物理专业外文翻译--能量转换和守恒1

外文原文：
1. Energy conversion and conservation
The conversion of mechanical energy to heat is by no means new to us. We are also familiar with other transformations 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 sunlight 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 conditions, they also transform themselves into each other.”In a word, all energies maybe converted from one form to another and what is more, they all can transform into heat by themselves. Heat is an energy of irregular motion of particles in a substance, at ordinary temperature it is less unable than any of the other energies.
However, at high temperatures 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 that 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 turn, convert the mechanical energy. All these transformations are taking place every minute and everywhere in our daily life and production.
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 were 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 be created nor destroyed, constitutes one of the most important laws in science, the law of conservation of energy. No one form of energy can be long conserved, but the total is conserved at any time. A machine may be designed to lift a much larger weight than the force that is applied, but it can never produce more work than was supplied to it. In other words, a machine cannot have an efficiency greater than one. Since man cannot create or destroyed energy, he must use the energy that is available to him.
Some devices were designed for the purpose of doing work without the need of 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 never be successful.
2.Generator and electricity
(1) Faraday and his Generator
The electric current in our homes is produced in power stations which usually 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.
What kind of engine can we use? Steam engines 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.
The water which is stored behind a dam flows out 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 country with useful current.
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 chemistry and electricity. Before his time scientists got their electric current from electric cells. Several cells together form a battery. An Italian, V olta, made the first battery and it produced a small current. Modern cells are boxes which contain acids and other materials such as metals or carbon rods. Faraday knew about V olta’s work, but he wanted to produce an electric current by using magnets.
An electric current which flows through a coil of wire round an iron rod produces magnetism 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 instruments 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 current. It was the first generator in the world.
All modern 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, 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 the turbines, it does all the necessary work, and no fuel is needed. Those countries which have big and powerful rivers are lucky because they can get
a lot of electric power from them.
(2) Direct and Alternating Currents
A direct current is, of course, useful. The electric system in a car uses the direct current. Besides, direct current is also used to meet some of he industrial requirements.
However, at present, most cities make use of another kind of electric current going first in one direction and then in another, we give it the name of an alternating current.
In spite of its being very useful a direct current system has one great disadvantage; namely, there is no easy, economical way in 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 it is possible to transform power at low voltage into power at high voltage, and vice versa.
In that manner, current can be generated at a voltage which is suitable for any given machine. In large power 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 or 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, or 380 V in factories, etc. (3) Voltage and Current
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 electric 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 be built in it. An electric field is usually set
up by applying a voltage between the two terminals of the conductor. Thus, the free electrons form an electric current in the conductor.
There are two kinds of electric currents: direct current (D.C.) and alternating current (A.C.). Direct current is 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 current 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 electric field applied is unchanged, the current produced is D.C. If the electric field applied is alternating, the current produced is A.C. Both A.C. and D.C. have their advantages and disadvantages and they are respectively used in different applications. Electric power is made at power stations, but it is usually needed far away. How is the current taken to far-off places?
Thick wires usually carry it across the country, and steel pylons hold the wires above the ground. The pylons 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 easily hold the cables up.
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 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 current is cheaper because the wires need not be thick.
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 voltage 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 volts, 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? The high voltage would drive a heavy current through our bodies to the earth.
The wires are placed high up so that nobody can touch them. 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.
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 separate the wires from the pylons, and we do this with insulators.
An insulator does not allow an electric current to flow through it; but a conductor lets it flow easily. Paper, air and glass are 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 have 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 them.
(4) Resistance
Resistance 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
cross-section, the material and the temperature of the wire.
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 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.
Now let’s study the effect of temperature on resistance. 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 conductor 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.
3.Electric equipments
(1) Electric wires
Electric wire is usually made of copper. Copper lets the electric current flow easily through it. We say that it has a low resistance. 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.
These wires carry the current to our lamps. There is a 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.
The thin 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 melts.
When the tungsten gets hot, it also gets bright. It shines and gives a good light. It also lasts a long time without breaking.
An American, Edison, invented the first small 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 cotton 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 resistance. So they did not get hot enough, and they gave no light. Edison did not stop trying; and after a lot of trouble he made his first lamp.
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 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.
Electric fires also have wires which get hot. These wires are 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.
In some houses an 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 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 might
kill himself. The water lets the current flow easily to his body. Then it can escape to the ground through his legs. The current can easily flow through his body; and it can go through his heart. Then his heart will stop beating. (2) Switches and fuses
An electric switch is often on a wall near the door of a room. Two wires lead to the lamp in the room. The switch 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.
Switches can control 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.
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 are sometimes placed in oil. Sparks do not easily jump though oil, and so the oil makes the switch safer.
A large current makes a wire hot. If the wire is very thin, even a small current makes it hot. This happens in an electric lamp.
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 one 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.
Fuses can stop this trouble. 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 “blows”. The wire is broken, and no current can flow. So the house does not catch fires go out because there is no current.
When a fuse blows, something is wrong. We must find the fault first. Perhaps 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.) If we do not repair the fault first, the new fuse will blow immediately.
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 melt; 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 may lose their lives. So it is always best to use proper fuse-wire. This will keep everyone and everything in the house safe.
(3) Autotransformers
A transformer in which the primary 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 transformer, 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, the ratio of voltage transformation is equal to the ratio of primary to secondary turns if the losses and exciting currents are neglected and Figure 11 represents an autotransformer winding with a total of 220 turns, with the sections ab and bc having 150 and 70 turns respectively. If a voltage of 440 V is applied to the winding ac, the voltage across each turn
will be 2 V. The voltage from a to b will then be 150×2,or 300 V.
When a non inductive load of 30 ohms is connected to winding ab, a current, I X, of 300/30 or 10A flows and the power output of the transformer is 300×10 or 3,00 W. Neglecting the transformer losses, the power input must be 3,000 W and the primary current 3,000/440 or 6.82 A.
An application of Kirchhoff’s current law to point a shows that when IX is 10 A and 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.
Thus the section of the winding that is common to both primary and secondary circuits carries 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 is 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 circuit power or KV A is transformed by an autotransformer, it is smaller and more efficient than a two-winding transformer of a similar rating.
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 or to a suitable switching device so that any one of several voltages may be selected.
Autotransformers are used when voltage transformations of near unity are required. Such an application of an autotransformer 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.
Autotransformers 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 appear across the secondary terminals. The requirements of safety codes should always be followed whenever autotransformers are applied.
(4) High-voltage fuses
High-voltage fuses 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 fuses and mountings are mentioned briefly in the following paragraphs.
Expulsion fuses consist of a fusible element mounted in a fuse tube and depend upon the vaporization of the fuse element and the 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 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 fuse, in which the arc is extinguished in a hole in a solid material. In one type of solid-material fuse a spring mechanism similar to that of the liquid fuse is used to separate the arcing terminals when the fuse blows. In this fuse, overload and low fault currents are interrupted in a small cylindrical chamber in the solid arc-extinguishing material, and large fault currents are interrupted in a lager chamber in the same fuse holder.
High-voltage fuses are often mounted in the 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 of a three-pole load-interrupter switch above and three solid-material fuses below.
Outdoor high-voltage fuses for low-capacity overhead lines are mounted in distribution fuse cutouts. Cutouts consist 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 housing. 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 the cutout door is closed. When the fuse link melts in clearing a short circuit, the cutout door drops open, thereby providing an indication of the blown fuse. The cutout is placed back in service by inserting a new fuse link in the fuse holder and closing the cutout door.
Fuse mountings for high interrupting-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 equipment where the system short-circuit currents are high.
(5) High-voltage Circuit Breakers
The term high-voltage circuit breaker as used here applies to circuit breakers intended 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 kV A. Breakers with even higher ratings are being developed.
During the early development of electrical systems, the vast majority of high-voltage breakers used were oil circuit breakers. However, air circuit breakers of the magnetic and compressed-air types have been developed and are now in common use.
The magnetic air circuit breaker is available in ratings up to and including 750,000 kV A at 13,800 V. In this type of breaker the current is interrupted between separable contacts in air with the aid of magnetic blowout coils. As the main current-carrying contacts part during the interruption of a fault, the arc is drawn out in a horizontal direction and transferred to arcing contacts. At the same time, the blowout coil is connected into the circuit to provide a magnetic field to draw the arc chutes. The arc accelerates upward, aided by the magnetic field and natural thermal effects, into the arc chutes where it is elongated and divided into small segments. The arc resistance increases until, as the current passes through zero, the arc is broken; after this it does not reestablish itself.
The general construction of the magnetic power circuit breaker is somewhat similar to the large air circuit breaker used on low-voltage circuits except that they are all electrically operated. These breakers are used extensively in metal-clad switchgear assemblies in industrial plants, steel mills, and power plants.
Compressed-air breakers (sometimes called air-blast breakers) depend upon a stream of compressed air directed toward the interrupting contacts of the breaker to interrupt the arc formed when current is interrupted. This type of breaker was introduced to the American market in 1940 and since that time it has become universally accepted for use in heavy-duty indoor applications. More recently, air-blast breakers have been developed for use in
extra-high-voltage outdoor stations with standard ratings up to 765,000 V.
Oil circuit-breaker contacts are immersed in oil so that the current interruption takes place under oil which by its cooling effect helps quench the arc. Since oil is an insulator, the live parts of oil circuit breakers may be placed closer together than they could be in air. The poles of small oil circuit breakers are all placed in one oil tank, but in the large high-voltage breaks each pole is in a separate oil tank. Tanks of small breakers are suspended from a framework so that the tanks may be lowered for inspection of the contacts. The tanks of very large oil circuit breakers rest directly on a foundation and have hand holes for access to the contact assembly.
The oil tanks of oil circuit breakers are usually sealed, the electrical connections between the external circuit and the contacts in the tank being made through porcelain bushings. The breaker contacts are opened and closed by means of insulated lift rods on which the movable contacts are mounted. The lift rods are connected to the operating mechanism by means of a mechanical linkage, so that the contacts of all poles of the breaker are opened and closed together.
Only the very small oil circuit breakers are manually operated. The larger oil circuit-breaker mechanisms are either pneumatically operated or spring operated. Pneumatic operators obtain the closing and tripping energy from compressed air provided by a small, automatically controlled air compressor that maintains enough compressed air for several operations of the breaker in an air receiver. Spring operators derive their energy from a spring that is compressed by a small electric motor.
Indoor oil circuit breakers are generally assembled into metal-clad switchgear units, although oil breakers are being replaced in many cases by
air circuit breakers. Oil circuit breakers are used under adverse atmospheric conditions such as in oil refineries where there is danger of explosion from any open arc. Outdoor oil circuit breakers are usually frame-mounted and are set individually on concrete footings, with open overhead connections being made to the breaker bushings. A typical frame-mounted outdoor oil circuit breaker is shown in Fig. 5-12.
(6) Protective relays
Low-voltage air circuit breakers ordinarily have self-contained series trip coils of either the instantaneous or time-delay types. The tripping energy is supplied by the flow of the short-circuit current through the trip coil. Power circuit breakers seldom use series trip coils but are equipped with trip coils designed to operate from a storage battery or a reliable source of alternating current. Auxiliary devices called protective relays, designed to detect the presence of short circuits on a system, are used to connect the breaker trip coils to the source of tripping power and thereby trip the breaker. Protective relays are said to be selective when they trip only the circuit breakers directly supplying the defective part of the system and no other circuit breakers. When relays and circuit breakers are selective, short circuits are removed from a system with a minimum of service interruption. Of course, it is also desirable to isolate the defective system element as quickly as possible. To this end, relays and circuit breakers have been developed that will clear a shot circuit in less than 0.1 s. however, selectivity being more important than speed, the tripping of some circuit breakers on a system is delayed intentionally to gain selectivity in clearing faults at certain locations on a system.
Protective-relay operating elements are connected to high-voltage circuits。