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...

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

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

3. 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 FIFTEEN

THE 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 our 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 in case it does not carry 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, this current can be detected by the heat to be generated when 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 these wires. When the current is sent through the wire which is too thin to carry it freely, then more electric power 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 some other times we must remove or, at least, decrease it as it represents a needless waste of useful power. Thus, when heat is developed in a transmission line, a generator or a motor, it is but a useless waste of electric power and over­heating is most undesirable and even dangerous. It is this waste that is generally called "heat loss" for it serves no useful purpose 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 able to make use of 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 a hundred and one other necessary and irreplaceable things which have been serving mankind for so many years. ln 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 power into heat.

The production of heat by an electric current is called heating effect. One might also name it light effect provided the heat in the conductor be 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 its 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 devices. All of them give off a little light and a lot of heat. However, it would be quite wrong to decide that electric power is the only possible source of heat. We are provided with some other sources besides electricity. For example, we can obtain heat from chemical reactions by using chemical power and from the sun by employing radiant power. It is quite possible to produce heat by motion and we do get heat from work by making use of mechanical power. Note that in every above-mentioned case, it is work that produces heat, directly or indirectly. Thus, on the one hand, heat can do work, and on the other, work can produce heat. It is the heat engine that turns heat into work.