高级技师英语.doc

Passage 1Switches 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. Passage 2 The Computer Classroom In some schools there is a computer classroom. For example, students can do their mathematics with a computer. The computer writes questions on the screens in front of the students, and the students answer on their keyboards. This is part of a lesson with a girl: Computer: (writing on screen) Hello! Whats your name and number? Girl: (pressing buttons) Mary, 208. Computer: Hello, Mary! Look at this: 2x + 4y =8 (Two x plus four y equals eight.) 2x - 4y =0 (Two x minus four y equals nought.) Girl: (after thinking) x=1; y=2 Computer: No. Try again. Girl: x=2; y=1 Computer: Thats right. Very good, Mary! Now look The computer knows Mary. This is not their first lesson. The computer can give Mary the right lesson for her, neither too fast, nor too slow. And the computer can do this with many students at the same time. Passage 3For Almost Everything A computer can do a lot of work. It can do thousands of things at high speed. For example, the police use computers. The computers have all the information about traffic,etc. The police can look at this information at any time. If a policeman finds a car in the country and there is nobody in it or near it, a computer can help him. Whose car is it? Where does it come from? He can ask the computer these things. Is there any information in the computer about the car? Yes, somebody many miles away lost it two weeks ago.Passage 4Communications In the basic electric sense, the term“communications”refers to the sending, reception and processing of information by electrical means. Radio communication was made possible by the invention of the triode. It has subsequently become even more widespread and refined through the invention and use of the transistor, integrated circuits and other semiconductor devices. A modern communications system is first concern with the collation, processing and storage of information before its transmission. The actual transmission then follows. Finally we have reception. In long-distance communications, a transmitter is required to process the incoming information so as to make it suitable for transmission and subsequent reception.Passage 5Time Constant When a voltage is applied across the terminals of a circuit with capacitance and resistance the voltage does not appear across the capacitor instantaneously. It takes time for the plates of a capacitor to reach their full charge. The time for the capacitor to become fully charged depends on the product of circuit resistance and capacitance. This product, RC, or resistance times capacitance, is called the time constant of capacitive circuit. The RC time constant gives the time in seconds for the voltage to reach 63%(actually 63.2%) of its maximum value. The greater the time constant, the longer the time for the capacitor to reach its maximum voltage.Passage 6Resistivity The current density J in a conductor depends on the electric field E and on the nature of the conductor. In general, the dependence of J on E can be quite complex. For some materials, especially, however, it can be represented quite well by a direct proportionality. For such materials the ratio of E to J is constant. We define the resistivity of a particular material as the ratio of electric field to current density: E = JThat is, resistivity is the electric field per unit current density. A “perfect”conductor would have zero resistivity, and a “perfect”insulator would have infinite resistivity. Metals and alloys having the lowest resistivities are the best conductors.Passage 7Series Circuit If several electric components, such as resistors, are connected so that the current is the same in every one, the components are said to be in a series circuit. Consider the simple series circuit comprising the battery and three resistors. The current I result in a potential difference between the terminals of each resistor. That is,V1 =R1I, V2=R2I and V3=R3IClear, the sum of these voltage is equal to the battery emf, orV=V1 + V2 + V3The equation above states that the algebraic sum of the potential differences around any complete circuit is equal to zero. The equivalent resistance of any number of resistors connected in series equals the sum of their individual resistance.Passage 8Voltage, Resistance and Current Voltage is the potential difference in an electric circuit. The opposition given by a conductor or an insulator to the flow of electrons is called resistance. There are two kinds of current: direct current(DC) and alternating current(AC). A direct current is a current flowing in a conductor always in one direction. An alternating current is a current periodically changing its direction of flow. From Ohms law we know that the current in an electric circuit is equal to the voltage divided by the resistance. In other words, we get the voltage when multiplying the current by the resistance. Passage 9Ohms Law In 1825 Ohm made the discovery that a simple correlation exists between three of the basic electric quantities, namely: resistance, current and voltage. In simple statement, the current flowing in a circuit is directly proportional to the voltage and inversely proportional to the resistance. This statement is, generally, spoken of as Ohms law. It is a common practice to give a shorter form to Ohms law by using letters, the letter I meaning current, V voltage, and R resistance. Ohms law can, then, be written: V I = R This relationship is considered to one of the most important in all work with electricity. However, we should keep in mind that this relation is true for metallic conductors. Passage 10Conductors Just as good electrical conductors, such as the metal, are also good conductors of heat, poor electrical conductors, such as ceramic and plastic materials, are also poor thermal conductors. The free electrons in a metal carrying charge in electrical conduction also play an important role in the conduction of heat; hence we expect a correlation between electrical and thermal conductivity. The discovery that the current density J is proportional to the electric field E for a metallic conductor at constant temperature was made by G.S. Ohm(17891854) and is called Ohms law. A material obeying Ohms law is called an ohmic conductor, or a linear conductor. If Ohms law is not obeyed, the conductor is called nonlinear.Passage 11Potential Difference To describe the situation of a charge in an electric field, the quantity “potential difference ” is introduce. The potential VAB between two points A to B is defined as the ratio of the work that must be done to take a charge q from A to B to the value of q : WAB VAB= q In a uniform electric field, the potential difference between two points is the product of the field intensity and their separation in a direction parallel to that of the field. A positive potential difference means that that the energy of the charge is greater at B than at A, while negative potential difference means that its energy is less at B than at A. Passage12Direct and Alternating Current The current that flows steadily in one direction is usually called a direct current. A direct current is, of course, usefully. It is the kind of current which is always associated with batteries. We know that the electrical system in an automobile and an air plane, the telegraph, the telephone and the trolly-bus use the direct current. Direct current is also used to meet some of the industrial requirements.For industry and many other purposes, however, most cities make use of another type of electric current which flows first in one direction and then in another. It was given the name of an alternating current. We know that the alternating current is the very current that makes radio possible. Passage13The transformer One cannot call a transformer a machine, for it has no moving parts. We know the transformer to be an apparatus that is designed for changing the alternating voltages and alternating currents by means of magnetic induction without any change of frequency. One of the great advantages of the alternating current is the ease and efficiency with which power at low voltage may be transformed into an almost similar amount of power at high voltage, and vice versa. Using a transformer, it is possible to transmit the alternating current to very distant places at which the power is required. Passage14Power Supply The power supply is an essential part of every electronic equipment. In its simplest form it may consist of no more than a transformer, rectifier, and smoothing circuit, but frequently much more sophisticated arrangements are required, especially in the industrial field of computers, digital instruments, D.C. amplifier, etc. Power supply can be defined as circuits that transform electrical input power, either A.C. or D.C., into D.C., output power. This definition distinguishes power supplies from other electronic power sources which are dealt with elsewhere under the following heading: D.C.-to-A.C. inverters, D.C.-to-D.C. converters, and static inverters. The term power supply is commonly used when referring to an electronic stabilizing circuit. Passage 15 Electronic Digital Computers In general there are two types of digital computers. The first is the special-purpose digital computer, which performs a fix and preset sequence of calculations. This type of computer may be constructed more efficiently in that it can be lighter and smaller and may consume less power, etc., than the general-purpose computer. Because of the advantages in construction, small special-purpose computers are used where such factors as weight, power consumption, etc., are critical. The second type of computer is defined as a general-purpose digital. The sequence of instructions the machine follows is generally read into this type of machine and stored in the memory of the machine. Since the sequence of operations performed by the general-purpose digital computer may be easily changed, the machine possesses great flexibility, and this is the type of the machine generally used in business and for scientific computations Passage16 Capacitance The simplest capacitor consists of a pair of parallel metal plates separated by air or other insulating material. When a potential difference V is placed across the plates, each acquires a charge Q of opposite sign to the other, and an electric field E appears between them. The magnitude of E is directly proportional to the charge Q. Because the field magnitude E is also proportional to the potential difference V between the plates, the ratio between Q and V is a constant for any capacitor. The value of this ratio for a given capacitor is C, and so Q C= V The unit of capacitance is the farad, which is so large a unit that, for practical purposes, it is usually replaced by the microfarad or picofarad Passage17Effect of Frequency on Reactance For a given value of capacitance, the amount of current flowing in an AC circuit depends on the frequency of the AC voltage. The higher the frequency, the greater the current flow. The current in a capacitive circuit increases with an increase in either frequency or capacitance. Then capacitive reactance, the opposition to current flow through a capacitance, must decrease when the frequency or capacitance increases. The formula used to obtain the value of capacitance reactance is Xc=1/2FC, where Xc is capacitive reactance, F the frequency in hertz, C the capacity in farads, and 2 a constant value of 6.28. Because Xc represents opposition or resistance to current flow, it is expressed in ohms. Passage 18Magnetism At present any of us knows that in magnetic materials, the molecules themselves are tiny magnets, each having a north and a south pole. If one halves a bar magnet, it will be found that each of the two halves is a complete magnet having a north pole and a south one. In other words, we shall have simply two smaller magnets instead of one a larger size, in case our bar magnet is cut into two. Either of these two magnets could, in turn, be cut into two with the same result as before. This process could go on indefinitely, each smaller piece always being a magnet just like the original bar magnet. If it were possible to divide the magnets until we reached the molecules, we should find each molecule to be a magnet. Passage 19CablesInsulated cables have many applications in the field of electric power. Small cables are used as extension cords around offices, home , and factories. Larger cables are used for connections to machines that are movable over restricted distances. In some instances portable cables carry quite heavy electrical loads, as for example the case of electrically driven drag lines,which required several thousand horespower for operation. Overhead cables find application in distribution circuit where tree conditions or proximity to buildings and other structures make the use of open-wire lines impracticable. Underground cables are used in many situations, including major transmisson circuits between large stations. Some cable power circuits operate at 765 kilovolts and carry loads of several hundred megawatts. Cables of even higher voltage will probably become available soon. Passage 20Self-induction Inductance exists in a circuit because current through a conductor always produces magnetic field. The lines of force in this magnetic field always encircle the conductor, which carries the current, forming concentric circles around the conductor. The strength of the magnetic field depends on the amount of current flow. When the circuit current increases or decreases, the magnetic field strength increases or decreases in the same direction. As the field strength increases, the lines of force increase in number and expand outward from the center of the conductor. Similarly, when the field strength decreases due to decreased current flow, the lines of force contract toward the center of the conductor. It is actually this expansion and contraction of the magnetic field as the current varies which causes an emf of self-induction, and the effect is known as self-inductance. 短文 1开关和保险丝 一个电开关通常装在一个房间门附近的墙上。两条电线连到房间里的电灯上。开关被安装在它们当中的一个上。开关能使这种电线产生断路，然后灯熄灭。开关也能使电线的两个部分再连接起来，那么我们就得到了灯光。开关能控制许多不同的东西。小开关控制灯和收音机，因为这些开关承受不了大电流。大开关控制电炉。其它开关能控制电马达。短文 2计算机教室 在一些学校里有一个计算机教室。例如，学生们能和一台计算机做他们的数学。计算机能在学生们前面的屏幕上显示问题，而学生用他们的键盘来回答问题。 这是一个女孩课程的一部分： 计算机：（显示在屏幕上）喂！你的名字和号码是什么？ 女孩： （压按键）玛丽，208。 计算机：喂，玛丽！注意看这： 2x + 4y = 8 (2个x加上 4y等于8) 2x - 4y = 0 (2个x减去4y等于0) 女孩： （思考后）x=1; y=2 计算机：不对，再试。 女孩： x=2; y=1 计算机：对了。很好，玛丽！ 现在看计算机认识玛丽。这不是她们的第一次课。计算机给了玛丽正确的课程，既不太快，也不太慢；并且计算机在同一时间里能为许多学生做这样的事。短文 3几乎可做每件事 一台计算机能做许多工作。它能高速的做成千上万的事情。例如，警察使用计算机。计算机存有关于交通的全部信息等。警察能在任何时间看到这些信息。如果一位警察在乡村发现一辆汽车，而在它的里面或附近没有人，那么计算机能帮助他。它是谁的车？ 它从哪儿来？ 他可以问计算机这些事情。在计算机中有任何关于这辆车的信息吗？ 是的，几公里外的某人在两周前丢失了它。短文 4通 讯 在电的基本意义上，“通讯”的术语是指用电的方法发送、接收和处理信息。三极管的发明使无线电的通讯成为了可能。通过发明和使用晶体管、集成电路和其它半导体装置，它后来成为更加广泛流传和完美。 一个现代化的通讯首先是关于核对、处理和发送之前的存贮；然后跟着实际的发送；最后我们接收。在远距离的通讯中，需要一个发射机来处理进来的信息，以便它适合发射和随后的接收。短文 5时间常数 当一个电压加在电容和电阻电路的端子上时，电容上的电压不会马上出现。电容器板要达到完全充电需要时间。 电容器要成为完全充电的时间取决于电路电阻和电容的乘积。这个乘积（RC或电阻乘以电容）被称为电容性电路的时间常数。这个RC时间常数给出了电压到达它最大值63%（实际上 63.2%）时的时间（以秒计）。时间常数越大，电容器达到它的最大电压的时间就越长。短文 6电 阻 导体中的电流密度取决于电场E 和导体的特性。一般来说，J对E的依赖关系是相当复杂的。然而，有一些特殊材料它完全可以用一种正比例关系来表达。对这样的材料，E和J 的比率是一个常数。 我们把一种特殊材料的电阻定义为电场和电流密度的比： E = J即 电阻率是每单位电流密度的电场。一种“理想”的导体电阻率将是零。而一种“理想”的绝缘体的电阻率无限大。金属和合金具有很低的电阻率，是最好的导体。短文 7串联电路 如果几个电的元件（例如电阻）被连接在一起，那么在每一电阻上的电流是一样的，这些元件被称为处在串联电路中。分析一下由电池和三个电阻组成的最简单的串联电路。电流I导致了每一个电阻端子之间的电位差。即， V1 = R1I， V2=R2I，和 V3=R3I很清楚，这些电压的总和等于电池电动势，或 V=V1+V2+V3上述的等式表明任何完整电路回路的电位差的代数和等于零。任何数量的电阻串联连接的等效电阻等于它们个别电阻的总和。 短文 8电压，电阻和电流 电压是一种电路中的电位差。一种导体或一种绝缘体对电子流的阻碍被称为电阻。 有两种电流：直流电（DC）和交流电（AC）。直流电是一种在导体中总是一个方向流动的电流。交流电是一种周期性变化其流动方向的电流。根据欧姆定律，我们知道电路中的电流等于电压除以电阻，换一句说，电流乘以电阻，我们得到电压。短文 9欧姆定律 欧姆在1825年发现：在三个基本电量之间存在着一种简单的相关性。即电阻，电流和电压。简单地说，电路中流过的电流与电压成正比，而与电阻成反比。这种叙述就是通常所说的欧姆定律。 经常使用