Introduction

Electrical supply comes in two basic forms. Direct current (DC) has a fixed polarity: the positive and negative terminals of the source never swap, so current flows in one constant direction (a battery is the everyday example). Alternating current (AC), by contrast, has a polarity that reverses periodically -- the terminals of the source keep swapping their sign in a regular, repeating pattern, so the current in the circuit repeatedly reverses direction as well. The previous chapter developed how an AC generator produces this kind of periodically-reversing emf by rotating a coil in a magnetic field. AC is used almost universally for electrical distribution because it is cheaper to generate and, crucially, far easier and more efficient to transport over long distances (its voltage can be stepped up for transmission and stepped back down for use, via transformers, with comparatively little loss) -- this is why the electricity reaching homes and offices is AC rather than DC.

Many common devices -- a fan, refrigerator, air conditioner, induction heater, coil heater -- are designed to run directly on AC and use components such as inductors and capacitors as an essential part of their operation. Others, like a TV, computer, or radio, need a steady DC supply internally and so first convert the incoming AC to DC using a rectifier. Either way, understanding how AC behaves when it passes through the basic circuit elements -- resistors, inductors and capacitors -- is essential, and that is exactly what this chapter builds up: first individually (one element at a time), then in combination, ending with the phenomenon of electrical resonance and its practical use in tuning and in the choke coil.