Electric power

Electric power is generated by converting heat, light, chemical energy, or mechanical energy into electrical energy. Most electrical energy is produced in large power stations by the conversion of mechanical energy or heat. The mechanical energy of falling water is used to drive turbine generators in hydroelectric stations, and the heat derived by burning coal, oil, or other fossil fuels is used to operate steam turbines or internal-combustion engines that drive electric generators. Also, the heat from the fissioning of uranium or plutonium is used to generate steam for the turbine generator in a nuclear power station.

Electricity generated by the conversion of light or chemical energy is used mainly for portable power sources. For example, a photoelectric cell converts the energy from light to electrical energy for operating the exposure meter in a camera, and a lead-acid battery converts chemical energy to electrical energy for starting an automobile engine.

Electrical power produced in large power stations generally is transmitted by using an alternating current that reverses direction 25, 50, or 60 times per second. The basic unit for measuring electric power is the watt – the rate at which work is being done in an electric circuit in which the current is one ampere and the electromotive force is one volt. Ratings for power plants are expressed in kilowatts (1,000 watts) or megawatts (one million watts). Electric energy consumption normally is given in kilowatt-hour – that is, the number of kilowatts used times the number of hours of use. Electricity is clean, inexpensive, and easily transmitted over long distances. Since the 1880’s, electricity has had an ever-increasing role in improving the standard of living. It now is used to operate lights, pumps, elevators, power tools, furnaces, refrigerators, air-conditioners, radios, television sets, industrial machinery, and many other kinds of equipment. It has been counted that in developed countries about 43% of the electric power is generally used for industrial purposes, 32% in homes, and 21 % in commercial enterprises.

XIV. Make the annotation.

LASERS

A device that has received a great deal of publicity is the laser (Light Amplification by Stimulated Emission of Radiation). This device produces a beam of light composed of waves that are both monochromatic (all of one wavelength) and coherent² (all in the same phase — that is, all the peaks coinciding³). These properties enable the beam to be used⁴ as a source of considerable energy at a sharply defined point, for welding, eye surgery, and similar applications. Because the beam is also extremely parallel, diverging⁵ very much less than ordinary light, it is used in space communications — a laser beam that has traveled the quarter of a million miles to the moon is still narrow enough to be useful.

The principle on which the laser works derives from an earlier device called the maser, which operates at microwave frequencies⁶ rather than optical frequencies. This principle is based on simulated emission, that is, the emission of a photon by an atom in an excited state as the result of the impact of a photon from outside of exactly equal energy. In this way the stimulating photon is augmented⁷ by the photon from the excited atom.

Thus if an atom in a substance is excited it will emit a photon to bring it back to the ground state⁸. It is stimulated (hit) by a photon containing energy, equal to the difference between the excited and ground states. If a high proportion of the atoms in a substance is pumped to an excited state there is an avalanche effect⁹. A stimulating photon from outside is doubled the first time it hits an excited atom, the two photons resulting then go on to double¹⁰ again by impacts with other excited atoms, and so on. All the photons have exactly equal energy, and are therefore associated with waves of identical wavelength.

A laser consists of a solid or gaseous active medium in which the majority of the atoms can be pumped to an excited state by exposing them to electromagnetic radiation of a different frequency to the stimulating frequency. The active medium consist of (or in the case of a gas is contained in) a transparent cylinder which acts as a resonant cavity—the stimulated waves of the same frequency making repeated passages up and down the cylinder. One end of the cylinder has a reflecting surface, and the other has a partially reflecting surface through which the laser beam emerges.

In a ruby laser, for example, the electrons in the chromium atoms of a cylindrical ruby crystal are pumped to an excited level by radiation from a flash tube, thus producing a pulsed beam. Continuous wave lasers can also be made using mixtures of inert gases.

Vocabulary notes

1. monochromatic adj — монохроматический

2. coherent adj — когерентный

3. that is, all the peaks coinciding —то есть все пики совпадают

4. enable the beam to be used — это дает возможность исполь­зовать луч

5. diverge v — расходиться (о лучах)

6. frequency n — частота

7. is augmented — (зд.) увеличивается

8. ground state — основное состояние

9. avalanche effect — «эффект лавины»

10. two photons resulting then go on to double — вследствие этого два фотона продолжают удваиваться

XV. Make the annotation.