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HEATING EFFECT OF AN ELECTRIC CURRENT



The production of heat is perhaps the most familiar among the principal effects of an electric current, either because of its development in the filaments of the electric lamps or, may be, because of the possible danger from overloaded wires.

As you know, of course, a metal wire carrying a current will almost always be at a higher temperature than the temperature of that very wire unless it carries any current. It means that an electric current passing along a wire will

heat that wire and may even cause it to become red-hot. Thus, the current can be detected by the heat developed while it flows along the wire.

The reader is certain to remember that the heat produced per second depends both upon the resistance of the conductor and upon the amount of current carried through it. As a matter of fact, if some current flowed along a thin wire and then the same amount of current were sent through a thicker one, a different amount of heat would be developed in both wires. When the current is sent through the wire which is too thin to carry it freely, then more electric energy will be converted into heat than in the case of a thick wire conducting a small current.

Let us suppose now that a small current is flowing along a thick metal conductor. Under such conditions the only way to discover whether heat has been developed is to make use of a sensitive thermometer because the heating is too negligible to be detected by other means. If, however, our conductor were very thin while the current were large the amount of generated heat would be much greater than that produced in the thick wire. In fact, one could easily feel it. Thus, we see that the thinner the wire, the greater the developed heat. On the contrary, the larger the wire, the more negligible is the heat produced.

Needless to say, such heat is greatly desirable at times but at other times we must remove or, at least, decrease it as it represents a waste of useful energy. In case heat is developed in a transmission line, a generator or a motor, it is a waste of electric energy and overheating is undesirable and even dangerous. It is this waste that is generally called "heat loss" for it serves no useful purposes and does decrease efficiency. Nevertheless, one should not forget that the heat developed in the electric circuit is of great practical importance for heating, lighting and other purposes. Owing to it we are provided with a large number of appliances, such as: electric lamps that light our homes, streets and factories, electrical heaters that are widely used to meet industrial requirements and other necessary and irreplaceable things which have been serving mankind for so many years.

In short, many of the invaluable electrical appliances without which life would seem strange and impossible at present can be utilized only because they transform elec­tric energy into heat.

The production of heat by an electric current is called heating effect.

One might also name its light effect provided the heat in the conductor is great enough to make it white-hot, so that it gives off light as well as heat. Take the fil­ament of an electric lamp as an example. We know it to glow because of heat. By the way, were we able to look inside a hot electric iron, we should see that its wires were glowing too. A similar statement could be applied as well to almost any electric heating device. All of them give off a little light and a lot of heat.

I. Learn the following words, groups of words. Translate the sentences.

1. a number of — ряд. Last week we carried out a number of tests.

2. appliance— прибор. What appliances are used in our homes?

3. to convert— превращать, преобразовывать. Water can be converted into steam.

4. to detect— обнаруживать, открывать. The fault was detected by the measuring instruments.

5. to develop heat— выделять тепло. When the current flows along the wire heat is developed.

6. desirable— желательный. We must heat this wire to the desirable temperature.

7. efficiency— эффективность; коэффициент полезного действия. The efficiency is an important feature of any machine. The efficiency of an old turbine is low.

8. loss — потеря. It is necessary to decrease power los­ses in transmission lines.

9. negligible— незначительный. Most of the nonmetals transmit a negligible amount of current.

10. to overheat— перегревать. Overheated wires can cause fire.

11.principal— основной, главный. The atom is the prin­cipal particle of all matter.

12. to remove— удалять, устранять. Nuclear fuel is re­moved from the reactor for replacement.

13.to represent— представлять. The heat losses are represented in the diagram.

14.waste— потеря, пустая трата. Heat produced in a transmission line is a waste of energy.

15.white-hot— раскаленный добела. The current can heat a substance white-hot.

 

II. Translate the following word combinations.

a) at least, thanks to, because of, as to, in case, at times, in short, by means of, in spite of, instead of, all over the world

b) из-за выделяемого тепла, при помощи электри­ческого нагреваемого прибора, благодаря химической реакции, в случае уменьшения кпд, что касается теп­ловой потери, по крайней мере внутри лампочки, иног­да это желательно, короче говоря, во всем мире, что касается нити накала, вместо механической энергии, несмотря на разность потенциалов

 

III. Define the meaning of the prefixes in the following words, translate them.

irreplaceable, supernatural, overloaded, reaction, in­valuable, discharge, indirectly, outstanding, semiconductor, impossible

 

IV. Translate the following groups of words.

a number of appliances, waste of time, negligible los­ses, desirable effects, to convert heat into work, overheat­ed furnace потеря тепла, желательное применение, основной вопрос, увеличивать к. п. д., удалять топливо, перегре­вать провода

 

V. Complete the following sentences.

a) 1. If I asked the teacher to explain grammar to me, she ... . 2. If he had been to England last year, he ... . 3. If I asked my sister to bring me the dictionary, she ... . 4. If she asked her friend to help her, he ... . 5. If they had been asked to carry the experiment out, they ... .

b) 1. ... if the weather were fine. 2. ... unless it rains. 3. ... if we are free on Sunday. 4. ... if we had had time yesterday. 5. ... if you brought me the textbook.

 

VI.Translate the following sentences paying attention to the words in bold type.

1. The fasterthe molecules of a substance move, the higheris the temperature of the substance. 2. The largerthe water pipe, the morewater passes through it. 3. The moreyou read, the moreyou learn. 4. The shorterthe wire, the lessis its resistance to current flow. 5. The greaterthe number of free electrons in a substance, the betterthat substance conducts electricity. 6. The higherthe tem­perature of a metal, the higheris its resistance.

 

VII. Answer the following questions.

1. How can electricity be detected? 2. What are the principal effects of an electric current? 3. Why does the current-carrying wire become red-hot? 4. What does the heat produced per second depend upon? 5. Why is heat developed in a transmission line undesirable? 6. What device turns heat into work? 7. What do we call the heating effect of an electric current? 8. When does the conductor become white-hot? 9. What takes place inside any electric heating device?

 

VIII. Ask your group-mate the following questions. Let him/her answer them.

a) 1. if it is possible to convert electric energy into heat. 2. if we can obtain heat from the sun by employing radiant energy. 3. if he is able to look inside a hot electric iron. 4. if it is desirable at times to remove heat. 5. if heat de­creases efficiency.

b) 1. What the three principal effects of an electric current are. 2. How the current passing along the wire can be detected. 3. Where different electrical appliances are used. 4. When overheating is most undesirable and even dangerous.

 

IX. Speak on the heating effect of an electric current.

 

X. Make up the annotation of the text and retell it.

 

 

LESSON SEVENTEEN

Read and translate the text.

 





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