There are three basic terms used in connection with the study of electricity: Current, Voltage and Resistance.

CURRENT

In a conductor, free electrons move or drift from atom to atom. This drift, or general movement of electrons along the conductor, is called an electric current. An electric current, therefore, is the movement or flow of electrons or negative charges through a conductor. Current is represented by the symbol "I".

Early experimenters recognized that an electric current was a movement of electrons or charges through a conductor. They decided to say that the flow of current was from positive to negative. This is referred to as "conventional current flow". Subsequent research has shown that in fact, electrons flow from negative to positive.

AC/DC

There are two types of electrical current: Direct current and alternating current. When the negative and positive poles of a current source are stationary, the flow of electrons is in one direction only and is referred to as direct current or DC. Batteries and DC generators supply this type of current. An automobile battery, for example, provides direct current for the engine starter.

When current is generated so that the positive and negative poles are constantly interchanging, then the current in the circuit is constantly altering direction, and is known as alternating current or AC. An AC alternator in an automobile provides this type of current, then it's changed to DC by the rectifying diodes. Most of the electricity used in homes and industries is AC because it can be transmitted over long distances at high voltage (pressure) with little loss of energy.

AMPERES

Amperes or amps for short, are the units of measurement for electrical current. Current flow through a conductor can be compared to the flow of water through a pipe. The flow of water past a given point is recorded in cubic meters per hour or cubic feet per hour) while the flow of electricity is measured in amperes per second. When a given quantity of electrons (6.28 x 10¹⁸) moves or flows past a given point in one second, then one ampere of current is flowing.

VOLTAGE

In order for electrons to flow or move through a conductor, a force or pressure is needed. This force is called voltage, potential difference, or electromotive force (EMF). Voltage can be compared to the pressure required to force water through a pipe. As with water, the greater the pressure or voltage, the greater the volume or current through a given pipe or conductor. The symbol for electromotive force is "E". Voltage is measured in units called volts.

One volt is required to force one amp through a resistance of one ohm.

RESISTANCE

Electrical resistance is defined as the opposition offered by materials to the flow of electrons or electric current through that material. As you already know, conductors have many free and mobile electrons, while insulators have few free electrons. Conductors offer low resistance or low opposition to the flow of electrons. Insulators offer high resistance or high opposition to the flow of electrons. The symbol for electrical resistance is "R". Resistance is measured in ohms. One ohm of resistance allows one amp to flow when the voltage is one volt.

SUMMARY

As you can see from the definitions, the amount of current that actually flows in a conductor is dependent both on voltage and on resistance. A high voltage forces more current through a given conductor than does a low voltage. A given voltage forces more current through a low-resistance conductor than through a high-resistance conductor.

OHM'S LAW

There is a close relationship between voltage, resistance and current in electrical circuits. Ohm's Law identifies the basic, most important relationship between these three. This law states:

The current in an electrical circuit is directly proportional to the applied electromotive force (voltage) and inversely proportional to the resistance.

If you increase the voltage in a given circuit, the current also increases. If the voltage is decreased, the current decreases.

If you increase the resistance in a given circuit, the current decreases.

Similarly, if you decrease the resistance the current increases.

Ohm's Law is a simple and powerful mathematical tool for helping us analyze electric circuits, but it has limitations, and we must understand these limitations in order to properly apply it to real circuits. For most conductors, resistance is a rather stable property, largely unaffected by voltage or current. For this reason, we can regard the resistance of most circuit components as a constant, with voltage and current being inversely related to each other.

The phenomenon of resistance changing with variations in temperature is one shared by almost all metals, of which most wires are made. For most applications, these changes in resistance are small enough to be ignored. In the application of metal lamp filaments, the change happens to be quite large. This is one example of "nonlinearity" in electric circuits.

SERIES CIRCUITS

A series circuit is a circuit in which all the components are connected in line or in series with each other. There is only one path for the electrons to flow from the source through the load and back to the source of supply.  Series circuit

PARALLEL CIRCUITS

This type of circuit combines the two previous circuits into one operating system.  Series Parallel circuit

RESISTANCE

Ohm's Law can be applied to each part of any circuit as well as to the entire circuit. The total circuit resistance of a series circuit is equal to the sum of the resistors in that circuit. The total resistance in a parallel circuit will always be less than the value of the smallest resistance in the circuit, since each resistor or load forms an independent branch circuit. Use this as a quick check when doing resistance calculations on parallel circuits.

If only the resistance values of the parallel circuit are known, the total resistance of a parallel circuit can be found by using the electrical calculators.

The total resistance of a series-parallel circuit will be the total resistance of the parallel part, plus the series resistance.

VOLTAGE DROP

Because every electrical load in a circuit offers some resistance, voltage is reduced as it moves the current through the load.

Voltage is electrical energy, and as it moves through a load, some of the electrical energy is changed to another form of energy, such as light, heat, or motion.

If you measure the voltage on both sides of a load, you can see how much voltage has been used to move the current through the load. This is called voltage drop.

If you measure the voltage drop at every load in a circuit and add the measurements, they will equal the original voltage applied. No voltage disappears; it is just changed into a different form of energy by the resistance of the load.

Voltage drop is also called IR drop, because it can be calculated using Ohms Law. Stating: E = I X R

ELECTRICAL FAULTS

The problems which stop or affect current flow fall into two categories: High-resistance faults and low resistance faults.

High-Resistance Faults

A high-resistance connection can be caused by corrosion, a damaged wire, a defective part, or a loose connection. Because of the increase in resistance, the current flow is less than what is required to properly operate the loads in the circuit.

Any resistance in the circuit will show a small voltage drop. Because there is some resistance across any conductor, there are maximum expected voltage drops for parts of a circuit that are not the load in a 12 volt system.

These are:
.2 volt across a high current wire or cable, such as a battery cable.
.3 volt across a switch.
.1 volt across a wiring connector.
.1 volt across a ground connection.

An open circuit results from a broken wire, no contact between connectors or a defective part. The resistance across an open circuit, or simply an "open", is infinite, so no current can flow.

Low-Resistance Faults

A shorted circuit, or "short" is the result of an unwanted connection between two conductors. It allows current to bypass all or part of the normal circuit and normal resistance. A short to ground is the result of an unwanted connection between a conductor and ground. All or part of the normal circuitry and resistance is bypassed. From the point where the short occurs, the current bypasses all remaining circuit conductors and loads and flows directly to ground. The short circuit path is simply a short to ground.

Because automotive electrical systems are single-wire systems using a common frame ground, most short circuits occur as shorts to ground.

SIMPLE TEST DEVICES

You can do a lot of troubleshooting using simple test equipment, like the following:

The Jumper Lead

The jumper wire is technically a continuity tester, but it does not give you any electrical output. If the circuit works properly after you install the jumper wire, you know that there is an open in the circuit.

A jumper wire must never be used to bypass an item that has resistance, or in a way that would ground a hot lead. This would reduce circuit resistance, allowing excessive current flow which could damage loads and conductors, or even start an electrical fire. Many jumper wires include an inline fuse or circuit breaker to protect the circuit.

12-Volt Test Light

This device is similar to a jumper wire but it contains a bulb that glows when there is power in the circuit or to the component being checked. This type of test lamp is used to check for open circuits and relies upon the vehicle's battery to power the circuit or component being tested. It is not polarity sensitive and can be connected either way in a circuit.

It is not a good idea, however, to use a low impedance 12-volt test light on electronic components, because the current draw added by the test light may damage the component.

WIRING DIAGRAMS

Wiring diagrams make tracing wires easier by identifying the wire colour and tracer colour. Wiring diagrams also allow you to separate circuits without touching them and to perform some basic testing in your head without using instruments.

Understanding the way the circuit functions and knowing what could happen if some portion were disconnected or some circuits were malfunctioning can save considerable time in testing.

There are four places that problems can occur in a circuit:

In the power source.
Between the load and the power.
In the load.
Between the load and the ground.

SAFETY PRECAUTIONS

With advanced technology, computerized and solid state components have replaced many of the old style controls and systems. Improper testing and repair procedures can cause incredible damage to sensitive and sophisticated equipment. Failure to follow special testing rules laid out by the vehicle manufacturer's service manual can be very expensive.

Remember that electrical testing techniques and automobile systems change rapidly.