First, let me orient you to the structure of this chapter — Page 158, Lesson 162

I want to walk you through the start of Chapter 11, which is titled "AC Electrics – Introduction to AC." This is where we begin our study of alternating current, or AC, as it applies to aircraft electrical systems. First, let me orient you to the structure of this chapter. The introduction begins on page 161, and from there we will cover the nature of alternating current, then the key terms we need to define. After that, we look at the relationship of current and voltage in an AC circuit, followed by resistance in AC circuits. Then we move into inductance in AC circuits, and a specific concept called inductive reactance on page 169. Capacitance in AC circuits comes next, with capacitive reactance on page 171. Then we cover impedance on page 172, resonant circuits on page 172, and a summary on page 173. The chapter then goes into power in AC circuits, starting on page 173, with power in a purely resistive circuit on page 174, power in a purely inductive circuit on page 174, power in a capacitive circuit on page 175, and power in a practical AC circuit on pages 175 and 176. Finally, we cover power factor on page 177, a power factor resume on page 177, questions on page 178, and answers on page 184. Now, let me introduce the core idea. Alternating current, or AC, is fundamentally different from the direct current, or DC, you may already know. In a DC circuit, current flows in one steady direction. In an AC circuit, the current periodically reverses direction. This chapter will teach you how that reversal happens, how we describe it with terms like frequency and amplitude, and how components like resistors, inductors, and capacitors behave when placed in an AC circuit. I want to start with the very first concept: the nature of alternating current. To understand AC, we begin with a simple generator. Imagine a loop of wire, which we call the armature, placed inside a magnetic field. When that loop is rotated, it experiences a changing magnetic flux, and that induces a voltage. This is the principle of electromagnetic induction. Let me show you this with a diagram. "Figure 11.1 shows that a loop of wire (armature) rotated in a magnetic field experiences" a changing flux, which induces an alternating voltage. As the armature rotates, the voltage it produces rises, falls, reverses polarity, rises in the opposite direction, and falls again, completing one full cycle. That cycle repeats with each rotation. Now, as the armature continues to rotate, the flux through the loop is constantly changing. "Figure 11.2 then the flux is constantly changing. In positions 1," we can see specific points in the rotation where the flux is at a maximum or minimum, and where the induced voltage is at its peak or zero. This gives us the familiar sine wave shape of an AC voltage. Later in the chapter, we will also look at what happens when a switch is opened in an inductive circuit. "Figure 11.7c the switch has been opened and there is a rapid collapse of the magnetic field" which induces a high voltage spike — an important effect to understand for aircraft electrical systems. So, to summarise the start of this chapter: we are moving from DC into AC. The key physical principle is that rotating a loop of wire in a magnetic field generates an alternating voltage. The diagrams in figures 11.1 and 11.2 will help you visualise that rotation and the resulting sine wave. We will build on this foundation as we define terms like frequency, period, peak voltage, and RMS voltage, and then see how resistors, inductors, and capacitors each behave differently in an AC circuit compared to DC. That is the opening of Chapter 11. We are ready to move into the detailed definitions and relationships next.