高级技师英语secret.docx

1、高级技师英语secretPassage 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 wir

2、e 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 i

3、s 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 nu

4、mber? 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 comp

5、uter 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 hig

6、h 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

7、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 elect

8、rical 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 th

9、e 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 recept

10、ion.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

11、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 th

12、e 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,

13、 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.

14、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 sa

15、me 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

16、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 8Vol

17、tage, 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 fl

18、owing 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 curr

19、ent 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 inversel

20、y 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 on

21、e 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 plas

22、tic 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 propor

23、tional 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 Dif

24、ference 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

25、 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 t

26、hat 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 electric

27、al 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 f

28、lows 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

29、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

30、 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 simples

31、t 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 e

32、lectrical 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 t