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 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. 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 used for measuring it.
One need not explain that a current is unable to flow in a circuit consisting of metallic wires alone. A source of an e.m.f. should be provided as well. The source under consideration may be a cell or a battery, a generator, a thermocouple or a photocell, etc.
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. The student 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, they are considered as good insulators. Nevertheless, one must not forget that all substances do allow the passage of some current provided the potential difference is high enough.
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 behaviour? The simple reason is that the air between the balls offers too great 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 a similar result. 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.
Exercises
I. Learn the following words, groups of words. Translate the sentences.
1. cell — элемент. A battery is an electrical cell.
2. electromotive force — электродвижущая сила. Electromotive force is the subject of the present article.
3. excess — избыток, излишек. How can we use the excess of generated electrical energy?
4. far apart — на расстоянии. We shall place these lamps far apart.
5. free — свободный. Free electrons move within all metals.
6. in case — в случае. The current flows in case the circuit is closed.
7. to influence — влиять. The temperature influences the conductance of metals.
8. movement — движение. The movement of free electrons constitutes the electrical current.
9. to offer resistance — оказывать сопротивление. All substances offer some resistance to the passing current.
10. potential difference — разность потенциалов. The electrical current flows if there is a potential difference.
11. provided — при условии. The resistance in electrolytes decreases provided their temperature increases.
12. reason — причина, основание. What is the reason for current leakage off the conductors?
13. size — размер. What is the size of a nuclear reactor?
14. to suspend — подвешивать. Insulator strings are suspended from the towers of transmission lines.
15. unit — единица; установка, агрегат. The ampere is the unit of current. A generator is a unit which generates electricity.
16. unless — если не. The current will not flow unless there is a potential difference in the circuit.
II. Translate the following words paying attention to prefixes.
disorder, interaction, impossible, unequal, misunderstanding, reread, unbalanced, immaterial, dissimilar, interplanetary, mispronounce, rearrange, reconstruct
III. Translate the following groups of words.
solar cell, to offer great resistance, large size, electrical units, free time, provided the distance is small
в некоторых случаях, электродвижущая сила, простой элемент, подвешивать лампы, избыток электронов, разность потенциалов, свободное движение
IV. Translate the following sentences.
a) 1. If an e.m.f. is applied to a path, the electrons will move towards the point of higher potential. 2. If people of the past had known that lightning was atmospheric electricity, they would not have invented numerous stories about it. 3. If a wire were held against an electrified body, electricity would flow along the wire to its 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 В in any direction, then A and В would be at the same potential. 3. Were the electromotive force alternating, the current would be alternating as well. 4. Should you break the circuit, no current would flow. 5. Had they used a larger water pipe for their last experiment, more water would have passed through that pipe 6. The current will flow around the circuit unless we break the circuit in some point.
V. Translate the following sentences and define the function of 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 high-quality 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 were known. 7. The electrons will jump through the air forming an electric spark provided the potential difference becomes great enough.
VI. 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 at the last lesson, I.... 6. If my brother goes to England, he....
b) 1. He 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....
VII. Translate the following sentences.
1. It was in 1800 that Volta first produced a continuous current. 2. It is the thermometer that measures the temperature of a substance. 3. It was M.V. Lomonosov who stated that heat phenomena were due to molecular motion. 4. It is the ampere that is the unit of current. 5. It is in Russia that atomic energy was first used for peaceful purposes. 6. It was A.S. Popov who was the inventor of the radio. 7. It is the difference of potential that causes the free electrons to flow from one point of the conductor to another.
VIII. For the words and expressions given in (a) find the Russian equivalents given in (b).
a) 1. in the same manner; 2. generally speaking; 3. to turn one's attention; 4. on the basis of; 5. electrical engineering; 6. for this reason; 7. to meet requirements; 8. by means of; 9. nevertheless; 10. on the contrary; 11. to offer resistance; 12. under such conditions; 13. as well as; 14. as a result; 15. in spite of; 16. under consideration
b) 1. в результате; 2. рассматриваемый; 3. при помощи; 4. в таких условиях; 5. оказывать сопротивление; 6. вообще говоря; 7. тем не менее; 8. таким же образом; 9. обращать внимание; 10. электротехника; 11. удовлетворять требованиям; 12. несмотря на; 13. на основе чего-л.; 14. наоборот; 15. также, тоже; 16. по этой причине
IX. 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.
X. Apply suitable adjectives to the following nouns.
difference, spark, force, wire, factor, resistance, conductor, ball
XI. Form new words of the same root, define what parts of speech they are.
active, differ, ease, conductance, resist, generator, turn
XII. Answer the following questions.
1. What force moves the electrons from one point to another? 2. What is an electromotive force? 3. Is the electromotive force measurable? 4. What unit is used for measuring an e.m.f.? 5. What devices may be used as a source of an e.m.f.? 6. What factors greatly influence the current flow in the circuit? 7. What does resistance depend on? 8. What materials are good insulators? 9. What factors can influence the resistance of an electrical circuit? 10. Does silver wire offer less resistance than an iron wire?
XIII. Speak on:
1. Electromotive force. 2. Resistance.
XIV. Make up the annotation of the text and retell it.
LESSON SIXTEEN
Read and translate the text.