Unit 4. Electricity in motion

When an electric charge is at rest, it is spoken of as static electricity, but when it is in motion, it is referred to as an electric current. In most cases, an electric current is described as a flow of electric charges along a conductor. Such is the case, for example, in the experiment of charging an electroscope from a distant point by means of a long copper wire and a charged rubber rod. This experiment is explained by stating that electrons already in the wire are pushed along toward the electroscope by the repulsion of electrons from behind. No sooner does this current start, however, that the negative charge of the rod is dissipated and the current stops flowing.

To make an electric-current flow continuously along a wire, a continuous supply of electrons must be available at one end and a continuous supply of positive charges at the other. This is like the flow of water through a pipe; to obtain a continuous flow a continuous supply of water must be provided at one end and an opening for its escape into some receptacle at the other. The continuous supply of positive charges at one end of a wire offers a means of escape for the electrons. If this is not provided for, electrons will accumulate at the end of the wire, their repulsion back along the wire stopping the current flow.

There are two general methods by which a continuous supply of electrical charge is obtained; one being by means of a battery, and the other being by means of an electric generator. The battery is known to be a device by which chemical energy is transformed into electrical energy and the generator as a device by which mechanical energy is transformed into electrical energy.

EXERCISES I. Find in the text synonyms for:

movement, to be called, for instance, with the help of; to be similar to, to get, to provide, instrument, to change into.

2. Find in the text antonyms for:

in motion, attraction, to finish, to discharge, to accumulate,

2. Translate the following sentences, paying special attention to the different meanings of the words and expressions in italics:

1. If a continuous supply of positive charge at one end of a wire is not provided for, electrons will accumulate at the end of the wire. 2. The continuous supply of positive charge at one end of a wire provided a means of escape for the electrons. 3. Electrons will accumulate at the end of the wire provided there is no continuous supply of positive charge. 4. The laboratory provided "with the most modern equipment gives all the possibilities for studying electricity in motion. 5. In the case just considered we had to do with electricity at rest. 6. In most cases an electric current is described as a flow of electric charge along a conductor. Such is the case, for example, in the experiment of charging an electroscope from a distant point by means of a long copper wire and a charged rubber rod. 7. In case one end of the wire is made positive, by removing electrons from it, the shortage of electrons at this end will cause free electrons to be attracted toward it. 8. If, as is usually the case, the number of turns of wire in the secondary exceeds the number of turns in the primary, the average voltage of the secondary will be many times higher than the direct-current voltage impressed on the primary. 9. Atoms may have the same number of protons and a different number of neutrons in which case they are atoms of the same elements but of different weights.

2. Translate the following negative sentences;

1. No sooner does this current start, however, than the negative charge of the rod is dissipated and the current stops flowing. 2.No additional device is needed to make the voltage increase. 3. No dangerous increase of the voltage is to be expected in the test under consideration. 4. No current is drawn from the cathode when cold. 5. No material is a perfect insulator. 6. No matter how badly distorted the current and voltage may be, however, the wattmeter readings will be a correct measure of the power actually applied.

2. State the functions of the Infinitives in the following sentences.Translate these sentences into Russian:

1. To magnetize a magnetic body or to create a magnetic flux in a circuit always requires an expenditure of energy. 2. To increase the reliability of large industrial centres, the latter are fed from several power plants joined into a common network. 3. An automatic control system may be regarded as consisting of two main parts, the plant to be controlled and the controlled unit. 4. The usual procedure is to start checking at the last audio amplifier and work back. 5. One knows to a high degree of certainty that being accelerated to a sufficiently high velocity an electron may have enough kinetic energy imparted to it to knock one or mote electrons out of any material with which it comes in contact, either a metal conductor or an insulator. 6, It is mostly impossible to make some types of meters function properly at the very high frequencies used in radio work. 7. The force that causes electrons to move is called the difference of potentials or e. m. f. 8. Control systems have already been shown to differ considerably as functions of control problems and plant characteristics. 9. In a battery the determining factors are kinds of electrolytes and the kind of the metals to be used for all plates.

2. Put questions to the text.

3. Write a summary of the text.

4. Translate into Englishf

Для того, щоб змусити електричний струм рухатись безперервно вздовж провідника, необхідно створити різність потенціалів на кінцях провідника. Є два методи, за допомогою яких можна отримати різність потенціалів: за допомогою батареї або електричного генератора.Відомо, що батарея перетворює хімічну енергію в електричну. Нам відомо, що генератор перетворює механічну енергію в електричну.

2. Supplementary reading.

TEXT 2

DIFFERENCE BETWEEN A.C. AND D.C.

A direct current (D.C.) flows continuously through a conducting circuit in one direction only, although it may not be steady so far as magnitude is concerned. It is unidirectional in character. An alternating current (A.C), on the other hand, continually reverses in direction, as its name implies. Starting from zero, it grows in one direction, reaches a maximum, dies down to zero again, after which it rises in the opposite direction, reaches a maximum, again dying down to zero. It is thus continually changing in magnitude as well as direction, and this continual change causes certain effects of far reaching importance.

It can be shown that high voltages are desirable for the economic transmission of a given amount of electric power. Take, for example, the transmission of 1000 kW. If the transmission voltage is 100 volts the current must be 10,000 amperes, but if the transmission voltage is 10,000 volts the current is only 100 amperes. The cross-section of the cables transmitting the power is determined by the current to be carried, and so in the former case the cables would need to be very much larger than in the latter case. It is true that the high-voltage cable would need to have more insulation, but even so, it would be very much cheaper than the larger low- voltage cable. A high voltage is therefore essential for the economic transmission of electric power. Again, a.c. generators can be designed and built for much higher voltages than can d.c. generators, the voltage of the latter being limited by the problem of sparking at the commutator, a component which is absent in the a.c. generator. Then there is the most important factor that it is easy to transform a.c. power from one voltage to another by means of the transformer, an operation that is denied to the d.c. system. The transformer also enables the voltage to be stepped down at the receiving end of the transmission line to values which can readily be used by the various consumers. If necessary, it can be converted to the d.c. form for actual use, although this is not often necessary. There are certain processes for which D.C. is either essential or at any rate desirable but the utilization of electric power in the a.c. form is growing steadily. At the present day, by far the greater part ( значно більша частина) of the generation, transmission, and utilization of electric power is carried out by means of A.C