外文翻譯--能量轉(zhuǎn)換和守恒

1、<p><b>　　附錄</b></p><p>　　1. Energy conversion and conservation</p><p>　　The conversion of mechanical energy to heat is by no means new to us. We are also familiar with other tr

2、ansformations of energy. Chemical energy is converted into heat when fuel burns. Electrical energy is transformed into heat and light in electrical lamps and electrical stoves. Radiant energy turns into heat when sunligh

3、t strikes an object which absorbs it. “All contradictory things are interconnected; not only do they coexist in a single entity in given conditions, but in other given conditio</p><p>　　However, at high temp

4、eratures heat energy may be converted into energy of more usable forms. Some people have made different kinds of machines to convert heat into mechanical energy. Diesel and gasoline engines are designed to convert heat t

5、hat is developed by the burning of fuel into mechanical energy for running tractors, trucks, and cars. The mechanical energy transformed from heat in a steam turbine is made to operate generators. And the generators, in

6、turn, convert the mechanical energy. Al</p><p>　　In any energy transformation, there is some loss, but no energy is destroyed. The part that is lost is simply wasted. If all of the energies that are wasted w

7、ere added to that used, the total would be found to be equal to the total supplied. The form may be changed, but the amount remains unchanged. The fact that energy can be changed from one form to another, but can neither

8、 be created nor destroyed, constitutes one of the most important laws in science, the law of conservation of energy. No on</p><p>　　Some devices were designed for the purpose of doing work without the need o

9、f supplying energy. These are the so-called perpetual-motion machines. We say that such machines are impossible because they violate the law of conservation of energy. The attempt has never been successful. And it will n

10、ever be successful.</p><p>　　Generator and electricity</p><p>　　(1) Faraday and his Generator</p><p>　　The electric current in our homes is produced in power stations which usually

11、contain several generators. These are machines which generate electric current when they are turned. So there has to be some kind of engine to turn them.</p><p>　　What kind of engine can we use? Steam engine

12、s are suitable, and so are oil engines. Sometimes the water of a great river can turn the generators, and so power stations are often built near dams.</p><p>　　The water which is stored behind a dam flows ou

13、t with great force when it is allowed to do so. We can use this force to turn machines which are called turbines. The water is led through big pipes to the turbines, and then they turn the generators. These supply the co

14、untry with useful current.</p><p>　　Michael Faraday (1791-1867) made the first generator. He was a great scientist. He studied gases and changed some of them into liquids. He made many discoveries in chemist

15、ry and electricity. Before his time scientists got their electric current from electric cells. Several cells together form a battery. An Italian, Volta, made the first battery and it produced a small current. Modern cell

16、s are boxes which contain acids and other materials such as metals or carbon rods. Faraday knew about Volta’s</p><p>　　An electric current which flows through a coil of wire round an iron rod produces magnet

17、ism in it. Faraday wanted to do the opposite: he wanted to produce a current in a wire by using magnetism. He tried to do this for a long time, but he failed completely until he moved a wire near the magnets. Then his in

18、struments showed that a small current was flowing in the wire. Either the magnet or the wire had to move. He made a small machine to turn a coil of wire near the magnets, and this generated a </p><p>　　All m

19、odern generators depend on Faraday’s work. The magnets in them are usually electromagnets; even in an electromagnet a little magnetism remains in the iron after the current is switched off. As soon as the generator turns

20、, a small current appears. This increases the magnetism, and so the current increases. This again increases the magnetism, and so on. In a few seconds there is quite a big current flowing in the wires. If a river turns t

21、he turbines, it does all the necessary work, and no fue</p><p>　　(2) Direct and Alternating Currents</p><p>　　A direct current is, of course, useful. The electric system in a car uses the direct

22、 current. Besides, direct current is also used to meet some of he industrial requirements.</p><p>　　However, at present, most cities make use of another kind of electric current going first in one direction

23、and then in another, we give it the name of an alternating current.</p><p>　　In spite of its being very useful a direct current system has one great disadvantage; namely, there is no easy, economical way in

24、which one can increase or decrease its voltage. The alternating current does not have this disadvantage, its voltage may be increased or decreased with little energy loss by the use of a transformer. Using a transformer

25、it is possible to transform power at low voltage into power at high voltage, and vice versa.</p><p>　　In that manner, current can be generated at a voltage which is suitable for any given machine. In large p

26、ower stations, the best suited voltage is often 6,300 or 10,500 V. Power being transmitted over long distances with less loss at high voltage than at low voltage, it is more economical to increase the voltage to 35,000 o

27、r 110,000 or even 220,000 V for transmission. Wherever the power is to be used, it is lowered to the voltage which satisfies that particular purpose, such as 220 V in homes, o</p><p>　　(3) Voltage and Curren

28、t</p><p>　　All metals are good conductors because there is a great number of free electrons in them. These free electrons usually do not move in a regular way so that there is no current. However, when an el

29、ectric field is set up, all the free electrons will be made to move in one direction. And an electric current is formed. Or to say, in order that an electric current can be produced in a conductor, an electric field must

30、 be built in it. An electric field is usually set up by applying a voltage between t</p><p>　　There are two kinds of electric currents: direct current (D.C.) and alternating current (A.C.). Direct current is

31、 an electric current the charges of which move in one direction only. It is constant in value, unless the circuit conditions, such as the applied voltage or circuit resistance are changed. The changes of an alternating c

32、urrent change their direction regularly. First they flow one way, then the other. The difference between A.C. and D.C. depends upon the voltage applied. If the electri</p><p>　　Electric power is made at powe

33、r stations, but it is usually needed far away. How is the current taken to far-off places?</p><p>　　Thick wires usually carry it across the country, and steel pylons hold the wires above the ground. The pylo

34、ns are so high that nobody can touch the wires at the top. The wires are not usually copper wires; they are made of aluminum, and thirty wires together form one thick cable. Aluminum is so light that the pylons can easil

35、y hold the cables up.</p><p>　　It would not he cheap to drive very large currents through these cables. Large currents need very thick wires. If thin wires are used, they get hot or melt, and so the currents

36、 ought to be as small as possible. Can we send a lot of power if we use a small current? We can do so if the voltage is high. We need a small current and a high voltage; or a large current with a low voltage. The small c

37、urrent is cheaper because the wires need not be thick.</p><p>　　The result is that the voltage has to be very high. The pressure in the aluminum cables may be 132,000 volts, and this is terribly high. The vo

38、ltage of a small battery is usually between 1 and 9 volts. The is the kind of battery which we carry about in our pockets. A car battery has a voltage of 6 or 12 volts. In a house the pressure in the wires may be 230 vol

39、ts, or something like that. Even 230 volts is high enough to kill a person, so what would happen if we touched one of the aluminum cables</p><p>　　The wires are placed high up so that nobody can touch them.

40、When they lead down to a house or a railway, the voltage is made lower. It can be changed easily; but if the voltage is lower, the current must be higher. If it is not, we will lose power. So the wires have to be thicker

41、.</p><p>　　The wires must never tough the steel pylons. If they did that, the current would escape to the earth through the steel. Steel is a good conductor of electricity, and so are most metals. We have to

42、 separate the wires from the pylons, and we do this with insulators.</p><p>　　An insulator does not allow an electric current to flow through it; but a conductor lets it flow easily. Paper, air and glass are

43、 examples of good insulators. Another is porcelain. Porcelain is such a good insulator that it is widely used, and the aluminum cables hang down from the pylons on several separated porcelain insulators. Parts of these h

44、ave to keep dry even when it rains, because water is a good conductor. So the insulators have a special shape and the rain cannot reach all parts of th</p><p>　　(4) Resistance</p><p>　　Resistanc

45、e is the opposition to the flow of electrons. The greater the resistance of a wire is, the less electric current will pass through it under the same voltage. The resistance of a wire depends mainly on the length, the cro

46、ss-section, the material and the temperature of the wire.</p><p>　　Copper is one of the best conductors that are used in electrical engineering. A long copper wire has a larger resistance than a short copper

47、 wire with the same cross-section. If two copper wires are equal in length, the wire with a larger cross-section will show smaller resistance.</p><p>　　Now let’s study the effect of temperature on resistance

48、. Measure the resistance of a conductor when a small current is passing though it, and then measure its resistance when a large current causes it red-hot. You will find the electrons meet more resistance when the conduct

49、or is hot than when it is cold. Accordingly a conductor which has a resistance of 100 ohms at 0℃ will have a resistance of about 150 ohms at 100℃. The higher its temperature is, the more resistance it shows.</p>&

50、lt;p>　　Electric equipments</p><p>　　(1) Electric wires</p><p>　　Electric wire is usually made of copper. Copper lets the electric current flow easily through it. We say that it has a low resi

51、stance. Some other metals also have a low resistance, but copper is the most useful. There are copper wires in millions of houses in the world.</p><p>　　These wires carry the current to our lamps. There is a

52、 thin wire inside an electric lamp; you can see it if you look carefully. A thin wire has a higher resistance than a thick one. It tries to stop the flow of current. Then it gets very hot.</p><p>　　The thin

53、wire is not made of copper; it is made of tungsten. All metals melt when they get hot. (Mercury melts at a lower temperature than our usual ones.) Tungsten does not melt easily. It has to be very hot indeed before it mel

54、ts.</p><p>　　When the tungsten gets hot, it also gets bright. It shines and gives a good light. It also lasts a long time without breaking.</p><p>　　An American, Edison, invented the first small

55、 electric lamp. He wanted a thin wire for his lamp, and tried to make one; but he had a lot of trouble. Thin wires easily melt if they are made of copper. He decided to use carbon because it does not melt. He tried cotto

56、n and hundreds of other materials to make his thin piece of carbon. But at first all of them broke. They were too thin and weak. They had to be thin because they had to shine brightly. Thick pieces do not have a high res

57、istance. So the</p><p>　　Our tungsten lamps are better than the old carbon lamps. They are brighter and they last longer. The tungsten does not easily melt or break. There is not much air inside an electric

58、lamp; we have to take it out. Air contains oxygen, and the hot tungsten could burn in it. Usually we put some gas in the place of the air.</p><p>　　Electric fires also have wires which get hot. These wires a

59、re thick, but they are not made of copper. They have a high resistance. A large current flows though them and makes them hot. So we can use electric fires in winter to keep us warm.</p><p>　　In some houses a

60、n electric current also makes the water hot. This is useful when we want a bath. The wires get hot like the wires of electric fires; but we must keep them away from the water. We have to separate the wires from the water

61、 with some special material. It is not safe to let an electric wire tough water. Water has a low resistance to an electric current. Sometimes a person touches an electric wire with a wet hand; he ought not to do this. He

62、 might kill himself. The water lets the curr</p><p>　　(2) Switches and fuses</p><p>　　An electric switch is often on a wall near the door of a room. Two wires lead to the lamp in the room. The s

63、witch is fixed in one of them. The switch can cause a break in this wire, and then the light goes out. The switch can also join the two parts of the wire again; then we get a light.</p><p>　　Switches can con

64、trol many different things. Small switches control lamps and radio sets because these do not take a large current. Larger switches control electric fires. Other switches can control electric motors.</p><p>　

65、　Good switches move quickly. They have to stop the current suddenly. If they move slowly, an electric spark appears. It jumps across the space between the two ends of the wire. This is unsafe and it heats the switches ar

66、e sometimes placed in oil. Sparks do not easily jump though oil, and so the oil makes the switch safer.</p><p>　　A large current makes a wire hot. If the wire is very thin, even a small current makes it hot.

67、 This happens in an electric lamp. </p><p>　　The electric wires in a house are covered with some kind of insulation. No current can flow through the insulation; so the current can never flow straight from on

68、e wire to the other. But the insulation on old wires can tough. A large current may flow; and if this happens, the wires will get very hot. Then the house may catch fire.</p><p>　　Fuses can stop this trouble

69、. A fuse is only a thin wire which easily melts. It is fixed in a fuse-holder. The fuse-holder is made of some material which cannot burn. A large current makes the fuse hot and then it melts away. We say that the fuse “

70、blows”. The wire is broken, and no current can flow. So the house does not catch fires go out because there is no current.</p><p>　　When a fuse blows, something is wrong. We must find the fault first. Perhap

71、s two wires are touching. We must cover them with new insulation of some kind. Then we must find the blown fuse and repair it. We put a new piece of fuse-wire in the holder. (Sometimes we can find the others are cold.) I

72、f we do not repair the fault first, the new fuse will blow immediately.</p><p>　　Some people get angry when a fuse blows. So they put a thick copper wire in the fuse-holder! Of course this does not easily me

73、lt; if the current rises suddenly, nothing stops it. The thick wire easily carries it. Then the wires of the house may get very hot, and the house may catch fire. Some of the people in it may not be able to escape. They

74、may lose their lives. So it is always best to use proper fuse-wire. This will keep everyone and everything in the house safe.</p><p>　　(3) Autotransformers</p><p>　　A transformer in which the pr

75、imary and secondary windings are connected electrically as well as magnetically is called an autotransformer. Figure shows a connection diagram of an autotransformer. If this transformer is to be used as a step-down tran

76、sformer, the entire winding ac forms the primary winding and the section ab forms the secondary winding. In other words, the section ab is common to both primary and secondary. As in the standard two-winding transformer,

77、 the ratio of voltage transform</p><p>　　When a non inductive load of 30 ohms is connected to winding ab, a current, IX, of 300/30 or 10A flows and the power output of the transformer is 300×10 or 3,00

78、W. Neglecting the transformer losses, the power input must be 3,000 W and the primary current 3,000/440 or 6.82 A.</p><p>　　An application of Kirchhoff’s current law to point a shows that when IX is 10 A and

79、 IH is 6.82 A then the current form b to a must be 3.18 A. Similarly, the current from b to c must be 6.82 A.</p><p>　　Thus the section of the winding that is common to both primary and secondary circuits ca

80、rries only the difference in primary and secondary currents. In effect, the transformer in the example transforms only 3.18×300=954W rather than the full circuit power of 3000W. The percentage of power transformed i

81、s 100×954/3,000 or 31.8 percent. This is the same as the percent voltage difference between the primary and secondary voltage or (440－300)/440=0.318 or 31.8 percent. Since only a part of the circui</p><p&

82、gt;　　For some applications that require a multivoltage supply, an autotransformer in which the winding is tapped at several points is used. The connections from the various taps are brought out of the tank to terminals o

83、r to a suitable switching device so that any one of several voltages may be selected.</p><p>　　Autotransformers are used when voltage transformations of near unity are required. Such an application of an aut

84、otransformer is in “boosting” a distribution voltage common application is in the starting of ac motors, in which case the voltage applied to the motor is reduced during the starting period.</p><p>　　Autotra

85、nsformers are not safe, however, for supplying a low voltage from a high-voltage source; for, if the winding that is common to both primary and secondary should accidentally become open, the full primary voltage will app

86、ear across the secondary terminals. The requirements of safety codes should always be followed whenever autotransformers are applied.</p><p>　　(4) High-voltage fuses</p><p>　　High-voltage fuses

87、are used both indoors and outdoors for the protection of circuits and equipment with voltage ratings above 600 V. These are many types of fuses and they are mounted in many different ways. Some of the more commonly used

88、fuses and mountings are mentioned briefly in the following paragraphs.</p><p>　　Expulsion fuses consist of a fusible element mounted in a fuse tube and depend upon the vaporization of the fuse element and th

89、e fuse-tube liner to expel conducting vapors and metals from the fuse tube, thereby extinguishing the arc formed when current is interrupted. Another type of fuse, called the liquid fuse, depends on a spring mechanism to

90、 separate quickly the ends of the melted fuse element in a nonflammable liquid to extinguish the arc. Still another type of fuse is the solid-material fu</p><p>　　High-voltage fuses are often mounted in the

91、same enclosure with disconnect switches to provide short-circuit protection and switching facilities for circuits and equipment. Typical equipment of this type removed from its enclosure is shown in circuits and consists

92、 of a three-pole load-interrupter switch above and three solid-material fuses below.</p><p>　　Outdoor high-voltage fuses for low-capacity overhead lines are mounted in distribution fuse cutouts. Cutouts cons

93、ist of a fuse support and fuse holder in which the fuse link is installed. One commonly used type of fuse cutout is the drop-out enclosed cutout. In this type of cutout, the fuse holder is enclosed within a porcelain hou

94、sing. The fuse holder which contains the fuse link is mounted on the inside of the hinged enclosure door and is so arranged that it is connected into the circuit when </p><p>　　Fuse mountings for high interr

95、upting-capacity high-voltage outdoor fuses are mounted on insulators, as shown in Fig.5-11. The size of the insulators and the spacing between phases is dependent upon the protection of circuits, transformers and other e

96、quipment where the system short-circuit currents are high.</p><p>　　(5) High-voltage Circuit Breakers</p><p>　　The term high-voltage circuit breaker as used here applies to circuit breakers inte

97、nded for service on circuits with voltage ratings higher than 600 V. High-voltage circuit breakers have standard voltage ratings of from 4,160 to 765,000 V and three-phase interrupting ratings of from 50,000,000 kVA. Bre