9.1: Introduction

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In this chapter we begin our study of alternating current, or AC, electrical circuit analysis. As such, this chapter is filled with foundational material that will be used in the remainder of this text. It is critical that the concepts presented here be fully understood in order to achieve success in later chapters. We start with the mathematical description of the most simple AC waveform, the sine wave. This includes parameters such as amplitude, frequency, period, phase and DC offset. From there we discover how to describe other waveforms in terms of combinations of sine waves, and also how to determine the effective, or DC equivalent, value of a sine wave. AC analysis is practically impossible to perform without the use of complex numbers, and discussion follows with their description and proper manipulation. Finally, we introduce the concepts of reactance and impedance, and how they relate to resistance. This includes examination using both frequency domain plots and phasor diagrams. Phasor diagrams are vector plots and can be used to show the relationships between various voltages in a circuit, as well as between currents or resistive/reactive values.

Many of the topics in this text will echo your studies in DC circuit analysis, such as Ohm's law, Kirchhoff's voltage and current laws, series-parallel analysis, nodal analysis, and the like. Thus many concepts will be familiar. The major practical difference is that all quantities in DC systems are scalars, that is, they have only magnitude. In contrast, AC quantities are vectors, and thus have both magnitude and direction (or more properly, phase). Consequent;y, mathematical operations such as addition or multiplication have to follow vector rules instead of scalar rules. Forgetting to do so is a common error for students new to the subject and one that will bring no end of grief. These rules will be reviewed in the section covering complex numbers. Make sure that you have this material mastered before proceeding.