For many truck enthusiasts the mechanical aspects of our vehicles are easy to accept even if they're not completely understood, but the same can't always be said for the electrical system. Electricity is one of those things so theoretical in nature that it can be difficult grasp. After all, it's not something you can see (unless you walk across the carpet in your socks and touch someone and see a big, blue spark). But on the other hand you don't have to see it to know it exists, as anyone who has made contact with the business end of a spark plug wire with an engine running will testify.

When most of our trucks were rolling off the assembly line the demands on the electrical system were simple-the engine's ignition and starting circuits, lighting, maybe a heater, and that was about it. But the electrical equipment that most of us take for granted today includes a long list of electron-gobbling devices such as air conditioning, engine cooling fans, halogen lights, power window and seats, air suspension compressors, elaborate stereo systems, and more. All that juice has to come from somewhere and that usually means a big-amp alternator. But before we get into selecting an alternator for your truck let's start with some terms and their definitions you should know:

Electricity: According to the electron theory that most of us heard about in junior high science, but couldn't have cared less about because we didn't have an old truck's electrical system to work on, atoms that make up all matter are made of protons, electrons, and neutrons. In simple terms, electricity is the flow of electrons from one atom to another through a conductor.

Voltage: Is electrical pressure, or the push, that sends electrons through a wire. It is measured in volts. 12 volts has greater electrical pressure than 6 volts.

Current: Is the volume of electrons flowing in a circuit. It's measured in amperes, or amps. If voltage is increased, more electrons flow. If you turn your car's headlights on with the engine off, as soon as you start the car and the charging system increases the system's voltage the lights get brighter; more voltage increases the current.

Resistance: Is the opposition to current flow. Resistance is like electrical friction and as a result it can create heat. In the case of a light bulb, the element gets so hot it glows white-hot which creates light. Increasing resistance reduces current flow; if your car's headlight switch has a dimmer for the dash lights, rotating the knob increases resistance, which reduces current flow and the lights dim. Resistance is measured in ohms and is represented by the symbol omega (O).

Conductor: Is something that current can flow through. However, some conductors are better than others. Copper has less resistance than aluminum, so copper is a better conductor of electricity.

Insulator: Is something current can't flow through. The porcelain on a spark plug is an example of an insulator. However, keep in mind some things can be a conductor or an insulator. As an example, air can be an insulator; the 12 volts of an automotive battery won't make a spark jump the gap of an ordinary spark plug; but after the coil boosts the voltage to 20,000-plus volts it will.

AC/DC: Alternating current and direct current refers to the direction the electricity travels-homes are AC, while batteries and automobiles are DC.

Types of Circuits
Circuits are the paths the electricity follows. As an example in automotive applications, electricity travels from the source (the battery) via conductors, through a switch to the load (the device that uses electricity, like a light bulb) to ground (the metal body or frame) that's used as a return path to the battery.

Closed: The preceding is an example of a complete circuit-there is a return path to the source. As a result, electrons flow through the circuit and operates the load, whatever that might be.