電氣工程及其自動化專業(yè)英語第一章課文翻譯

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這兩個變量是電路中最基本的兩個變量,它們刻劃了電路的各種關(guān) 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.電荷和電流電荷的概念是用來解釋所有電氣現(xiàn)象的基本概念。也即,電路中最基本 的量是電荷。 電荷是構(gòu)成物質(zhì)的原子微粒的電氣屬性, 它是以庫侖為單位來度量 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. 我們從基礎(chǔ)物理得知一切物質(zhì)是由被稱為原子的基本構(gòu)造部分組成的, 并且每個原子是由電子, 質(zhì)子和中子組成的。 我們還知道電子的電量是負的并且 在數(shù)值上等于 1.602100×10-12C ,而質(zhì)子所帶的正電量在數(shù)值上與電子相等。 質(zhì)子和電子數(shù)量相同使得原子呈現(xiàn)電中性。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讓我們來考慮一下電荷的流動。 電荷或電的特性是其運動的特性, 也就是, 它可 以從一個地方被移送到另一個地方,在此它可以被轉(zhuǎn)換成另外一種形式的能量。 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當我們把一根導線連接到某一電池上時 (一種電動勢源 , 電荷被外力驅(qū)使移動; 正電荷朝一個方向移動而負電荷朝相反的方向移動。 這種電荷的移動產(chǎn)生

9、了電流。 我們可以很方便地把電流看作是正電荷的移動, 也即, 與負電荷的流動方向相反,如圖 1-1所示。這一慣例是由美國科學家和發(fā)明家本杰明-富蘭克林引入的。 雖然我們現(xiàn)在知道金屬導體中的電流是由負電荷引起的, 但我們將遵循通用的慣例, 即把電流看作是正電荷的單純的流動。 于是電流就是電荷的時率,它是以安培為單位來度量的。從數(shù)學上來說,電流 i 、電荷 q 以及時間 t 之 間的關(guān)系是:The charge transferred between time t 0 and t is obtained byintegrating both sides of Eq. (1-1. We obtain

10、從時間 t0到時間 t 所移送的電荷可由方程(1-1兩邊積分求得。我們算得: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我們通過方程 (1-1 定義電流的方式表明電流不必是一個恒值函數(shù), 電荷可以不同的方式隨時間而變化

11、,這些不同的方式可用各種數(shù)學函數(shù)表達出來。電壓,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 在導體中朝一個特定的方向移動電荷需要一些功或者能量的傳遞, 這個功是由外部的電動勢來完成的。圖 1-1所示的電池就是一個典型的例子。這種電動勢也被稱為電壓或電位差。電路中 a 、 b 兩點間的電壓等于從 a 到 b 移動單位電荷所需的能量(或所需做的功 。數(shù)學表達式為: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 是單位為焦耳的能量而 q 是單位為庫侖的電荷。 電壓 Uab 是以伏特為單位來度量的,它是為了紀念意大利物理學家 Alessandro Antonio Volta 而命名的,這位意大利物理學家發(fā)明了首個伏達電池。 于是電壓 (或電壓差 等于將單位電荷在元件中移動所需的能量,它是以伏特為單位來度量的。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圖 1-2顯示了某個元件 (用一個矩形框來表示 兩端 a 、 b 之間的電壓。 正號 (+ 和負號(-被用來指明參考方向或電壓的極性, Uab 可以通過以下兩種方法來 解釋。 1在 Uab 伏特的電位中 a 點電位高于 b 點, 2 a 點電位相對于 b 點而言 是 Uab ,通常在邏輯上遵循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雖然電流和電壓是電路的兩個基本變量, 但僅有它們兩個是不夠的。 從實際應(yīng)用 來說, 我們需要知道功率和能量。 為了把功率和能量同電壓、 電流聯(lián)

18、系起來, 我 們重溫物理學中關(guān)于功率是消耗或吸收的能量的時率, 它是以瓦特為單位來度量 的。我們把這個關(guān)系式寫成: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 是以瓦特為單位的功率, w 是以焦耳為單位的能量, t 是以秒為單位的時 間,從方程(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 通常是時間的函數(shù), 方程 (1-6 中的功率 p 是個時間變量于是被稱 為瞬時功率, 某一元件吸收或提供的功率等于元件兩端電壓和通過它的電流的乘 積。 如果這個功率的符號是正的, 那么功率向元件釋放或被元件吸收。 另一方面, 如果功率的符號是

21、負的, 那么功率是由元件提供的。 但我們?nèi)绾蔚弥螘r功率為 正或為負?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.在我們確定功率符號時, 電流的方向和電壓的極性起著主要的作用, 這就是我們 在分析圖 1-3(a 所顯示的電流 i 和電壓 u 的關(guān)系時特別謹慎的重要原因。為 了使功率的符號為正, 電壓的極性和電流的方向必須與圖 1-3(a 所示的一致。 這種情況被稱為無源符號慣例,對于無源符號慣例來說,電流流進電壓的正極。 在這種情況下, p =ui 或 ui>0, 表明元

24、件是在吸收功率。 而如果 p =-ui 或 ui<0, 如圖 1-3(b 所示時,表明元件是在釋放或提供功率。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事實上, 在任何電路中必須遵循能量守恒定律。 由于這個原因, 任一電 路中在任何瞬間功率的代數(shù)和必須等于零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這再一次證明了提供給電路的功率必須與吸收的功率相平衡這一事實。 從方程 (1-7可知,從時間 t0到時間 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.電路僅僅是元件之間的相互結(jié)合。 我們發(fā)現(xiàn)電路中存在有兩種元件:無源元件和 有源元件。 有源元件能夠產(chǎn)生能量而無源元件卻不能, 無源元件有電阻、 電容和 電感器等。 最重要的有源元件是通常向與它們相連的電路釋放能量的電壓和電流 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 一個理想的獨立源是產(chǎn)生完全獨立于其它電路變量的特定電壓或電流的有源元 件。 一個獨立電壓源是一個二端口元件, 如一個電池或一臺發(fā)電機, 它們在其端 部維持某個特定的電壓。該電壓完全獨立于流過元件的電流,在其端部具有 u 伏電壓的電壓源的符號如圖 1-4(a 所示,極性如圖所示,它表明 a 端比 b 端 高 u 伏。如果 u>0,那么 a 端的電位高于 b 端,當然,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.在圖 1-4(a 中,電壓

32、u 可以是隨時間而變化,或者可以是恒定的,在這種情 況下我們可能把它標為 U ,對于恒定電壓源我們通常使用另一種符號,例如在兩 端只有 U 伏電壓的電池組,如圖 1-4(b 所示。在恒定源的情況下我們可以交 替地使用于圖 1-4(a 或圖 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.我們可能已經(jīng)注

33、意到這一點,即圖 1-4(b 中的極性標號,是多余的因為我們 可以根據(jù)長天線的位置符,確定電池極性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一個獨立電流源是二端元件在兩端之間特定的電流流過, 該電流完全獨立于元件 兩端的電壓,一個獨立電流源的符合如圖 1-5所示。圖中 i 是特定電流,該電 流的方向由箭頭標明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.獨立源通常指的是向外電路釋放功率而非吸收功率, 因此如果 u 是電源兩端的電 壓而電流 i 直接從其正端流出,那么該電源正在向?qū)﹄娐丰尫殴β?由式 p =ui 算出。 否則它就在吸收功率。 例如圖 1-6(a 中電池正在向外電路釋放功率 24w , 在圖 1-6(b 中,電池就在充電情況,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