Learning Objective: Determine voltage, amperage,
and resistance. Explain Ohms law and describe the
types of electrical circuits used in vehicles.
Electricity is measured in two waysby 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
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.
Ohms 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
Ohms law formula can be expressed in three ways:
1. To find voltage: E = IR