Ohm’s Law and Circuits
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FIGURE 36-23
Current flow is the result of voltage and resistance.

Ohm’s law helps us to understand the relationship between volts, amps, and ohms. They always have to balance out. If they do not, you made a mistake. Ohm’s law tells us that it takes 1 volt to push 1 amp through 1 ohm of resistance. That gives us the relationship among the three units. Volts and resistance are physical things. Volts are the surplus of electrons creating electrical pressure. Resistance is the physical restriction of the conductor. Amps are the amount of electrons moved. That means that amps are the result of both the volts and resistance. For example, if resistance doubles and voltage stays the same, current flow must be cut in half. To illustrate this relationship, consider the following: If you steadily push (voltage) a loaded wheelbarrow twice as far up a hill (ohms), you will only move half the amount of dirt in a given amount of time Figure 36-23. It is the same with the electrical circuit: If the pressure (volts) stays the same but the restriction (ohms) doubles, then the electrons (amps) will be reduced by half. Conversely, if the voltage is doubled and the resistance stays the same, then amperage will double.

If you understand the preceding, then you are well on your way to diagnosing electrical problems. Here is a question. If the amps in a circuit are lower than they should be, what are the two possible causes? First, the source voltage could be low. In that case, you would use a voltmeter to measure the source voltage at the battery. The second possibility is that the resistance in the circuit is too high. In that case, you would use a voltmeter to see if there is an excessive voltage drop on both sides of the circuit that would be caused by high resistance. If the voltage drop is within specifications, you would use an ohmmeter to measure the resistance of the load.

Most of the time when circuits fail it is because the current flow is too low or nonexistent. Yes, it is possible that the battery is dead or discharged, but more likely there is high resistance as a voltage drop either in the feed side or the ground side of the circuit. Or the electrical device has too much resistance or is open. Performing a voltage drop test on both sides of the circuit will quickly identify if there is an excessive voltage drop present. And a resistance check of the electrical device will indicate a high resistance or open condition in the component. Knowing the specifications that go along with these steps will enable you to repair more than half of the electrical problems on a vehicle.

Circuits are made up of components and interconnecting conductors, such as wires, arranged to manage and control the flow of electrons to perform specific electrical tasks. Understanding circuits and how to take electrical measurements is essential to perform electrical repair activities. Circuits come in two basic configurations: series circuits and parallel circuits. The two types can also be combined into what is called a series-parallel circuit. Understanding how electricity behaves within each of these circuits will help you know how the circuit operates and how to approach diagnosis. It will also allow you to apply Ohm’s law correctly to each type of circuit. We will look at the types of circuits further after finishing our discussion of Ohm’s law.