电气工程及其自动化专业英语第一章课文翻译

1、第一章第一篇 sectiongTwo variables u(t and i(t are the most basic concepts in an electric circuit, they characterize the various relationships in an electric circuitu(t和 i(t这两个变量是电路中最基本的两个变量,它们刻划了电路的各种关 系。Charge and CurrentThe concept of electric charge is the underlying principle for explaining all elect

2、rical phenomena. Also, the most basic quantity in an electric circuit is the electric charge. Charge is an electrical property of the atomic particles of which matter consists, measured in coulombs (C.电荷和电流电荷的概念是用来解释所有电气现象的基本概念。也即,电路中最基本 的量是电荷。 电荷是构成物质的原子微粒的电气属性, 它是以库仑为单位来度量 的。We know from elementar

3、y physics that all matter is made of fundamental building blocks known as atoms and that each atom consists of electrons, protons, and neutrons. We also know that the charge e on an electron is negative and equal in magnitude to 1.60210×10 19C, while a proton carries a positive charge of the sa

4、me magnitude as the electron. The presence of equal numbers of protons and electrons leaves an atom neutrally charged. 我们从基础物理得知一切物质是由被称为原子的基本构造部分组成的, 并且每个原子是由电子, 质子和中子组成的。 我们还知道电子的电量是负的并且 在数值上等于 1.602100×10-12C ,而质子所带的正电量在数值上与电子相等。 质子和电子数量相同使得原子呈现电中性。We consider the flow of electric charges. A

5、 unique feature of electric charge or electricity is the fact that it is mobile; that is, it can be transferred from one place to another, where it can be converted to another form of energy让我们来考虑一下电荷的流动。 电荷或电的特性是其运动的特性, 也就是, 它可 以从一个地方被移送到另一个地方,在此它可以被转换成另外一种形式的能量。 When a conducting wire is connected

6、 to a battery (a source of electromotive force, the charges are compelled to move; positive charges move in one direction while negative charges move in the opposite direction. This motion of charges creates electric current. It is conventional to take the currentflow as the movement of positive cha

7、rges, that is, opposite to the flow of negative charges, as Fig.l-1 illustrates. This convention was introduced by Benjamin Franklin (l706l790, the American scientist and inventor. Although we now know that current in metallic conductors is due to negatively charged electrons, we will follow the uni

8、versally accepted convention that current is the net flow of positive charges. Thus, Electric current is the time rate of charge, measured in amperes (A. Mathematically, the relationship among current i , charge q , and time tis当我们把一根导线连接到某一电池上时 (一种电动势源 , 电荷被外力驱使移动; 正电荷朝一个方向移动而负电荷朝相反的方向移动。 这种电荷的移动产生

9、了电流。 我们可以很方便地把电流看作是正电荷的移动, 也即, 与负电荷的流动方向相反,如图 1-1所示。这一惯例是由美国科学家和发明家本杰明-富兰克林引入的。 虽然我们现在知道金属导体中的电流是由负电荷引起的, 但我们将遵循通用的惯例, 即把电流看作是正电荷的单纯的流动。 于是电流就是电荷的时率,它是以安培为单位来度量的。从数学上来说,电流 i 、电荷 q 以及时间 t 之 间的关系是:The charge transferred between time t 0 and t is obtained byintegrating both sides of Eq. (1-1. We obtain

10、从时间 t0到时间 t 所移送的电荷可由方程(1-1两边积分求得。我们算得:The way we define current as i in Eq. (1-l suggests that current neednot be a constant-valued function, charge can vary with time in severalways that may be represented by different kinds of mathematical functions我们通过方程 (1-1 定义电流的方式表明电流不必是一个恒值函数, 电荷可以不同的方式随时间而变化

11、,这些不同的方式可用各种数学函数表达出来。电压,能量和功率To move the electron in a conductor in a particular directionrequires some work or energy transfer. This work is performed by anexternal electromotive force (emf, typically represented by the battery in Fig.l-1. This emf is also known as voltage or potential difference.

12、The voltage u ab between two points a and b in an electric circuit is theenergy (or work needed to move a unit charge from a to b; mathematically 在导体中朝一个特定的方向移动电荷需要一些功或者能量的传递, 这个功是由外部的电动势来完成的。图 1-1所示的电池就是一个典型的例子。这种电动势也被称为电压或电位差。电路中 a 、 b 两点间的电压等于从 a 到 b 移动单位电荷所需的能量(或所需做的功 。数学表达式为:where w is energy i

13、n joules (J and q is charge in coulombs (C. The voltageu ab is measured in volts (V, named in honor of the Italian physicistAlessandro Antonio Volta (l745l827, who invented the first voltaicbattery. Thus, Voltage (or potential difference is the energy requiredto move a unit charge through an element

14、, measured in volts (V.式中 w 是单位为焦耳的能量而 q 是单位为库仑的电荷。 电压 Uab 是以伏特为单位来度量的,它是为了纪念意大利物理学家 Alessandro Antonio Volta 而命名的,这位意大利物理学家发明了首个伏达电池。 于是电压 (或电压差 等于将单位电荷在元件中移动所需的能量,它是以伏特为单位来度量的。Fig.l-2 shows the voltage across an element (represented by a rectangularblock connected to points a and b . The plus (+ a

15、nd minus (- signs areused to define reference direction or voltage polarity. The u ab can be dq i dt = (1-10t t q idt = (1-2ab dw u dq =3interpreted in two ways: point a is at a potential of u ab volts higher than point b; the potential at point a with respect to point b is u ab. It follows logicall

16、y that in general图 1-2显示了某个元件 (用一个矩形框来表示 两端 a 、 b 之间的电压。 正号 (+ 和负号(-被用来指明参考方向或电压的极性, Uab 可以通过以下两种方法来 解释。 1在 Uab 伏特的电位中 a 点电位高于 b 点, 2 a 点电位相对于 b 点而言 是 Uab ,通常在逻辑上遵循Although current and voltage are the two basic variables in an electric circuit, they are not sufficient by themselves. For practical pu

17、rposes, we need to know power and energy. To relate power and energy to voltage and current, we recall from physics that power is the time rate of expending or absorbing energy, measured in watts (W. We write this relationship as虽然电流和电压是电路的两个基本变量, 但仅有它们两个是不够的。 从实际应用 来说, 我们需要知道功率和能量。 为了把功率和能量同电压、 电流联

18、系起来, 我 们重温物理学中关于功率是消耗或吸收的能量的时率, 它是以瓦特为单位来度量 的。我们把这个关系式写成:Where p is power in watts (W, w is energy in joules (J, and t is time in seconds (s. From Eq. (1-1, Eq. (1-3, and Eq. (1-5, it follows that 式中 p 是以瓦特为单位的功率, w 是以焦耳为单位的能量, t 是以秒为单位的时 间,从方程(1-1 、 (1-3和(1-5可以推出Because u and i are generally functi

19、on of time, the power p in Eq. (1-6 is a time-varying quantity and is called the instantaneous power. The power absorbed or supplied by an element is the product of the voltage across the element and the current through it. If the power has a plus sign, power is being delivered to or absorbed by the

20、 element. If, on the other hand, the power has a minus sign, power is being supplied by the element. But how do we know when the power has a negative or a positive sign ?由于 u 和 i 通常是时间的函数, 方程 (1-6 中的功率 p 是个时间变量于是被称 为瞬时功率, 某一元件吸收或提供的功率等于元件两端电压和通过它的电流的乘 积。 如果这个功率的符号是正的, 那么功率向元件释放或被元件吸收。 另一方面, 如果功率的符号是

21、负的, 那么功率是由元件提供的。 但我们如何得知何时功率为 正或为负?Current direction and voltage polarity play a major role in determining the sign of power. It is therefore important that we pay attention to the relationship between current i and voltage u in Fig.1-3(a. The voltage polarity and current i direction must conform wi

22、th those shown in Fig.1-3(a in order for the power to have a positive sign. This is known ab ba u u = (1-4-dw p dt=5p ui = (1-6as the passive sign convention. By the passive sign convention, current enters through the positive polarity of the voltage. In this case, p = ui or ui 0 implies that the el

23、ement is absorbing power. However, if p = -ui or ui 0, as in Fig.1-3(b, the element is releasing or supplying power.在我们确定功率符号时, 电流的方向和电压的极性起着主要的作用, 这就是我们 在分析图 1-3(a 所显示的电流 i 和电压 u 的关系时特别谨慎的重要原因。为 了使功率的符号为正, 电压的极性和电流的方向必须与图 1-3(a 所示的一致。 这种情况被称为无源符号惯例,对于无源符号惯例来说,电流流进电压的正极。 在这种情况下, p =ui 或 ui>0, 表明元

24、件是在吸收功率。 而如果 p =-ui 或 ui<0, 如图 1-3(b 所示时,表明元件是在释放或提供功率。In fact, the law of conservation of energy must be obeyed in any electric circuit. For this reason, the algebraic sum of power in a circuit, at any instant of time, must be zero事实上, 在任何电路中必须遵循能量守恒定律。 由于这个原因, 任一电 路中在任何瞬间功率的代数和必须等于零This again c

25、onfirms the fact that the total power supplied to the circuit must balance the total power absorbed. From Eq. (l-7, the energy absorbed or supplied by an element from time t 0 to time t is这再一次证明了提供给电路的功率必须与吸收的功率相平衡这一事实。 从方程 (1-7可知,从时间 t0到时间 t 被元件吸收或由元件提供的功率等于Section2An electric circuit is simply an

26、interconnection of the elements. There are two types of elements found in electric circuits: passive elements and active elements. An active element is capable of generating energy while a passive element is not. Examples of passive elements are resistors, capacitors, and inductors. The most importa

27、nt active elements are voltage or current sources that generally deliver power to the circuit connected to them.电路仅仅是元件之间的相互结合。 我们发现电路中存在有两种元件:无源元件和 有源元件。 有源元件能够产生能量而无源元件却不能, 无源元件有电阻、 电容和 电感器等。 最重要的有源元件是通常向与它们相连的电路释放能量的电压和电流 源。Independent sourcesAn ideal independent source is an active element that

28、provides a specified voltage or current that is completely independent of other circuit variables.An independent voltage source is a two-terminal element, such as a battery or a generator, which maintains a specified voltage between its terminals. The voltage is completely independent of the current

29、 through the element. The symbol for a voltage source having u volts across its terminals is 0p = (1-70t t w pdt = (1-8shown in Fig.1-4(a. The polarity is as shown, indicating that terminal a is u volts above terminal b . Thus if u > 0, then terminal a is at a higher potential than terminal b . T

30、he opposite is true, of course, if u < 0 一个理想的独立源是产生完全独立于其它电路变量的特定电压或电流的有源元 件。 一个独立电压源是一个二端口元件, 如一个电池或一台发电机, 它们在其端 部维持某个特定的电压。该电压完全独立于流过元件的电流,在其端部具有 u 伏电压的电压源的符号如图 1-4(a 所示,极性如图所示,它表明 a 端比 b 端 高 u 伏。如果 u>0,那么 a 端的电位高于 b 端,当然,如果 u<0,反之亦然。 In Fig.1-4(a, the voltage u may be time varying, or i

31、t may be constant, in which case we would probably label it U . Another symbol that is often used for a constant voltage source, such as a battery with U volts across its terminals, is shown in Fig.1-4(b. In the case of constant sources we shall use Fig.1-4(a and 1-4(b interchangeably.在图 1-4(a 中,电压

32、u 可以是随时间而变化,或者可以是恒定的,在这种情 况下我们可能把它标为 U ,对于恒定电压源我们通常使用另一种符号,例如在两 端只有 U 伏电压的电池组,如图 1-4(b 所示。在恒定源的情况下我们可以交 替地使用于图 1-4(a 或图 1-4(b We might observe at this point that the polarity marks on Fig.1-4(b are redundant since the polarity could be defined by the positions of the longer and shorter lines.我们可能已经注

33、意到这一点,即图 1-4(b 中的极性标号,是多余的因为我们 可以根据长天线的位置符,确定电池极性An independent current source is a two-terminal element through which a specified current flows. The current is completely independent of the voltage across the element. The symbol for an independent current source is shown in Fig.1-5, where i is the

34、specified current. The direction of the current is indicated by the arrow一个独立电流源是二端元件在两端之间特定的电流流过, 该电流完全独立于元件 两端的电压,一个独立电流源的符合如图 1-5所示。图中 i 是特定电流,该电 流的方向由箭头标明Independent sources are usually meant to deliver power to the external circuit and not to absorb it. Thus if u is the voltage across the sourc

35、e and its current i is directed out of the positive terminal, then the source is delivering power, given by p = ui , to the external circuit. Otherwise it is absorbing power. For example, in Fig.1-6 (a the battery is delivering 24 W to the external circuit. In Fig.1-6(b the battery is absorbing 24 W

36、, as would be the case when it is being charged.独立源通常指的是向外电路释放功率而非吸收功率, 因此如果 u 是电源两端的电 压而电流 i 直接从其正端流出,那么该电源正在向对电路释放功率,由式 p =ui 算出。 否则它就在吸收功率。 例如图 1-6(a 中电池正在向外电路释放功率 24w , 在图 1-6(b 中,电池就在充电情况,吸收功率 24w 。Dependent sourcesAn ideal dependent (or controlled source is an active element in which the sourc

37、e quantity is controlled by another voltage or current.Dependent sources are usually designated by diamond-shaped symbols, as shown in Fig.1-7Since the control of the dependent source is achieved by a voltage or current of some other element in the circuit, and the source can be voltage or current, it follows that there are four possible types of dependent sources, namely: （1）A voltage-controlled voltage source (VCVS. （2）A current-controlled voltage sourc