电气自动化专业英语section 1-3.

1、Chapter 1Fundamentals of Electric CircuitsSection 3 Ohms Lawn Textn New Words and Expressionsn Exercisesn Endn Process of TranslationSection3 Ohms Law The circuit element used to model the current-resisting behavior of a material is the resistor. The resistor is the simplest passive element. Georg S

2、imon Ohm (17871854), a German physicist, is credited with formulating the current-voltage relationship for a resistor based on experiments performed in 1826. This relationship is known as Ohms Law. Section3 Ohms Law Ohms Law states that the voltage across a resistor is directly proportional to the c

3、urrent flowing through the resistor. The constant of proportionality is the resistance value of the resistor in ohms. The circuit symbol for the resistor is shown in Fig.1-8. For the current and voltage shown, Ohms law is Fig.1-8 Circuit symbol for the resistor )(tRiu (1-9)Section3 Ohms Lawwhere R 0

4、 is the resistance in ohms. Rearranging Eq. (1-9) into the form R = u(t) / i(t), we see that l ohm = l V/AThe symbol used to represent the ohm is the capital Greek omega () . Since R is constant, Eq.(1-9) is the equation of a straight line. For this reason, the resistor is called a linear resistor.

5、A graph of u(t) versus i(t) is shown in Fig.1-9, which is a line passing through the origin with a slope of R. Obviously, a straight line is theSection3 Ohms Lawonly graph possible for which the ratio of u(t) to i(t) is constant for all i(t). Resistors whose resistances do not remain constant for di

6、fferent terminal currents are known as nonlinear resistors. For such a resistor, the resistance is a function of the current flowing in the device. Fig.1-9 Voltage-current characteristic for a linear resistor Section3 Ohms LawA simple example of a nonlinear resistor is an incandescent lamp. A typica

7、l voltage-current characteristic for this device is shown in Fig.1-10, where we see that the graph is no longer a straight line. Since R is not a constant, the analysis of a circuit containing nonlinear resistor is more difficult. Fig.1-10 Typical voltage- c ur r ent characteristic for a nonlinear r

8、esistor Section3 Ohms Law In reality, all practical resistors are nonlinear because the electrical characteristics of all conductors are affected by environmental factors such as temperature. Many materials, however, closely approximate an ideal linear resistor over a desired operating region. We sh

9、all concentrate on these types of elements and simply refer to them as resistors. Section3 Ohms Law Since the value of R can range from zero to infinity, it is important that we consider the two extreme possible values of R. An element with R = 0 is called a short circuit, as shown in Fig.1-11 (a).

10、For a short circuit u = iR = 0 （1-10）showing that the voltage is zero but the current could be anything. In practice, a short circuit is usually a connecting wire assumed to be a perfect conductor. Thus, a short circuit is a circuit element with resistance approaching zero. Section3 Ohms Law Similar

11、ly, an element with R = is known as an open circuit, as shown in Fig.1-11 (b). For an open circuit 0limRuiR(1-11) Fig.1-11(a) Short circuit ( R = 0 ); (b) Open circuit ( R = ) Section3 Ohms Lawindicating that the current is zero though the voltage could be anything. Thus, an open circuit is a circui

12、t element with resistance approaching infinity. Another important quantity which is useful in circuit analysis is known as conductance, defined by The conductance is a measure of how well an element will conduct electric current. The unit of conductance is the siemens (S). uiRG1(1-12)ReturnNew Words

13、 and ExpressionsOhm 欧姆（电阻，电抗，阻抗的单位） Ohms Law 欧姆定律resistance n. 电阻,电阻器（件,装置）阻力，阻尼 抵抗，抵制，反对 resistance load 电阻(性)负载 magnetic resistance 磁阻 measuring resistance 标准电阻,测量用电阻rearrange v. 重新整理（安排,布置,排列）,调整New Words and Expressionslineara. 线性的，一次的 （直）线的，直线型的，线状的 linear resistor线性电阻nonlinear a. 非线性的，非直线的 non

14、linear resistor非线性电阻incandescenta. 白炽（热）的，炽热的 incandescent lamp 白炽灯 voltage-current characteristic 伏安特性short circuit短路open circuit开路 New Words and Expressionsconductancen. 电导，导电性，导纳 传导（性，率，系数），导率siemens n. 西门子（电导单位） ReturnProcess of Translationn 理解阶段理解阶段 主要是通过辨明词义，弄清各种语法成分及其相互关系，并把前后句子贯串起来理解，形成对原文的完

15、整印象，真正掌握原文的内容和实质。 n 表达阶段表达阶段 是在理解的基础上，以忠实于原意为前提，灵活地运用各种翻译方法和技巧，写出符合汉语规范、表达习惯以及翻译标准的译文。 n 校核阶段校核阶段 是理解与表达的进一步深化，是对原文内容进一步核实，对译文语言进一步推敲，进行必要的润色和修改，使译文符合标准规范。 ReturnExercises I. Choose the best answer into the blank 1. The circuit element used to model the current-resisting behavior of a material is .

16、A. the capacitorB. the inductorC. the resistorD. the dependent source2. Ohms Law states that the voltage across a resistor is the current flowing through the resistor. A. equal to B. directly proportional to C. inversely proportional to D. different from Exercises I. Choose the best answer into the

17、blank 3. A resistor whose resistances vary with its current is known as . A. a linear resistor B. a constant resistor C. a nonlinear resistor D. a time-varying resistor 4. is a circuit element with resistance approaching infinity. A. An open circuit B. A short circuit C. A resistive circuit D. An in

18、ductive circuit Exercises I. Choose the best answer into the blank 5. An electric heater draws 10 A from a 120V line. The resistance of the heater is . A. 1200 B. 120 C. 12 D. 1.2 Exercises II. Answer the following questions according to the text 1. What is Ohms law?2. Does the resistance value of a

19、 linear resistor vary with the current through it?3. What feature of the voltage-current characteristic curve is for a linear resistor?4. What is a short circuit?5. What relationship between resistance and conductance is there? Exercises III. Translate the following into Chinese It must be emphasized that the linear resistor is an idealized circuit element; it is a mathematical model of a physical