BEFORE any use can be made of electricity or of any electrical machine or device, it must form part of an electrical circuit. Even complex machines may be modelled by simple elements that, when assembled into a circuit in the right way, can be analysed and so predict the machine’s behaviour. Accordingly, circuits are the foundation of any study of electrical or electronic engineering. We begin by defining simple circuit elements, then we shall incorporate them into circuits for analysis with the help of a number of laws and theorems. There are not many laws to remember — Ohm’s law and Kirchhoff’s laws are almost the only ones — but from these a number of theorems have been deduced to assist in circuit analysis. In this chapter we shall primarily be concerned with direct currents and voltages (DC for short), but the principles developed will serve for analysing circuit behaviour with alternating currents and voltages (or AC).
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