电气英语试讲课件制作

1、Section 4 Kirchhoffs Lawsn Textn New Words and Expressionsn Exercisesn Endn Grammar Characteristic 基尔霍夫定律 The network variables may have many interrelationships among themselves. Some of these relationships are due to the nature of the variables. 网络变量可能会有很多它们之间的相互关系。有些关系是由这些变量的性质所决定的。 A different cl

2、ass of relationship occurs because of the restriction that some specific type of network element places on the variables. 一些不同类型之间的关系是由于某些特定类型的网络元件对变量的限制所产生的。 Still another class of relationship is one between several variables of the same type which occurs as the result of the network configuration

3、, i.e., the manner in which the various element of the network are interconnected. 另一类关系是介于相同类型的多变量之间的关系， 这些变量是由于网络结构即网络的不同元件相互连 接 的方式而产生的。 Such a relation is said to be based on the topology of the network. 这样一种关系就被称为是基于网络拓扑结构的关系。 Kirchhoffs Current and Voltage Laws are laws that are based on the c

4、onnective features of a network. These laws say nothing about the elements themselves. 基尔霍夫电流和电压定律都是基于网络连接特性的定律。这些定律都没涉及到与元件本身的特性。 Kirchhoffs Current Law 基尔霍夫电流定律基尔霍夫电流定律 Kirchhoffs Current Law is based on the law of conservation of charge, which requires that the algebraic sum of charges within a s

5、ystem can not change。 基尔霍夫电流定律基于电荷守恒定律，电荷守恒定律要求一个系统中电荷的代数总和不变。 Kirchhoffs Current Law (KCL) states that the algebraic sum of currents entering a node (or a closed boundary) is zero. Mathematically, KCL implies that 基尔霍夫定律表明流进一个节点（或者一个边界)电流代数和为零，从数学上，KCL表明： where N is the number of branches connected

6、 to the node and in is the nth current entering (or leaving) the node. 式中N为连接到节点的支路数而in是流入（或流出 ）节点的第n条支路电流。 By this law, currents entering a node may be regarded as +, while currents leaving the node may be taken as -.(1-13)01Nnni01Nnni 根据这个定律，流入一个节点的电流可以认为是“”电流，而流出节点的电流可以看成是“”电流。 Consider the node

7、in Fig.1-12. Applying KCL gives 考虑图112的节点，应用KCL得到 Since current i1, i3, i4 are entering the node, while currents i2, and i5 are leaving it. By rearranging the Eq.(1-14), we get 由于电流i1，i3，i4流入节点，而电流i2和i5流出节点，重新整理方程（114），我们可以得到(1-14)(1-14)(1-15)(1-15)0)()(54321iiiii0)()(54321iiiii52431iiiii52431iiiii

8、An alternative form of KCL: The sum of the currents entering a node is equal to the sum of the currents leaving the node. KCL定律的另一种形式是：流入节点的电流之和等于流出节点的电流之和。 Note that KCL also applies to a closed boundary. This may be regarded as a generalized case, because a node may be regarded as a closed surface

9、 shrunk to a point. 注意KCL定律也可以应用于闭合边界。这可以被视为定律的广义应用情形，这是由于节点可以被看成是由某个闭合面收缩成一点而形成的。 In two dimensions, a closed boundary is the same as a closed path. As typically illustrated in the circuit of Fig.1-13, the total current entering the closed surface is equal to the total current leaving the surface.

10、在二维情况下，一个闭合的界面等同于一个闭合的线路。图113所示的电路就是一个典型的例子。流入闭合面的总电流等于流出闭合面的总电流。Fig.1-12 Current at a node illustrating KCL Fig.1-13 Applying KCL to a closed boundary Kirchhoffs Voltage Law (KVL) Kirchhoffs voltage law is based on the principle of conservation of energy. 基尔霍夫电压定律（KVL） 基尔霍夫电压定律基于能量守恒原理。 Kirchhoffs

11、Voltage Law (KVL) states that the algebraic sum of all voltages around a closed path (or loop) is zero. Expressed mathematically, KVL states that where M is the number of voltages in the loop and um is the mth voltage. (1-16)01Mmmu 基尔霍夫电压定律（KVL）表明环绕闭合线路（或回路）的电压的代数和为0，从数学上来说，KVL表达为： 式中M是回路电压总数而且um是第m

12、个电压。 To illustrate KVL, consider the circuit in Fig.1-14. The sign on each voltage is the polarity of the terminal encountered first as we travel around the loop. We can start with any branch and go around the loop either clockwise or counterclockwise. Suppose we start with the voltage source and go

13、 clockwise around the loop as shown; then voltages would be u1, +u2, + u3, u4, and + u5, in that order。(1-16)01Mmmu 为了解释KVL，让我们研究图114所示的电路。每个电压的符号就是当我们环绕回路时首先遇到的端部的极性。我们可以从任何一个电压开始并且可以顺时针或逆时针方向环绕回路。假设我们从电压源开始并如图所示顺时针环绕回路，按照顺序，那么电压将是u1、u2、u3、u4以及u5。 For example, as we reach branch 3, the positive ter

14、minal is met first; hence we have + u3. For branch 4, we reach the negative terminal first; hence, u4. Thus, KVL yields 举例说，当我们到达支路3时，我们首先遇到正极，于是，得到 u3，对于支路4，我们首先遇到负极，于是，得到u4 。因此，应用KVL，得出：(1-17)054321uuuuu054321uuuuu(1-17) Rearranging terms gives重新整理以上各项，得到： Which may be interpreted as Sum of voltag

15、e drops = Sum of voltage rises 上式可以解释为：电压降之和等于电压升之和。(1-18)41532uuuuuFig.1-14 A single-loop circuit illustrating KVL This is an alternative form of KVL. Notice that if we had traveled counterclockwise, the result would have been u1, u5, +u4, u3, and u2, which is the same as before except that the sig

16、ns are reversed. Hence, Eq. (1-17) and Eq. (1-18) remain the same. 这是KVL定律的另一种形式，注意如果我们逆时针环绕回路结果将是u1、u5、u4、u3以及u2，结果与前面相同，除了符号相反外。因此，方程（117）和方程（118）是一样的。 New Words and Expressions network n. 网络，电路，电网 linear time-varying network 线性变参数网络 network analyzer 网络分析器 configurationn. 结构，构造 topologyn. 拓扑，拓扑学，拓

17、扑结构 noden. 节点，结点，交点，叉点 （波）节，结，节 branchn. 支路，支线，支脉 分支（路，线，流）部门，分部，分行 dimension n. 维（数），度（数） 尺寸，线度 量纲，因次 loop n. 回路，闭合电路，环路，循环，环 clockwise a.; ad. 顺时针方向（的） counterclockwise a.; ad. 逆时针方向（的） Exercises I. Choose the best answer into the blank 1. Kirchhoffs Laws are laws that are based on the . A. nature

18、 of the network element B. topology of the network C. nonlinear or linear elements D. types of network elements2. Kirchhoffs Current Law can be stated in . A. only a wayB. two ways C. three waysD. four ways 3. Kirchhoffs Current Law is based on . A. Ohms Law B. the law of conservation of charge C. t

19、he law of conservation of energy D. the Coulombs law4. Kirchhoffs Voltage Law is based on . A. Ohms Law B. the law of conservation of charge C. the law of conservation of energy D. the Coulombs law 5. The algebraic sum of all voltages around a loop is . A. zero B. indefinite value C. sum of voltage

20、rises D. sum of voltage drops II. Answer the following questions according to the text 1. Are Kirchhoffs Laws valid in a nonlinear circuit? 2. How does Kirchhoffs Voltage Law state in two ways? 3. If currents leaving a node are regarded as +, what sign must current entering the node be taken as? 4.

21、Widely speaking, can a closed boundary be regarded as a node for KCL? 5. Is the sign of each voltage in KVL equation related to going around a loop clockwise or counterclockwise? III. Translate the following into Chinese If a circuit has two or more independent sources, one way to determine the valu

22、e of a specific variable (voltage or current) is to use nodal or mesh analysis. Another way is to determine the contribution of each independent source to the variable and then add them up. The latter approach is known as the superposition. The superposition principle states that the voltage across

23、(or current through) an element in a linear circuit is the algebraic sum of the voltages across (or currents through) that element due to each independent source acting alone. Drunk One day, a father and his little son were going home. At this age, the boy was interested in all kinds of things and w

24、as always asking questions. Now, he asked, Whats the meaning of the word Drunk, dad? Well, my son, his father replied, look, there are standing two policemen. If I regard the two policemen as four then I am drunk. But, dad, the boy said, theres only ONE policeman! 醉酒 一天，父亲与小儿子一道回家。这个孩子正处于那种对什么事都很感兴趣的年龄，老是有提不完的问题。他向父亲发问道：“爸爸，醉