1.1 Introduction

← Back to Fundamentals of Electric Circuits Overview

1.1 Introduction

Electric circuit theory and electromagnetic theory are the tw o funda mental theories upon which all branches of electrical engineering are built. Many branches of electrical engineering, such as po wer, 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 al ways an e xcellent starting point for a be ginning student in electrical engineering education. Circuit theory is also v aluable to students specializing in other branches of the ph ysical 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 kno wn as an element.

An electric circuit is an interconnection of electrical elements.

A simple electric circuit is sho wn in Fig. 1.1. It consists of three basic elements: a battery, a lamp, and connecting wires. Such a simple circuit can e xist by itself; it has se veral 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 co ver 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.

Electric circuits are used in numerous electrical systems to accomplish different tasks. Our objecti ve in this book is not the study of various uses and applications of circuits. Rather, our major concern is the analysis of the circuits. By the analysis of a circuit, we mean a study of the behavior of the

Figure 1.1 A simple electric circuit.

Figure 1.2

Electric circuit of a radio transmitter.

circuit: How does it respond to a gi ven input? How do the interconnected elements and devices in the circuit interact?

We commence our study by defining some basic concepts. These concepts include char ge, current, v oltage, circuit elements, po wer, and energy. Before defining these concepts, we must first establish a system of units that we will use throughout the text.

1.2 Systems of Units

As electrical engineers, we must deal with measurable quantities. Our measurements, ho wever, must be communicated in a standard language that virtually all professionals can understand, irrespecti ve of the country in which 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 base units from which the units of all other ph ysical quantities can be de rived. Table 1.1 shows six base units and one derived unit (the coulomb) that are related to this text. SI units are commonly used in electrical engineering.

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

TABLE 1.1

Six basic SI units and one derived unit relevant to this text.

Quantity	Basic unit	Symbol
Length	meter	m
Mass	kilogram	kg
Time	second	s
Electric current	ampere	A
Thermodynamic temperature	kelvin	K
Luminous intensity	candela	cd
Charge	coulomb	C

TABLE 1.2

The SI prefixes.

Multiplier	Prefix	Symbol
1018	exa	E
1015	peta	P
1012	tera	T
109	giga	G
106	mega	M
103	kilo	k
102	hecto	h
10	deka	da
10−1	deci	d
10−2	centi	c
10−3	milli	m
10−6	micro	μ
10−9	nano	n
10−12	pico	p
10−15	femto	f
10−18	atto	a