Part 1: Fundamental Concepts: First Circuit

Now that you have a basic understanding of what voltage and current are, let’s take a look at a very simple electrical device, a flashlight. The flashlight we are discussing is one of the old school types with and filament style light bulb instead of a modern LED flashlight. A flashlight has batteries (usually two), a switch to turn it on and off, and a lightbulb that illuminates when the switch is on. This is shown pictorially in the figure below. The lines drawn between the batteries, switch, and light bulb are meant to represent the wires in the flashlight. Although flashlights don’t look like this (with all of the parts spread out and connected with wires), functionally, this is how they are made.

Figure 1. Pictorial of a flashlight.

A schematic is a symbolic way of drawing the electrical components used in a circuit and how they are connected to each other. The schematic shown below in Figure 2, is for the flashlight shown pictorially in Figure 1. The symbol labeled B1 represents the batteries in the flashlight, S1 represents the switch you use to turn it on, and DS1 is the lightbulb. The lines that connect the three symbols together are conductors (pieces of wire).

There are two schematics shown in this figure. The left hand schematic is shown with the switch off (normally referred to as the switch being open) while the right hand schematic is shown with the switch on (normally referred to as the switch being closed).

In the left hand figure, when the switch is open, there is a gap in the circuit so no current can flow and the light bulb is dark. The voltage across the battery is 3V. The voltage across the lightbulb is 0V because of the open switch. When the switch is closed, as shown in the right hand figure, the electrons are free to move through the circuit and there is a current flowing which causes the light bulb to illuminate. The voltage across both the battery and lightbulb is 3V.

Figure 2. Schematic of a flashlight with the switch open and closed.

The letter “I” signifies the current flowing in the direction of the arrow. Notice that when the switch is closed in the right hand schematic, current flows in a circle from the positive connection to the battery, through the switch and light bulb, and returns to the negative connection of the battery. The battery acts like a voltage source. It provides the potential that causes the electrons, or in this case the holes, to move. As a result the current flows in a circle, or loop, starting out at the battery and returning to it.

As mentioned previously, the lines connecting the schematic symbols are conductors. For most circuits, wires are treated as ideal. That is to say, they act as perfect conductors and they can carry an electric current perfectly. The voltage that is present on one end of a conductor is the same as the voltage on the other end. As discussed in later chapters, this is not always the case. For our flashlight, it is true, and the voltage across the battery is the same as the voltage across the light bulb because the connecting wires are treated as perfect conductors.

One question that might come to mind is, how much current is flowing?