1、Chapter 1Fundamentals of Electric CircuitsSection 2 Circuit Elementsn Textn New Words and Expressionsn Exercisesn Endn Translation StandardSection2 Circuit ElementsAn electric circuit is simply an interconnection of the elements. There are two types of elements found in electric circuits: passive el
2、ements 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 circuit connected
3、 to them. Section2 Circuit ElementsIndependent sourcesAn 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 or a generator, whi
4、ch 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 Elementsindicating that terminal a
5、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 often used for a co
6、nstant 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 ElementsWe might observe at this point that the polarity marks on Fig.1-4(b) are redundant since the
7、 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 ElementsAn independent current source is a two-terminal element through which
8、 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 independent current sou
9、rces Section2 Circuit ElementsIndependent 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 by p = ui, to the exter
10、nal 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 ElementsFig.1-6 Symbols for independent sources(a) A source d
11、elivering power; (b) A source absorbing power Section2 Circuit ElementsDependent sourcesAn 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 diamond-shaped symbols, as shown in Fig.1-7. Fig.1-7 Symbols for dependent sources(a) Dependent voltage source; (b) Dependent current source
