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Electrical Basics
By Jim Mellema

It seems that many otherwise good mechanics ask a lot of questions about electricity. Maybe it’s hard to understand because we can’t see it? This text will give an understanding of how and why electricity works, and how to measure it, and how to use that knowledge to keep the electrical system of your car functioning.

Glossary

Voltage           (E) Has been described as the “pressure in a water hose”, which is a semi good analogy. When I learned electronics in the Navy, voltage was described as a “difference in potential.” Note the word “difference.” To have voltage, 2 points must be electrically different. One of those points is often ground, the other any place where your cars battery voltage is connected. If your battery is laying on the floor and you measure it, it should read 12 volts. Note, we have a potential here, the battery is on the floor, not connected to anything but the meter. The potential exists to get some work done, and we can measure that, but nothing is happening yet.

Current          (I) Current (measured in AMPS or Amperes) is what you have when electricity is actually doing work. If you have current, your electricity is doing something for you. If you have current, you have a complete circuit from the battery’s positive terminal, through a load that is performing some work, and then connected to the battery’s negative terminal to complete the circuit. Electricity cannot ever do any work without a complete circuit.

Resistance      (R) Resistance is anything that “resists” the flow of electricity. These are typically the things that actually do the work for us, such as a headlight or a heater blower. These are the loads in the system. Wires have resistance, too, but a very small amount. Anything that conducts electricity has at least some resistance to doing so. A resistance, by it’s nature, will convert some of the electrical energy into action, and some into heat. Heat is always a product of using electricity. Resistance is measured in OHMs.

OHM’s law     Defines how electricity behaves. Voltage equals Current times Resistance, always and without exception. If our voltage is 12, and we are moving 6 Amps of electricity, our resistance has to be 2 ohms. You can use algebra to shuffle the formula around so the one you don’t know is alone. Since E = I * R, then R = E / I and I = E / R.

Power             (W) is measured in Watts, and is defined as the AMOUNT of electricity used. We can’t measure power, but we can figure it. Power = voltage times current. Again, we can use simple algebra to shuffle the formula. As an example, stay with me here. Our low beam headlight is a 55 watt  bulb. 55 watts divided by 12 volts = 4.6 amps. If we apply OHMs law, we can determine the resistance of the headlight bulb. 12 volts divided by 4.6 amps = 2.6 Ohms.

These definitions and behaviours define how electricity works. They are fact and they are absolute, just like gravity makes things fall down not up. Future articles will discuss how to measure and apply these facts in circuits, and how to use these facts to troubleshoot malfunctions.

 
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