WARNING
To avoid personal injury and/or vehicle damage, refer to the service precautions at the beginning of this section.
Electricity is based on the principle that electrons are attracted to protons. Electron movement can be created when the atomic structure of a material is forced to become imbalanced. Atoms are made up of an equal quantity of positively and negatively charged parts. The nucleus contains protons, with a positive charge, and neutrons with a neutral charge. The negative charge or electrons constantly orbit around the nucleus in valence rings.
If an electron were to be separated from an atom it would assume a net positive charge and become a positive ion. On the other hand, if an element were to "acquire" an electron it would have a net negative charge and become a negative ion. If we were to store these positive and negative ions in a container we would have a power source, or battery.
Electricity is the flow of electrons from a greater potential (more electrons) to a lesser potential (less electrons). If a path were provided for the electrons from the negative ions to flow to the positive ions each ion could then maintain its balanced condition. The pressure that the electrons exert when returning to its source is called voltage. When voltage (V) is measured with a voltmeter, the value displayed represents the attractive force, or electromotive force available to get the atoms in balance again.
Amperage is a measure of the actually quantity of electrons that flow from the net negative charge to the net positive charge. This movement of electrons, or current, is what actually does the work in an electrical circuit. Current flow is measured in units of Amperes, or Amps (A). Amperage is a time-based unit. One Amp is equal to 6.28 x 1028 electrons moving past one point in one second. When an ammeter is connected in series with a circuit, the actual quantity of electrons that flow through the circuit are measured.
Resistance is an element’s ability to oppose current flow. The resistance of an element depends upon its atomic structure- specifically how many electrons are held in orbit in the outermost or valence ring. to eight electrons can occupy the valence ring of an atom. When fewer electrons are present in the valence ring there is more "room" for electrons to flow across the surface of an atom.
Electrically speaking, elements can be categorized as conductors, insulators and semi-conductors. Conductors are elements with between one and three electrons in their valence ring. Insulators are elements that contain between five and eight valence ring electrons. Semi-conductors are elements that contain four electrons in their valence ring.
Impedance is something that restricts flow. If you put a number of connections along an electrical circuit, each connection becomes a source of resistance slowing the flow of electricity to its final destination. A common analogy would be a coke bottle; if you turn a coke bottle upside down to empty its contents it takes a longer period of time to do so. There is resistance within the bottle to empty a large volume of fluid through a low volume orifice. Now, take the same bottle turned on its side so there is air space present at the mouth of the bottle while pouring out the fluid. The process takes less time due to low impedance (less resistance). The resistance we find in an electrical circuit is measured in Ohms. The resistance of a circuit varies depending on the amount and type of components used in the circuit. an Ohmmeter is used to measure resistance. Current is applied to the component from a power source (battery) in the meter. The voltage that returns to the meter is converter to a resistive value.
The main factors which determine resistance are the material used, the size and cross section of the wire, the length of the wire and the temperature that these items operate. Some materials have more resistance than others. Those with high resistance are said to be insulators. Rubber materials (or rubber-like plastics) are some of the most common insulators used in vehicles as they have a very high resistance to electricity. Very low resistance materials are said to be conductors. Copper wire is among the best conductors. Most automotive wiring is made of copper. Silver is actually a superior conductor to copper and is used in some relay contacts, but its high cost prohibits its use as common wiring. Airbag systems commonly use gold plated terminal to ensure that current will readily flow through the system. Gold, while cost prohibitive, will not react to air and contaminants that can contribute to unwanted voltage drops.
Larger diameter wires provide more surface area for current flow. The larger the wire size being used, the less resistance the wire will have. Solid conductors provide less surface area when compared to stranded conductors. Stranded conductors have the capacity to carry greater currents when compared to solid conductors of the same gauge (size). This is because the individual strands of a stranded conductor contribute to greater surface area. This is why components which use large amounts of electricity have larger wires supplying current to them. All elements offer some degree of resistance. While copper wire, as an example, is a conductor, it too has a resistive value. For a given thickness of wire, the longer the wire, the greater the resistance. The shorter the wire, the less the resistance. When determining the proper wire for a circuit, both size (gauge) and length must be considered to design a circuit that can handle the current needed to provide enough power to the component being powered. With many materials, the higher the temperature, the greater the resistance (positive temperature coefficient). Some materials exhibit the opposite trait of lower resistance with higher temperatures (negative temperature coefficient). These principles are used in many of the sensors on the engine. As voltage flows through the wiring, these varying properties effect the overall performance of the electrical system, but the current is what actually does the work.
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.
| This example illustrates a simple circuit. When the switch is closed, power from the positive (+) battery terminal flows through the fuse and the switch, and then to the light bulb. The light illuminates and the circuit is completed through the ground wire back to the negative (–) battery terminal. In reality, the two ground points shown in the illustration are attached to the metal frame of the vehicle, which completes the circuit back to the battery
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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 frame of the vehicle. Since the negative (–) battery cable is attached to the frame (made of electrically conductive metal), the frame 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.
Electricity is the flow of electrons-the subatomic particles that constitute the outer shell of an atom. Electrons spin in an orbit around the center core of an atom. The center core is comprised of protons (positive charge) and neutrons (neutral charge). Electrons have a negative charge and balance out the positive charge of the protons. When an outside force causes the number of electrons to unbalance the charge of the protons, the electrons will split off the atom and look for another atom to balance out. If this imbalance is kept up, electrons will continue to move and an electrical flow will exist.
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 and the wire is the pipe.
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). You can compare amperage to the volume of water flowing through a pipe. 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.
Water pressure is measured in units such as pounds per square inch (psi); The electrical pressure is measured in units called volts (v). When a voltmeter is connected to a circuit, it is measuring the electrical pressure.
The actual flow of electricity depends not only on voltage and amperage, but also on the resistance of the circuit. The higher the resistance, the higher the force necessary to push the current through the circuit. The standard unit for measuring resistance is an ohm. Resistance in a circuit varies depending on the amount and type of components used in the circuit. The main factors which determine resistance are:
There is a direct relationship between current, voltage and resistance. 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) time’s resistance (R): E=I x R
Other 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.
The amount of work that the electricity can perform is expressed as power. The unit of power is the watt (w). The relationship between power, voltage and current is expressed as:
Power (w) is equal to amperage (I) time’s voltage (E): W=I x E
This is only true for direct current (DC) circuits; the alternating current formula is a tad different, but since the electrical circuits in most vehicles are DC type, we need not get into AC circuit theory.
