Learning Objective: Determine voltage, amperage, and resistance. Explain Ohm's law and describe the types of electrical circuits used in vehicles.

Electricity is measured in two ways - by the amount of current (number of electrons) flowing and by the push, or pressure, that causes current to flow. The push, or pressure, is caused by actions of the electrons. They repel each other. When electrons are concentrated in one place, their negative charges push against each other. If a path is provided for the electrons, they will flow away from the area where they are concentrated.

The pressure to make them move is called voltage. If there are many electrons concentrated in one spot, we say that there is high voltage. With high voltage, many electrons will flow, provided there is a path or conductor through which they can flow. The more electrons that flow, the greater the electric current. Electric current is measured in amperes. Resistance is the movement of electrons through a substance. Resistance is a fact of life in electric circuits. We want resistance in some circuits so that too much current (too many electrons) will not flow. In other circuits, we want as little resistance as possible so that high current can flow.

There is a definite relation between current (electron flow), voltage (current pressure), and resistance. As the electric pressure goes up, more electrons flow. Increasing the voltage increases the amperes ofcurrent. However, increasing the resistance decreases the amount of current that flows. These relationships can be summed up in a statement known as Ohm's law.

Electrons are caused to flow by a difference in electron balance in a circuit; that is, when there are more electrons in one part of a circuit than in the other, the electrons move from the area where they are concentrated to the area they are lacking. This difference in electron concentration is called potential difference, or voltage. The higher the voltage goes, the greater the electron imbalance becomes. The greater this electron imbalance, the harder the push on the electrons (more electrons repelling each other) and the greater the current of electrons in the circuit. When there are many electrons concentrated at the negative terminal of a generator (with a corresponding lack of electrons at the positive terminal), there is a much stronger repelling force on the electrons; con- sequently, many more electrons are moving in the wire. This is exactly the same as saying that the higher the voltage, the more the electric current will flow in a circuit, all other things, such as resistance, being equal.

Current flow, or electron flow, is measured in amperes. While we normally consider that one ampere is a rather small current of electricity (approximately what a 100-watt light bulb would draw), it is actually a tremendous flow of electrons. More than 6 billion electrons a second are required to make up one ampere.

A copper wire conducts electricity with relative ease; however, it offers resistance to electron flow. This resistance is caused by the energy required to free the outer shell of electrons and the collision between the atoms of the conductor and the free electrons. It takes electromotive force (emf) or voltage to overcome the resistance met by the flowing electrons. The basic unit of resistance is the ohm. The resistance of a conductor varies with its length, diameter, composition, and temperature. A long wire offers more resistance than a short wire of the same diameter; this is due to the electrons having farther to travel. Some materials can lose electrons more readily than others. Copper loses electrons easily, so there are always many free electrons in a copper wire. Other materials, such as iron, do not lose their electrons as easily, so there are fewer free electrons in an iron wire. However, fewer electrons can push through an iron wire; that is, the iron wire has more resistance than the copper wire. A wire with a small diameter offers more resistance than a wire with a large diameter. In the small diameter wire, there are fewer free electrons, and thus fewer electrons can push through. Most metals show an increase in resistance with an increase in temperature, while most nonmetals show a decrease in resistance with an increase in temperature.

Ohm's law is used to figure out the current (I), the voltage (E), and the resistance (R) in a circuit. This law states that voltage is equal to amperage times ohms. Or, it can be stated as the mathematical formula: E = I x R. For the purpose of solving problems, the Ohm's law formula can be expressed in three ways:

1. To find voltage: E = IR

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