D.C. Theory involves the analysis of electrical circuits, consisting of voltage sources (such as batteries), conductors and components called resistors. First we will consider the fundamental nature of voltage, current and resistance. Then we will describe how the basic circuit laws (Ohm's Law, & Kirchhoff's Laws) show the relationship between these quantities. This leads on to techniques for adding resistors together when connected in series or parallel. We will finish the section by explaining how circuit theorems such as Thevenin's, Norton's & the superposition theorems apply these circuit laws to solve particular types of problems with electrical circuits. We will also consider energy and power in electrical circuits as well as concepts such as internal resistance of power supplies and the effects of temperature on resistance.

We will begin by considering a very simple circuit. The diagram below shows a simple electrical circuit (cct.). It consists of a battery, some connecting wires, a switch and a bulb. We can actually learn a great deal about electricity just by investigating how this simple circuit works.

After completing this section you should be able to:

- Explain what electrical current is.
- Explain what is meant by the terms charge and charged particle.
- Calculate values for current (I), quantity of charge (Q) or time (t) using the equation I=Q/t .
- Describe how the electrical current in a circuit is related to, the voltage applied to the circuit and it's resistance.
- Calculate values for current (I), voltage (V) or resistance (R) using Ohm's law.

*In the later sections you will learn more about voltage and resistance.*