1、Section 1 Current and Voltagen Textn New Words and Expressionsn Exercisesn EndSection1 Current and Voltage Two variables u(t) and i(t) are the most basic concepts in an electric circuit, they characterize the various relationships in an electric circuit. Charge and Current The concept of electric ch
2、arge is the underlying principle for explaining all electrical 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). Section1 Current and Voltage We know fro
3、m elementary 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 1.602101019C, while a proton carries a positive charge of th
4、e same magnitude as the electron. The presence of equal numbers of protons and electrons leaves an atom neutrally charged. Section1 Current and Voltage We consider the flow of electric charges. A unique feature of electric charge or electricity is the fact that it is mobile; that is, it can be trans
5、ferred from one place to another, where it can be converted to another form of energy. When a conducting wire is connected 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.
6、This motion of charges creates electric current. It is conventional to take the currentSection1 Current and Voltageflow as the movement of positive charges, that is, opposite to the flow of negative charges, as Fig.l-1 illustrates. This convention was introduced by Benjamin Franklin (l706l790), the
7、American scientist and inventor. Although we now know that current in metallic conductors is due to negatively charged electrons, we will follow the universally accepted convention that current is the net flow of positive charges. Thus, Electric current is the time rate of charge, measured in ampere
8、s (A). Mathematically, the relationship among current i, charge q, and time t is Section1 Current and Voltage The charge transferred between time t0 and t is obtained by integrating both sides of Eq. (1-1). We obtain The way we define current as i in Eq. (1-l) suggests that current need not be a con
9、stant-valued function, charge can vary with time in several ways that may be represented by different kinds of mathematical functions. tqiddtttiq 0d (1-1)(1-2)Section1 Current and Voltage Voltage, Energy, and Power To move the electron in a conductor in a particular direction requires some work or e
10、nergy transfer. This work is performed by an external electromotive force (emf), typically represented by the battery in Fig.l-1. This emf is also known as voltage or potential difference. The voltage uab between two points a and b in an electric circuit is the energy (or work) needed to move a unit
11、 charge from a to b; mathematically Section1 Current and Voltage where w is energy in joules (J) and q is charge in coulombs (C). The voltage uab is measured in volts (V), named in honor of the Italian physicist Alessandro Antonio Volta (l745l827), who invented the first voltaic battery. Thus, Volta
12、ge (or potential difference) is the energy required to move a unit charge through an element, measured in volts (V). qwuabdd(1-3)Section1 Current and Voltage Fig.l-2 shows the voltage across an element (represented by a rectangular block ) connected to points a and b. The plus (+) and minus (-) sign
13、s are used to define reference direction or voltage polarity. The uab can be interpreted in two ways: point a is at a potential of uab volts higher than point b; the potential at point a with respect to point b is uab. It follows logically that in general Fig.1-2 Polarity of voltage uab baabuu(1-4)S
14、ection1 Current and Voltage Although current and voltage are the two basic variables in an electric circuit, they are not sufficient by themselves. For practical purposes, we need to know power and energy. To relate power and energy to voltage and current, we recall from physics that power is the ti
15、me rate of expending or absorbing energy, measured in watts (W). We write this relationship as twpdd(1-5)Section1 Current and VoltageWhere 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 Because u and i are g
16、enerally function 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
17、 absorbed by the element. If, uip (1-6)Section1 Current and Voltageon 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？ Current direction and voltage polarity play a major role in determining the s
18、ign of power. It is therefore important that we pay attention to the relationship between current i and voltage u in Fig.1-3(a). Fig.1-3 Reference polarities for power using the passive sign convention Section1 Current and VoltageThe voltage polarity and current i direction must conform with those s
19、hown in Fig.1-3(a) in order for the power to have a positive sign. This is known as 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 element is absorbing power. However, if p = -ui
20、or ui 0, as in Fig.1-3(b), the element is releasing or supplying power. Section1 Current and Voltage 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 This again confirms t
21、he 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 t0 to time t is 0p(1-7)tttpw 0d (1-8)ReturnNew Words and Expressionscurrentn. 电流 alternating current 交流电流 direct current 直流电流 current de
22、nsity 电流密度 active current 有功电流 reactive current 无功电流voltagen. 电压 voltage drop 电压降落 voltage gain 电压增益 voltage divider 分压器 voltage stabilizer 稳压器New Words and Expressionsvariable n. 变量；a.可变的，可调的，易变的 complex variable 复变量 dependent variable 因变量 independent variable 自变量 input variable 输入变量 charge n. 电荷，充
23、电；v. 充电，带电，起电 charge distribution 电荷分布 charge particle 带电粒子 test charge 试验电荷 positive charge 正电荷 negative charge 负电荷New Words and Expressionscoulomb n. 库仑（电荷的单位） coulomb meter 库仑计circuit n. 电路 circuit diagram 电路图 analogue circuit 模拟电路 short circuit 短路 open circuit 开路 active circuit 无源电路 passive circ
24、uit 无源电路underlying a. 基本的，根本的 潜在的，在下（面）的，下伏的New Words and Expressionselectron n. 电子 magnituden. 大小，尺寸，数量，数值electromotivea. 电动的，起电的 electromotive force 电动势 amperen. 安，安培（电流的单位） ampere meter 安培计 ampere turn 安匝 volt ampere 伏安 integratev. 积分，求积分workn. 功 New Words and Expressionspotential a.势的，位的，电位的；n.势
25、（能）,位(能), 电势(位,压) potential difference 电位差，电势差 earth potential 地电势（位） potential energy 势（位）能 reference potential 参考（基准）电位rectangular a. 矩形的，直角的，正交的joulen. 焦尔（能量、热量、功的单位）voltn. 伏特（电压、电位、电势的单位） electron volt 电子伏特 New Words and Expressionsvoltaic a. 动流的，电压的，电镀的，伏打(式)的 voltaic battery伏打电池conventionn. 习惯
26、，惯例，常规 by convention 按照惯例reference direction 参考方向polarityn. 极性powern. 功率，效率 动力，电力，能力，电 源 乘方，幂wattn. 瓦特（功率的单位） time-varyinga. 时变的 New Words and Expressionsinstantn. 瞬间，瞬时，即刻，时，时刻；a.立刻 的，直接的，紧急的，迫切的instantaneous a.瞬时（作用）的，瞬息的，即刻的 instantaneous power瞬时功率passivea. 无源的 消极的，被动的；n. 无源 passive network 无源网络a
27、lgebraica. 代数的ReturnExercises I. Choose the best answer into the blank 1. It is conventional to take the current flow as the movement of . A. negative chargesB. positive charges C. any charge D. protons2. Electric current is the time rate of charge, measured in . A. wattsB. volts C. joulesD. amperes
28、 Exercises I. Choose the best answer into the blank 3. The energy required to move a unit charge through an element is . A. currentB. powerC. voltage D. potential4. The plus (+) and minus (-) signs in an electric circuit diagram are used to define . A. voltage polarityB. current directionC. power fl
29、owD. absorbed power Exercises I. Choose the best answer into the blank 5. According to the passive sign convention, if the power has a plus sign, power is by the element. A. suppliedB. absorbedC. generatedD. transferred Exercises II. Answer the following questions according to the text 1. Is current
30、 in any electric circuit always a constant-valued function? Why?2. How does current change when the time rate of charges is greater?3. How to interpret the uab = -1 volt in two ways? 4. What relation is between power and energy?5. Why do current direction and voltage polarity play a major role in de
31、termining the characteristics of power? ReturnSection 1 EndSection 2 Circuit Elementsn Textn New Words and Expressionsn Exercisesn Endn Translation StandardSection2 Circuit Elements An electric circuit is simply an interconnection of the elements. There are two types of elements found in electric ci
32、rcuits: 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 important active elements are voltage or current sources that generally deliver power to the
33、 circuit connected to them. Section2 Circuit Elements Independent sources An ideal independent source is an active element that 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
34、 or a generator, which maintains a specified voltage between its terminals. The voltage is completely independent of the current through the element. The symbol for a voltage source having u volts across its terminals is shown in Fig.1-4(a). The polarity is as shown, Section2 Circuit Elementsindicat
35、ing that terminal a is u volts above terminal b. Thus if u 0, then terminal a is at a higher potential than terminal b. The opposite is true, of course, if u 0. In Fig.1-4(a), the voltage u may be time varying, or it may be constant, in which case we would probably label it U. Another symbol that is
36、 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.Section2 Circuit Elements We might observe at this point that the polarity marks on Fig.1-4(b) ar
37、e redundant since the polarity could be defined by the positions of the longer and shorter lines. Fig.1-4 Symbols for independent voltage sources(a) used for constant or time-varying voltage;(b) used for constant voltage (dc). Section2 Circuit Elements An independent current source is a two-terminal
38、 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 specified current. The direction of the current is indicated by the arrow. Fig.1-5 Symbols for i
39、ndependent current sources Section2 Circuit Elements 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 source and its current i is directed out of the positive terminal, then the source is delivering power, given
40、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, as would be the case when it is being charged. Section2 Circuit Elements Fig.1-6 Symbols for independe
41、nt sources(a) A source delivering power; (b) A source absorbing power Section2 Circuit ElementsDependent sources An ideal dependent (or controlled) source is an active element in which the source quantity is controlled by another voltage or current. Dependent sources are usually designated by diamon
42、d-shaped symbols, as shown in Fig.1-7. Fig.1-7 Symbols for dependent sources(a) Dependent voltage source; (b) Dependent current source Section2 Circuit Elements Since 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 v
43、oltage 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 source (CCVS).（3）A voltage-controlled current source (VCCS).（4）A current-controlled current source (CCCS). Dependent sources are
44、 useful in modeling elements such as transistors, operational amplifiers and integrated circuits. Section2 Circuit Elements It should be noted that an ideal voltage source (dependent or independent) will produce any current required to ensure that the terminal voltage is as stated, whereas an ideal
45、current source will produce the necessary voltage to ensure the stated current flow. Thus an ideal source could in theory supply an infinite amount of energy. It should also be noted that not only do sources supply power to a circuit, but they can absorb power from a circuit too. For a voltage sourc
46、e, we know the voltage but not the current supplied or drawn by it. By the same token, we know the current supplied by a current source but not the voltage across it. ReturnNew Words and Expressionsactivea. 有源的，有功的，有效的 活动的，主动的，积极的 活性的，放射的，激励的 active element 有源元件resistorn 电阻，电阻器 capacitorn. 电容器induct
47、orn. 电感器sourcen. 电源，（光，能，动力，信号，辐射，根，来，起）源 voltage source 电压源 current source 电流源 ideal independent source 理想独立源New Words and Expressionsconstant voltage source恒定电压源independent source独立源dependent source 受控源controlled source受控源 voltage-controlled voltage source电压控制电压源current-controlled voltage source电流
48、控制电压源voltage-controlled current source电压控制电流源current-controlled current source电流控制电流源 New Words and Expressionsgenerator n. 发电机 （脉冲，信号，气体）发生器，振荡器，加速器 AC generator 交流发电机diamond-shaped a. 菱形的 transistor n. 晶体管，半导体管operational a. 运算的，计算的 操作的，工作的，业务的，运转的amplifier n. 放大器 integrated circuit 集成电路 New Words
49、 and Expressionstokenn. 标记，象征，记号 特征，证明 by the same token 同理，同样；另外，还有 ReturnTranslation Standardn 准确明白准确明白n 通顺严密通顺严密n 简练全面简练全面专业英语翻译是对原文的一种再创作，其标准为专业英语翻译是对原文的一种再创作，其标准为Translation Standardn 掌握英语语法知识掌握英语语法知识n 具有一定深度的专业知识和较为广博的科具有一定深度的专业知识和较为广博的科学知识学知识n 有较高的汉语修养有较高的汉语修养为达到以上翻译标准，必须做到：为达到以上翻译标准，必须做到：n 掌
50、握翻译方法和技巧掌握翻译方法和技巧ReturnExercises I. Choose the best answer into the blank 1. An independent voltage source is a , which maintains a specified voltage between its terminals. A. one-terminal element B. two-terminal element C. three-terminal element D. four-terminal element2. Resistors are elements. A.
