# Лекция: DIRECT CURRENT AND ALTERNATING CURRENT

When a cell or any other generator is connected so as to form a continuous path of conductors the electrons begin to move through conductors forming an electric current or an electric circuit. This phenomenon is known to be the electricity flow through a circuit. If broken anywhere, the electric circuit will stop carrying a current. There are two main types of current: direct and alternating. When electrons flow in one direction only, the current is called a direct current. When electrons flow first in one direction and then in another in a periodic manner, the current is called an alternating current.

Alternating current flows in cycles. The number of cycles per second is accepted to be termed the frequency of current. In a 60-cycle alternating current circuit, the current flows in one direction 60 times per second and in the other direction 60 times per second.

Two frequencies are in use nowadays: the standard for Europe is 50 cycles per second, while the standard for the USA is 60 cycles per second. A standard frequency has a great advantage since different electrical systems can be interconnected.

Notes

cell — элемент

circuit — электрическая цепь

to term — называть, выражать

frequency — частота, частотность

to interconnect —связывать, объединять энергосистемы

cycle per second — герц

II. Read the text and define whether the following statements are true or false.

If broken anywhere, the electric current will continue carrying a current.

When electrons flow first in one direction and then in another in a periodic manner, the current is called a direct current.

The standard frequency for the USA is 60 cycles per second.

III. Expand the sentences.

1. There are two main types of current.

2. Alternating current flows in cycles.

3. Two frequencies are in use nowadays.

IV. Now decide which of the following statements express important ideas or supporting details.

1. a) One of the main types of current is termed alternating.

b) The electrons moving through conductors form an electric current.

c) An alternating current is a current that changes its directions of flow through a circuit in a periodic manner.

2. a) The number of cycles per second is called the frequency of current.

b) The standard frequency for Europe is 50 cycles per second.

c) There are two frequencies in use nowadays.

V. Find the words – carriers of the primary and the secondary information in the text.

VI. Define the function of commas in the text.

VII. Find out the means of connection of simple, compound and complex sentences.

IX. Arrange the sentences in the logical order according to the text.

1. In a 60-cycle alternating current circuit, the current flows first in one direction and then in another 60 times per second.

2. A standard frequency has a great advantage.

3. When electrons flow in one direction only the current is called a direct current.

IX. Give the main points of the text in 4-5 sentences.

Text C

I. Translate from English into Russian.

Laser

In the “War of Worlds” written before the turn of the century H. Wells told a fantastic story of how Martians almost invaded our Earth. Their weapon was a mysterious “sword of heat”. Today Wells’ sword of heat has come to reality in the laser. The name stands for light amplification by stimulated emission of radiation.

Laser, one of the most sophisticated inventions of man, produces an intensive beam of light of a very pure single colour. It represents the fulfilment of one of the mankind’s oldest dreams of technology to provide a light beam intensive enough to vaporize the hardest and most heat-resistant materials. It can indeed make lead run like water, or, when focused, it can vaporize any substance on earth. There is no material unamenable to laser treatment and by the end of 2000 laser will have become one of the main technological tools.

The applications of laser in industry and science are so many and so varied as to suggest magic. Scientists in many countries are working at a very interesting problem: combining the two big technological discoveries of the second half of the 20-th century – laser and thermonuclear reaction – to produce a practically limitless source of energy. Physicists of this country have developed large installations to conduct physical experiments in heating thermonuclear fuel with laser beams. There also exists an idea to use laser for solving the problem of controlled thermonuclear reaction. The laser beam must heat the fuel to the required temperature so quickly that the plasma does not have time to disintegrate. According to current estimates, the duration of the pulse has to be approximately a thousand-millionth of a second. The light capacity of this pulse would be dozen of times greater than the capacity of all the world’s power plants. To meet such demands in practice scientists and engineers must work hard as it is clear that a lot of difficulties are to be encountered on route.

The laser’s most important potential may be its use in communications. The intensity of a laser can be rapidly changed to encode very complex signals. In principle, one laser beam, vibrating a billion times faster than ordinary radio waves, could carry the radio, TV and telephone messages of the world simultaneously. In just a fraction of a second, for example, one laser beam could transmit the entire text of the Encyclopaedia Britannica.

Besides, there are projects to use lasers for long distance communication and for transmission of energy to space stations, to the surface of the Moon or to planets in the solar system. Projects have also been suggested to place lasers aboard Earth satellites nearer to the Sun in order to transform the solar radiation into laser beams, with this transformed energy subsequently transmitted to the Earth or to other space bodies. These projects have not yet been put into effect, because of the great technological difficulties to be overcome and therefore the great cost involved. But there is no doubt that in time these projects will be realized and the laser beam will begin operating in outer space as well.

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