f.

A steam 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 dependant upon the direction of the current.

If the force causing the electron flow is indirect, the current is called direct (d.c.). If the force changes its direction periodically the current is called alternative (a.c.).

Alternating current is the current that changes direction periodically. The electrons leave one terminal of the power supply, flow out along the conductor, stop, and then flow back toward the same terminal. A voltage that caused current reverses its polarity periodically. This is properly called an alternating voltage. The power supply that provides the alternating voltage actually reverses the polarity of its terminals according to a fixed periodic pattern. A given terminal will be negative for a specific period of time and drive electrons out through the circuit. Then, the same terminal becomes positive and attracts electrons back from the circuit. This voltage source cannot be a battery. It must consist of some types of rotating machinery.

II. Guess the meaning of the following international words:

1) physicist, 2) ampere, 3) symbol, 4) second, 5) polarity, 6) period, 7) battery.

III. Translate into Russian the words and expression from the text:

1) increment of electrons; 2) measuring; 3) to drive; 4) directly proportional; 5) conductor; 6) strength; 7) causing force; 8) terminal; 9) to flow; 10) to reverse.

IV. Give the English equivalents for the words below:

1) переменный ток, 2) за секунду, 3) количество электронов, 4) поток электронов, 5) магнитное поле, 6) направление, 7) зависеть, 8) усиление, 9) источник напряжения, 10) ротационный механизм.

V. Complete the sentences using the text:

1. The electric current is …

2. The unit for measuring current is …

3. A steam of electrons in a circuit will develop …

4. The current is called direct if …

5. The current is called alternating if…

6. Alternating voltage is …

7. Alternating voltage source cannot be …

VI. Answer the questions:

1. Why do electrons move along the circuit?

2. What does the strength of the magnetic field depend upon?

3. What does the direction of the field depend upon?

4. What is the way of alternating current electrons?

5. How does the alternating voltage power supply reverse the polarity of terminals?

VII. Talk on the properties of the electric current and its types.

UNIT 6

I. Read the text.

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. Solid metals conduct electricity with ease while non-metals do not allow it to flow freely. Thus, there are conductors and insulators.

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

This difference is expressed by what is called electrical conductivity of the body. It depends upon the atomic constitution of the body. Substances through which electricity is easily transmitted are called conductors. Any material that strongly resists the electric current flow is known as an insulator.

Conductance, that is the conductor's ability of passing electric charges, depends on the four factors: thesize of the wire used, its length and temperature as well as the kind of material to be employed.

A large conductor will carry the current more readily than a thinner one. 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.

There is a great difference in the conducting ability of various substances. Almost all metals are good electric current conductors. The best conductors are silver, copper, gold and aluminum. Nevertheless, copper carries the current more freely than iron; and silver, in its turn, is a better conductor than copper. Copper is the most widely used conductor. The electrically operated devices are connected to the wall socket by copper wires.

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

The higher the opposition is, the better the insulator is. 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 generally used to cover the wires are rubber, asbestos, glass, plastics and others. The best insulators are oil, rubber and glass.

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 off the conductor.

II. Give the Russian equivalents for the words and word combinations below:

1) conductors; 2) insulators; 3) transmit; 4) resistance; 5) passage of current; 6) socket; 7) to connect to; 8) cord; 9) high voltage transmission line; 10) leak off.

III. Find in the text the sentences with the following related words and translate them:

conducting – conductor – conductivity – conductance.

IV. Make up sentences corresponding to the information given in the text

Copper is used to cover desk lamp cords
Silver one of the most important insulators of all
Rubber the most widely used conductor
Glass a better conductor than copper
Iron not so good conductor as copper
Air the insulator usually used on the city street poles and high voltage transmission lines

V. State questions to the underlined words:

1) Solid metals conduct electricity with ease.

2) Conductance depends on the four factors.

3) There are many kinds of insulation used to cover the wires.

4) Insulators keep electricity from leaking off the conductor.

5) Conductors play an important role in electrical engineering.

VI. Say whether these sentences are true or false:

1) Electrical conductivity of a body depends upon its atomic constitution.

2) There is no difference in the conducting ability of various substances.

3) The longer the wire is the weaker its opposition is.

4) The kind of the insulating material depends upon the purpose it is meant for.

5) Conductors are substances through which electricity is easily transmitted.

6) Insulators do not allow the electric current to flow freely.

VII. Talk on the conducting ability of various substances and their application in electrical engineering. Use the table in Task IV.

UNIT 7

I. Read the text.

SEMICONDUCTORS

There are materials that really occupy a place between the conductors of the electric current and the non-conductors. They are called semiconductors. These materials conduct electricity less readily than conductors but much better than insulators.

Semiconductors 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. 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.

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 of 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. Generally speaking, they are so sensitive that a heated object can be detected by its radiation.

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. On the one hand they may be used to transform light and heat energy directly into electric energy without any complex mechanism with moving parts, and on the other hand, they are capable of generating heat or cold from electricity.

Russian engineers and scientists turned their attention to semiconductors many 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. Semiconductor thermocouples created in Russia convert heat directly into electricity just as a complex system consisting of a steam boiler, a steam engine and a generator does it.

II. Give the English equivalents for the words and word combinations below:

1) полупроводник; 2) химическое соединение; 3) сплав; 4) освобождать; 5) свойство; 6) увеличивать(ся); 7) охлаждение; 8) чувствительный к; 9) выставлять; 10) луч; 11) направлять на; 12) дистанционное управление; 13) находить, обнаруживать; 14) защита; 15) ускорение; 16) решить инженерную проблему; 17) термоэлемент.

III. Guess the meaning of the following international words:

element, organic, mineral, crystal, phenomenon, automatic, control, process, reproduction, conversion, boiler.

IV. Join the beginnings and ends:

Semiconductors are sensitive to… … conductors of the electric current and non-conductors.
Semiconductors convert heat into … … dependence of conductivity on heat and light.
Semiconductors occupy a place between … … heat and light.
Semiconductors conduct electricity … …into electricity without machines.
Great possibilities for various uses of semiconductors are connected with … … less effectively than metals.
As a semiconductor is heated … … its conductivity increases as well.

V. Insert words and expressions:

1) Semiconductors include a great variety of (химические соединения), (сплавы металлов).

2) Minerals and crystals appear to possess some unexpected (свойства). Their conductivity increases with (нагревание) and falls with (охлаждение).

3) With the help of a ray of light directed at a semiconductor, we can effect (дистанционное управление).

4) The semiconductor devices are applied for (автоматический контроль) of a variety of processes, for the (воспроизведение) of sound, (ускорение) of some chemical reactions.

5) (Термоэлементы) created in Russia convert heat directly into electricity.

VI. Answer the questions:

1) What do semiconductors include? 2) How does the atomic structure of semiconductors influence their properties? 3) What phenomena influence semiconductors? 4) What are the semiconductor devices applied for? 5) How do semoconductors help in solving engineering problems?





Рекомендуемые страницы:


Читайте также:

Последнее изменение этой страницы: 2016-03-25; Просмотров: 1480; Нарушение авторского права страницы


lektsia.com 2007 - 2020 год. Все материалы представленные на сайте исключительно с целью ознакомления читателями и не преследуют коммерческих целей или нарушение авторских прав! (0.009 с.) Главная | Обратная связь