关于《电路》已经说的够多了，不想再多费口舌。在此，引两段经典《电路》教科书中的文字，这些都是出现在教材的前端，是必须领会的东西，可惜一般都被忽略了。
一）引自《电路》（邱关源）
本书的主要内容是介绍电路理论的入门知识并为后续课程的学习准备必要的基础。电路理论研究电路中发生的电磁现象，并用电流、电压、电荷、磁通等物理量来描述其中的过程。电路理论主要是计算电路中各部件、器件的端子电流和端子间的电压，一般不涉及内部发生的物理过程。本书讨论的对象不是实际电路而是实际电路的电路模型。实际电路的电路模型由理想电路元件相互连接而成，理想元件是组成电路模型的最小单元，是具有某种确定电磁特性并有精确数学定义的基本结构。在一定的工作条件下，理想电路元件及它们的组合足以模拟实际电路中部件、器件中发生的物理过程。在电路模型中各理想元件的端子是用“理想导线”（理想导线的电阻为零，且假设当导线中有电流时，导线内、外均无电场和磁场）连接起来的。根据元件对外端子的数目、理想电路元件可以分为两端、三端、四端元件等。
集总（参数）元件假定：在任何时刻，流入二端元件的一个端子的电流一定等于从另一端子流出的电流，且两个端子间的电压为单值量。由集总元件构成的电路称为集总电路，或称具有集总参数的电路。
附注：
集总参数（Lumped Parameter）元件是指有关电、磁场物理现象都由元件来“集总”表征。在元件外部不存在任何电场与磁场。如果元件外部有电场，进、出端子的电流就有可能不同；如果元件外部有磁场，两个端子之间的电压就可能不是单值得。
二）《Electric Circuits》（Nilsson）
Circuit Theory
In a field as diverse as electrical engineering, you might well ask whether all of its branches have anything in common. The answer is yes—electric circuits. An electric circuit is a mathematical model that approximates the behavior of an actual electrical system. As such, it provides an important foundation for learning—in your later courses and as a practicing engineer—the details of how to design and operate systems such as those just described. The models, the mathematical techniques, and the language of circuit theory will form the intellectual framework for your future engineering endeavors.
Note that the term electric circuit is commonly used to refer to an actual electrical system as well as to the model that represents it. In this text, when we talk about an electric circuit, we always mean a model, unless otherwise stated. It is the modeling aspect of circuit theory that has broad applications across engineering disciplines.
Circuit theory is a special case of electromagnetic field theory: the study of static and moving electric charges. Although generalized field theory might seem to be an appropriate starting point for investigating electric signals, its application is not only cumbersome but also requires the use of advanced mathematics. Consequently, a course in electromagnetic field theory is not a prerequisite to understanding the material in this book. We do, however, assume that you have had an introductory physics course in which electrical and magnetic phenomena were discussed.
Three basic assumptions permit us to use circuit theory, rather than electromagnetic field theory, to study a physical system represented by an electric circuit. These assumptions are as follows:
1. Electrical effects happen instantaneously throughout a system. We can make this assumption because we know that electric signals travel at or near the speed of light. Thus, if the system is physically small, electric signals move through it so quickly that we can consider them to affect every point in the system simultaneously. A system that is small enough so that we can make this assumption is called a lumped-parameter system.
2. The net charge on every component in the system is always zero. Thus no component can collect a net excess of charge, although some components, as you will learn later, can hold equal but opposite separated charges.
3. There is no magnetic coupling between the components in a system. As we demonstrate later, magnetic coupling can occur within a component.
That's it; there are no other assumptions. Using circuit theory provides simple solutions (of sufficient accuracy) to problems that would become hopelessly complicated if we were to use electromagnetic field theory. These benefits are so great that engineers sometimes specifically design electrical systems to ensure that these assumptions are met. The importance of assumptions 2 and 3 becomes apparent after we introduce the basic circuit elements and the rules for analyzing interconnected elements.
However, we need to take a closer look at assumption l.The question is, "How small does a physical system have to be to qualify as a lumpedparameter system?" We can get a quantitative handle on the question by noting that electric signals propagate by wave phenomena. If the wavelength of the signal is large compared to the physical dimensions of the system, we have a lumped-parameter system. The wavelength A is the velocity divided by the repetition rate, or frequency, of the signal; that is, λ = c/f. The frequency f is measured in hertz (Hz). For example, power systems in the United States operate at 60 Hz. If we use the speed of light (c = 3×10⁸ m/s) as the velocity of propagation, the wavelength is 5×10⁶ m. If the power system of interest is physically smaller than this wavelength, we can represent it as a lumped-parameter system and use circuit theory to analyze its behavior. How do we define smaller? A good rule is the rule of 1/lOth: If the dimension of the system is l/10th (or smaller) of the dimension of the wavelength, you have a lumped-parameter system. Thus, as long as the physical dimension of the power system is less than 5×10⁵ m, we can treat it as a lumped-parameter system.
On the other hand, the propagation frequency of radio signals is on the order of 10⁹ Hz.Thus the wavelength is 0.3 m. Using the rule of l/10th, the relevant dimensions of a communication system that sends or receives radio signals must be less than 3 cm to qualify as a lumped-parameter system. Whenever any of the pertinent physical dimensions of a system under study approaches the wavelength of its signals, we must use electromagnetic field theory to analyze that system. Throughout this book we study circuits derived from lumped-parameter systems.
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