See Figure 1
For any 12 volt, negative ground, electrical system to operate, the electricity must travel in a complete circuit. This simply means that current (power) from the positive terminal (+) of the battery must eventually return to the negative terminal (-) of the battery. Along the way, this current will travel through wires, fuses, switches and components. If, for any reason, the flow of current through the circuit is interrupted, the component fed by that circuit will cease to function properly.
Perhaps the easiest way to visualize a circuit is to think of connecting a light bulb (with two wires attached to it) to the battery - one wire attached to the negative (-) terminal of the battery and the other wire to the positive (+) terminal. With the two wires touching the battery terminals, the circuit would be complete and the light bulb would illuminate. Electricity would follow a path from the battery to the bulb and back to the battery. It's easy to see that with longer wires on our light bulb, it could be mounted anywhere. Further, one wire could be fitted with a switch so that the light could be turned on and off.
The normal automotive circuit differs from this simple example in two ways. First, instead of having a return wire from the bulb to the battery, the current travels through the chassis of the vehicle. Since the negative (-) battery cable is attached to the chassis and the chassis is made of electrically conductive metal, the chassis of the vehicle can serve as a ground wire to complete the circuit. Secondly, most automotive circuits contain multiple components which receive power from a single circuit. This lessens the amount of wire needed to power components on the vehicle.
THE WATER ANALOGY
Electricity is the flow of electrons - hypothetical particles thought to constitute the basic "stuff" of electricity. Many people have been taught electrical theory using an analogy with water. In a comparison with water flowing through a pipe, the electrons would be the water.
The flow of electricity can be measured much like the flow of water through a pipe. The unit of measurement used is amperes, frequently abbreviated as amps (a). When connected to a circuit, an ammeter will measure the actual amount of current flowing through the circuit. When relatively few electrons flow through a circuit, the amperage is low. When many electrons flow, the amperage is high.
Just as water pressure is measured in units such as pounds per square inch (psi), electrical pressure is measured in units called volts (v). When a voltmeter is connected to a circuit, it is measuring the electrical pressure. The higher the voltage, the more current will flow through the circuit. The lower the voltage, the less current will flow.
While increasing the voltage in a circuit will increase the flow of current, the actual flow depends not only on voltage, but also on the resistance of the circuit. Resistance is the amount of force necessary to push the current through the circuit. The standard unit for measuring resistance is an ohm (W or omega). Resistance in a circuit varies depending on the amount and type of components used in the circuit. The main factors which determine resistance are:
The preceding definitions may lead the reader into believing that there is no relationship between current, voltage and resistance. Nothing can be further from the truth. The relationship between current, voltage and resistance can be summed up by a statement known as Ohm's law.
Voltage (E) is equal to amperage (I) times resistance (R): E=I x ROther forms of the formula are R=E/I and I=E/R
In each of these formulas, E is the voltage in volts, I is the current in amps and R is the resistance in ohms. The basic point to remember is that as the resistance of a circuit goes up, the amount of current that flows in the circuit will go down, if voltage remains the same.