Joule’s Law states that the power (P) of a DC device is the voltage (V) across that device multiplied by the current (I) through the device:
P = V I
So, the higher the current, the more power something uses. That means lower resistances consume more power. Power is measured in Watts. As an example, think of a 60 W halogen headlight bulb. Since the voltage of an automobile’s electrical system is 12 V, the headlight bulb pulls 5 amps (A) of current. (12 V * 5 A = 60 W).
Automotive Grounding Systems
You can see from the diagram above that current flows in a loop (or circuit). Electricity must flow into and out of something in order for it to work. In your house, this is accomplished by running two or three wires to all of your appliances. In a car, we only run one wire. How does that work?
Well, most cars are made up of metal—lots of metal. So, instead of running two wires to everything, we just run one wire to it and ground the second wire to the chassis. The battery and alternator are also grounded to the chassis. So, electricity flows from the battery, through the device (like a fuel pump), through the chassis, and back to the battery.
What a lot of people don’t realize is that wires themselves (and the automobile’s chassis) have a resistance, too. I’m going to redraw that diagram above with a second resistance (Rw) added in. Rw is going to represent the resistance of the wire and the chassis.
Usually, the resistance of a wire and the chassis is really small. For example, a 20-gauge wire has a resistance of only 0.69 ohms in 100 feet of wire. That’s such a small resistance that it is insignificant when dealing with most automotive electrical systems.