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What are the different meanings of the following words?

current, finger, iron, power, needle, pressure

9. Make up 4 sentences using the infinitive constructions according to the models given below.

Models:

1. I heard my friend speak over the radio.

2. My teacher wants me to speak English well.

10. Speak on the work of the following scientists using the words given below:

Franklin: to prove, unlike, charge, to rub, dissimilar, object, rubber, negative, glass, positive

Volta: continuous, current, to produce, the first, unit, electric, pressure, volt, voltaic, pile

Lomonosov: to state, heat, phenomena, molecular, motion, atomic, theory, law, conservation, matter

LESSON THIRTEEN

ELECTRIC CIRCUIT

The purpose of the following article is to deal with the properties of the electric circuit. 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.

Generally speaking, the current may pass through solid conductors, liquids, gases, vacuums or any combinations of these. It may flow in turn over transmission lines from the power station, through transformers, cables and switches, through lamps, heaters, motors, and so on,—back through other switches, cables, transformers and transmission lines to the generator in the power station.

There are many kinds of circuits, such as: open circuits, closed circuits, series circuits, parallel circuits and short cir­cuits.

If the circuit is broken or, as we generally say, "opened" anywhere, the current stops everywhere. Hence, we break the circuit when we switch off our electric, devices.

The path along which electrons travel must be complete or no electric power can be supplied to the load from the source. Thus, we close the circuit when we switch on our electric lamp.

A simple electric circuit is illustrated in Fig. 7. In this figure a 4-cell battery has been used, the switch being in an open position. If the switch is in a closed position, a current will flow around the circuit in the direction shown by the arrows.

To understand the difference between the following circuit connections is not difficult at all. When electrical device are connected so that the current is not divided at any point they are said to be connected in series. Under such condition the current flow is the same in all parts of the circuit, as the is only a single path along which it may flow. Quite the opposite, the parallel circuit provides two or more paths for the passage of current. The circuit is divided in such a w« that part of the current goes through one path and pass through another.

 

Fig. 7. A simple electric circuit

We produce a short circuit or, as we sometimes call it, the "short" when we allow the current to return to the source of supply without control and without doing the work that we want it to do. In short, this phenomenon mostly results in cable faultand wire fault while under certain conditions it may even cause fires.

It is interesting to note that some substances like metals, for example, conduct electricity with ease; on the contrary, others, such as rubber, do not allow it to move freely. Thus, we obtain conductors and insulators, there being a marked difference between them, of course.

So far nothing was said about conductance, that is, the conductor's ability to pass electric charges. It appears to de­pend on four things, namely: its size, its length, the kind of material to be used, and its temperature.

It is not difficult to understand that a large water-pipe can pass much more water than a small one. We equally expect a large conductor to carry current' more readily than a thinner one. Fig. 8 illustrates this feature better than words alone! Indeed, we see that the larger the wire, the greater is its conductance because electricity meets less resistance then.

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 materials. Hence, the longer the wire, the greater is its opposition, that is resistance, to the passage of current.

Fig. 8. Comparing water flow and current flow.

There is a great difference in the conductance of various substances. For example, almost all metals are supposed to conduct current. Nevertheless, copper appears to carry the current more freely than iron, silver conducting better than copper. It is therefore said to have a greater conductance than copper. The reader probably remembers that insulators also differ in their insulating properties.

As to temperature, we are familiar with the following feature. As the temperature rises, opposition to the passage of current increases as well. Hence, conductance depends on the temperature of the wire.

It is quite wrong to think that conducting materials are the only materials to play an important part in electrical engineering. As a matter of fact, to meet our everyday power requirements, we are certain to need both conductors and insulators.

Active Words and Expressions

Cable кабель

Carry переносить

closed circuit замкнутая цепь

conductor проводник

electrical engineering электротехника

fault авария, неисправность

feature свойство, характеристика

insulator изолятор, диэлектрик

load нагрузка

open circuit разомкнутая цепь

short cir­cuit короткое замыкание

supply (v) снабжать

switch переключатель

Exercises

1. Translate the following sentences and define the functions of the infinitive:

I. The current is known to flow when the circuit is complete. 2. To stop the flow of current is to break the circuit in some point. 3. Copper and silver are considered to be the best conductors of electricity. 4. Various switches are gener­ally used to open or to close the circuits. 5. Ampere supposed the current to flow from the positive pole of the source to the negative pole. 6. It is quite possible to generate a. c. And then transform it into d. с 7. Yablochkov was the first to apply a. c. in practice. 8. A battery is the simplest device to produce direct current. 9. We know the circuit to be a path of an electric current. 10. We may expect a short circuit to result in wire fault and cable fault.

2. Answer the following questions:

1. What do we deal with in this article? 2. What is an electric circuit? 3. What kinds of circuits do you know? 4. When is the "short" produced? 5. What feature of the con­ductor is illustrated in Figure 8? 6. What does conductance depend on? 7. What do we mean by the term "short circuit?" 8. What does the term "closed circuit" mean? 9. Who deter­mined the difference between the electric current and the static charges? 10. Why does the current flow when the cir­cuit is closed?

3. Define the meaning of the following words:

circuit, electron, conductor, insulator, switch, current, conductance

4. Compare:

1. Conductors and insulators.

2. Closed circuits and open circuits.

3. Series circuits and parallel circuits.

5. Work in pairs. Put your questions and let youe fellow-students answer them.

1. if heat is a form of power.

2. if all substances are made up of molecules.

3. if the molecules are travelling in various directions in a substance,

4. if the circuit is a complete path which carries the cur­rent from the source and back to the source.

5. if all metals conduct electricity with ease.

6. if we close the circuit when we switch off the electric light.

7. if we open the circuit when we switch on the electric light.

8. if conductance depends on the temperature of the wire.

6. Translate the following sentences paying special attention to the words in bold type:

1. The faster the molecules of a substance move, the higheris the temperature of the substance. 2.The largerthe water-pipe, the morewater passes through it. 3. The more you »read, the more you learn. 4. The shorterthe wire, the lessis its resistance to current flow. 5. The greater the number of free electrons in a substance, the better that substance con­ducts electricity.

7. Fill in the blanks with the following words and expressions:
as, as well, as well as

1. It is necessary to remember the term "circuit" ... it is almost impossible to work with electricity without working with circuits. 2. Copper wires are mostly used ... conductors of electricity. 3. A "short" may cause wire fault ... cable fault. 4. In addition to travelling through solids, the electric cur­rent can flow through liquids and gases ... 5. ... a cold object and a hot one are brought into contact, the former gets warm­er and the latter gets colder. 6. ... a conductor becomes warmer, it is unable to pass electric charges as freely as it did before its temperature began to rise. 7. The magnitude of the current ... the voltage and resistance may vary from a minute amount to a very large quantity.

8. Fill in the blanks with suitable prepositions if necessary and
make up sentences with the following infinitives:

to answer ... ; to apply ... ; to be interested ... ; to con­tribute ... ; to consist ... ; to depend ... ; to invite ... ; to pay attention ... ; to play a part ... ; to go ... ; to be followed ...

9. Translate the following sentences:

1. Я не могу не ответить на этот вопрос. 2. Он не может не упомянуть об этом. 3. Мы не могли не вспомнить о пожаре, вызванном коротким замыканием. 4. Она не может, не думать о своей исследовательской работе. 5. Я не могу не знать, что короткое замыкание может вызвать поврежде­ние провода.

10. Speak on:

1. The properties of the electric circuit.

2. The conductors of electricity.

3. The insulators of electricity.

LESSON FOURTEEN

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