A circuit is a circular path in which electrical current flows. In Chapter 2, Basic Electronics we described electrical current as the flow of electrons. The circuit is the path in which these electrons flow, powering the components that make up the circuit. The point where the electrons enter the circuit, and the current starts, is called the source. The point where the electrons exit the circuit is called the return. Everything between the source and the return, the components and wires that make up the remainder of the circuit, is called the load.
In a closed circuit, the current has a complete path from the source to the return allowing the current to flow through the circuit. The following diagram shows a closed circuit where a switch is closed allowing the current to flow through the circuit. In this circuit, we are powering a light bulb. This makes the...