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 concerned. It is unidirectional in character. An alternating current (A.C.), on the other hand, continually reverses in direction. 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 in direction, and this continual change causes certain effects of far reaching importance.
It is known that high voltages are desirable for the economic transmission of a given amount of electric power. 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. 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 large low – voltage cable. A high voltage is therefore essential for the economic transmission of electric power.
There is the most important factor that it is easy to transform a.c. power from one voltage to another by means of transformer. The transformer also enables the voltage to be stepped down at the receiving end of the transmission line. 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.
III. Remember these words:
magnitude, to rise (rose, risen), opposite, step, cross-section, to determine, enable, steady, reach, amount, to receive.
IV. Translate into Russian the following word - combinations:
closed circuit, open circuit, pushbutton, voltaic pile, electric current, potential difference, positive charge, the magnitude of the current, the stream of electrons, chemical action, cross-section, unit of time, salt solution, positive charge.
V. Put the verb in brackets in the appropriate tense - Past Continuous or Past Simple.
1. We _____ (do) a Google search and _____ (find) the answer very quickly.
2. When he _____ (work) in Nigeria, there _____ (be) a serious accident on the oil rig.
3. When Trevor Baylis _____ (hear) about communication problems in Africa, he _____ (decide) to build a radio without batteries.
4. He _____ (not wear) a hard hat when the accident _____ (happen), so he hurt his head.
5. The motor _____ (run) when he _____ (remove) the guards. It was a
stupid thing to do.
6. The accident _____ (be) his own fault. He _____ (use) a grinder without wearing eye protection when it _____ (happen).
7. While the generator _____ (run), a fault ____ (develop) in the main bearing.
8. As they _____ (build) the wind farm, protestors ___ (damage) some equipment.