Make an outline of the text.

LESSON SEVEN

POTENTIAL DIFFERENCE, ELECTROMOTIVE FORCE AND RESISTANCE

The amount of electrons at a point is known as the poten­tial at that point. As was previously stated, there is always a disorderly movement of free electrons within all substances, especially metals. Thus, if at one end of a conductor there is an excess of electrons and at the other end—a deficiency, the excess of electrons will flow in a disorderly manner towards the point of deficiency. It is the difference of potential that caus­es the free electrons to flow from one point of the conductor to another. Of course, the greater the difference, the greater is the flow.

Let us suppose 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 В in any direction, it would mean that both the former and the latter were at the same potential.

The electromotive force is the very force that makes the electrons move along a conductor, the letters emf standing for the above term. In effect, it is the emf that moves electri­cal charges from one point in an electric circuit towards anoth­er. In case this emf is direct, the current, too, is direct. On the other hand, were the electromotive force alternating, the current would be alternating, too.

One need not explain to the reader that a current is unable flow in a circuit consisting of metallic wires without a source of emf. The source under consideration may be a cell, a battery, a generator, a thermocouple or a photo cell, с Needless to say, the emf is measurable and it is the volt that is the unit to be used for measuring it.

In addition to electromotive force and potential difference Terence 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 reader most probably remembers that all substances offer a certain amount of opposition, that 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. Take glass and rubber as an example! They offer a very high resist­ance and, hence, are considered to be good insulators. Never­theless, one must not.forget, too, that all substances do allow the passage of some cur­rent, provided the potential difference is high enough.

In Fig. 9 two oppositely charged balls are suspended far apart in the air. In spite of our having a difference" of poten­tial, no current flows. How can we explain this strange be­haviour? 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. As a matter of fact, it is not necessary at all to connect both balls in the manner described in order to obtain similar result. All that we have to do, is to increase the charges. The potential difference becoming 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 its size, length, type and temperature. In short, the thinner or longer the wire, the greater is the offered resistance. Besides, could we use a silver wire, it would offer less resistance than an iron one. The higher the temperature of a metal, the higher is its resistance.

 

Exercises

1.Learn the following active words:

deficiency                      недостаток, дефицит

disorderly                     беспорядочный

electromotive force       электродвижущая сила

(e.m.f.)                         (ЭДС)

excess (n,a)                   избыток, избыточный

influence (v)                  влиять

offer (v)                    оказывать

potential difference       разница потенциалов

photocell                       фотогальванический элемент

resistance                      сопротивление

same                         тот же самый, одинаковый

suspend                    подвешивать      

thermocouple           термопара

             

Conditional Sentences

I	If atomic power replaces the present sources of power, we shall get more power than we produce
II	If atomic power replaced the present sources of power, we should get more power.
III	If Professor Rihman had thought of the possible danger, lightning would not have killed him

 

2. Translate the following sentences:

A. 1. If an emf is applied to a path which allows the electrons to pass, they will move toward the point of higher potential, and then a current is said to flow in the circuit.
2. If people of the past had known that lightning was atmospheric electricity, they would not have invented numerous stories. 3. If a wire is held against an electrified body, electri­city would flow along the wire to the other end. 4. If coal were not used as fuel, we should get more valuable products.

B. 1. Were that liquid heated, it would greatly expand. 2.Were there no flow of current between A. and B. in any direction, then A. and B. would be at the same potential. 3. Were the electromotive force alternating, the current would be alternating as well. 4. Should we break the circuit, no current would flow. 5. Should they use a large water pipe, more water would pass through it.

3. Translate the following sentences paying special attention to the word provided:

1. These electrical devices are provided with rubber in­sulators. 2. These electrical devices provided with rubber insulators were produced at a large factory. 3. These electrical devices can work for a long time, provided they are made of good material. 4. The electric current flows provided there is a complete circuit. 5. Lightning did not strike the house as it was provided with a lightning conductor. 6. Ohm's law pro­vided the possibility of determining resistance provided the voltage and current are known. 7. The electrons will jump through the air forming an electric spark, provided the poten­tial difference becomes great enough. 8. The students will be able to translate difficult articles, provided they have dictionaries.

4. Fill in the blanks with prepositions and translate the follow­ing text:

The great French physicist, Ampere, was an absent-mind­ed man. One day he was waiting... his friend. The appointed hour arrived, passed and his friend did not come. As Ampere had to go... he took a piece... chalk and wrote... his door: "I have gone.... I shall return... two hours." And he went away.

He returned two hours later. While he was going upstairs he worked out a very difficult problem.

"If my friend had come... the appointed hour," he said... himself, "I should have told him... this problem. I shall speak... him... it now. Perhaps he will be able to solve it." So when he came... his own door and saw the words written... it, Ampere decided that he was... his friend's door. "Oh," said he, "he has gone...! I am very sorry! Were he... home, we should discuss my problem." And he wrote the following words: "Very sorry that I have not found you... home." Then he went downstairs again.

Had Ampere not been an absent-minded man he would have never made such a mistake.

5. Complete the following sentences:

a)

1. If my friend comes to Moscow, we...

2. If I had time tomorrow, I...

3. If my friend had not entered the Institute last year, he...

4. If I were you, I...

5. If the teacher had given me the dictionary, I...

6. If my brother goes to England, he...

b)

1. Не would have done it, unless...

2. We should have gone to the country yesterday, pro­vided...

3. I should finish my work in time tomorrow unless...

4. She will answer all the teacher's questions, provided...

6. Fill in the blanks with among or between:

1. The Voltaic pile consisted of copper and zinc placed one above another, with flannel moistened in salt water... them. 2. A wire was connected... the first disc of copper and the last disc of zinc. 3. There is a great difference... conductors and insulators of electricity. 4. There are many good conductors; copper and silver are... the best ones. 5. I found your dictionary... my books. 6. The scientist de­termined the difference... the two methods.

7. Translate the following words and expres­sions into English:

обращать внимание; на основе чего-л.; электротехника; по этой причине; удовлетворять требованиям; совсем не трудно; тем не менее; наоборот; оказывать сопротивление; в таких условиях; происходить

8. Put all possible questions to the following sentences:

1. The excess electrons will flow towards the point of deficiency.

2. The parallel circuit provides two or more paths for current flow.

9. Apply suitable adjectives to the following nouns.
Model: natural phenomena

difference, spark, force, wire, factor, resistance, conductor, ball

10. Form new words of the same root, define what parts of speech they are:

active, to differ, real, ease, conductance, resist

11. Speak on:

1. Potential difference.

2. Electromotive force.

3. Resistance.

LESSON EIGHT

GENERATORS

Great generators in our power stations, which rotate at a constant speed day and night, summer and winter, are able to generate electric power only because of the electromagnets.

It is calculated that these machines produce more than 99.99 per cent of all the world's electric power.

Although used to operate certain devices requiring small currents for their operation, batteries and cells are unlikely to supply light, heat, and power on our modern large scale. It is dynamo-electric machines that are used for this purpose. These are machines by means of which mechanical power is turned directly into electric power with a loss of only a few per cent.

There are two types of dynamos, namely, the al­ternator and the generator (Fig. 11). The former pro­vides a. c. while the latter supplies d. c, similar to the current from a battery. Both of them must be turned by some outside source of me­chanical power, such as, water power, for example. 'Where cities are far from water power, the mechanical power for the generator mostly comes from burning coal, which operates a steam turbine which turns the generator. Both-generators and alternators consist of the following principal parts: an armature and an electromagnet. The for­mer generally serves as a rotor in d. с generators while it is the magnetic system that serves this purpose in alternators. The latter is usually called a stator for it is in a static condi­tion "In order to get a strong e. m. f., the rotors in large alternators rotate at a speed of thousands of revolutions per minute (г. р. т.). In this connection one could add that the speed of rotation will control the output voltage of the gen­erator in the following manner:

1. The faster the armature is rotated, the more voltage the generator will produce.

 2. The slower the rotation, the less the output voltage. Electric power stations which supply electricity over longdistances are at present always equipped with large alter­nators. These supply us with much greater quantities of cur­rent than anything ever supposed by Volta, Oersted and Ampere. In order to produce electricity under the most economical con­ditions, the generators must be as large as possible. In addition to it, they should be kept as fully loaded as possible all the time. As you are likely to remember, the Kuiby­shev and Volgograd stations have 125,000 kil­owatt generators and the Bratsk hydroelec­tric station has twenty 225,000 kilowatt generators.

 

Russian scientists are known to develop 500,000 kilo­watt generators which are to be installed at the Kras­noyarsk station.

Large installations with turbo-generators of 200,000 to 300,000 kW power are being constructed in different parts of Russia. The eight generators of the Konakovo thermo-electric station not far from Moscow are planned to have 2,400,000 kW of rated capacity.

Our industry is most likely to produce even greater power installations for thermo-electric stations which play such an important part in energy supply of our country..

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

Fig. 11 shows the principles the construction of an ele­mentary d. с generator is based upon. We see the armature, the electromagnet, the shunt winding, the commutator and the load.

Exercises

1. Learn the following active words:

alternator                 генератор переменного тока

armature                        якорь

calculate                        считать

equip                   оснащать, оборудовать

operate                работать, функционировать

revolutions per minute  обороты в минуту

rotate                   вращаться

shunt                       шунт, параллельная цепь

thermo-electric station термоэлектростанция

tur­bine                     турбина

winding                    обмотка

2. Translate the following sentences:

1.Heat developed in a transmission line or a generator is a needless waste of power. 2. An electric lamp uses the light given off by its filament heated by the current. 3. The experiments made by Oersted attracted Ampere’s attention. 4. Electric power stations supplying power on a large scale are equipped with large alternators.

3.Translate the following sentences and combine the principal and the subordinate clauses with suitable conjunctions:

1. There is a great number of engineering problems the Russian scientists work at. 2. It is quite impossible to name a scientific problem Lomonosov did not turn his attention to. 3.There are many electric appliances we cannot do without. 4.The experiment I told you about will take much time.

4. Answer the following questions:

1. Do you want to translate this article? 2. Do you want me to translate this article? 3. Do you know this scientist? 4. Do you know this scientist to have explained the new phenomenon? 5. Did you hear my report? 6. Did you hear my friend make a report? 7. Did you see my brother? 8. Did you see my brother enter the room?

5. Form adjectives using the suffixes -able, -fut, -less, -ous:

control, continue, danger, value, replace, need, power, peace, use, life

6. Form adverbs using the suffix -ly:

exact, electrical, general, gradual, negative, natural, opposite, previous, usual, easy

7. Read the following abbreviations:

а. c, d.c, r.p.m., e.m.f., 317°F, 45°C, i.e.

8. Translate the following sentences:

1. Чем сильнее магнитное поле, тем больше ток. 2. Чем больше ток, текущий по проводнику, тем выше температура проводника. 3. Чем меньше труба, тем меньше воды про­ходит через нее. 4. Чем короче проводник, тем меньше со­противление проходящему току. 5. Чем больше скорость движения между магнитным полем и проводниками, тем больше ток.

9. Define the following terms:

1. electromotive force; 2. electric circuit; 3. heating effect of an electric current; 4. magnetic effect of an electric current; 5. electromagnet; 6. generator