Electric Circuits 



Introduction

Electric circuit theory and electromagnetic theory are the two fundamental theories upon which all branches of electrical engineering are
built. Many branches of electrical engineering, such as power, electric
machines, control, electronics, communications, and instrumentation,
are based on electric circuit theory. Therefore, the basic electric circuit
theory course is the most important course for an electrical engineering student, and always an excellent starting point for a beginning student in electrical engineering education. Circuit theory is also valuable
to students specializing in other branches of the physical sciences
because circuits are a good model for the study of energy systems in
general, and because of the applied mathematics, physics, and topology involved.
In electrical engineering, we are often interested in communicating
or transferring energy from one point to another. To do this requires an
interconnection of electrical devices. Such interconnection is referred
to as an electric circuit, and each component of the circuit is known as
an element.

An electric circuit is an interconnection of electrical elements.

A simple electric circuit is shown in Fig. 1.1. It consists of three
basic elements: a battery, a lamp, and connecting wires. Such a simple
circuit can exist by itself; it has several applications, such as a flashlight, a search light, and so forth.
A complicated real circuit is displayed in Fig. 1.2, representing the
schematic diagram for a radio receiver. Although it seems complicated,
this circuit can be analyzed using the techniques we cover in this book.
Our goal in this text is to learn various analytical techniques and
computer software applications for describing the behavior of a circuit
like this.




Systems of Units

As electrical engineers, we deal with measurable quantities. Our measurement, however, must be communicated in a standard language that virtually all professionals can understand, irrespective of the country where the measurement is conducted. Such an international measurement language is the International System of Units (SI), adopted by the General Conference on Weights and Measures in 1960. In this system, there are seven principal units from which the units of all other physical quantities can be derived. Table 1.1 shows the six units and one derived unit that are relevant to this text. The SI units are used throughout this text. One great advantage of the SI unit is that it uses prefixes based on the power of 10 to relate larger and smaller units to the basic unit. Table 1.2 shows the SI prefixes and their symbols. For example, the following are expressions of the same distance in meters (m):


600,000,000 mm     600,000 m         600 km