Current, Voltage, and Resistance
All materials are composed of one or more elements. An
element is a material made up of one type of atom. Elements
are often identified by the number of protons and electrons in
one atom of the element. A hydrogen atom, for example, has
only one electron and one proton. An aluminum atom has 13
electrons and 13 protons. An atom with an equal number of
electrons and protons is electrically neutral.
Electrical ChargesElectrons in the outer band of an atom can be easily displaced
by the application of external force. When an electron is forced
out of an atom, the atom it leaves behind has more protons
than electrons. This atom now has apositive charge. Atoms or
molecules of a material can also have an excess of electrons,
giving the material anegative charge. A positive or negative
charge is caused by an absence or excess of electrons. The
number of protons remains constant.
Attraction and Repulsion of Electric ChargesThe old saying, “opposites attract,” is true when dealing with
Electric Chargeselectric charges. Charged bodies have an invisible electric field
around them. These invisible lines of force cause attraction or
repulsion. When two like-charged bodies are brought together,
their electric fields repel one body from the other. When two
unlike-charged bodies are brought together, their electric fields
The interaction of electrical charges is dependent upon
both the both the amount of each charge and the distance
between charges. The greater the amount of each charge the
more charged objects attract or repel one another. However
this interaction is inversely proportional to the square of the
distance between charges.
CurrentThe flow of free electrons in a material from one atom to the
next atom in the same direction is referred to ascurrent and
is designated by the symbolI. The amount of current flowing
is determined by the number of electrons that pass through a
cross-section of a conductor in one second.
Keep in mind that atoms are very small. It takes about 1024
atoms to fill one cubic centimeter of a copper conductor. This
means that trying to represent even a small value of current as
electrons would result in an extremely large number.
For this reason, current is measured inamperes, often
shortened toamps. The letter A is the symbol for amps. A
current of one amp means that in one second about 6.24 x 1018
electrons move through a cross-section of conductor. These
numbers are given for information only and you do not need
to be concerned with them. It is important, however, to
understand the concept of current flow.
attract one body to the other.
Because in practice it is common to find wide variations in the
magnitude of electrical quantities, electrical units often have
metric unit prefixes that represent powers of ten. The following
chart shows how three of these prefixes are used to represent
large and small values of current.
Direction of Current FlowSome sources distinguish between electron flow and current
flow. The conventional current flow approach ignores the flow
of electrons and states that current flows from positive to
negative. To avoid confusion, this book uses theelectron flow
concept which states that electrons flow from negative to
VoltageThe force required to make electricity flow through a conductor
is called adifference in potential, electromotive force (emf),
orvoltage. Voltage is designated by the letter E or the letter V.
The unit of measurement for voltage is thevolt which is also
designated by the letterV.
A voltage can be generated in various ways. A battery uses an
electrochemical process. A car’s alternator and a power plant
generator utilize a magnetic induction process. All voltage
sources share the characteristic of an excess of electrons at
one terminal and a shortage at the other terminal. This results in
a difference of potential between the two terminals.
For adirect current (DC) voltage source, the polarity of the
terminals does not change, so the resulting current constantly
flows in the same direction.
ResistanceA third factor that plays a role in an electrical circuit is
resistance. All material impedes the flow of electrical current
to some extent. The amount of resistance depends upon
the composition, length, cross-section and temperature of
the resistive material. As a rule of thumb, the resistance of a
conductor increases with an increase of length or a decrease of
cross-section. Resistance is designated by the symbolR. The
unit of measurement for resistance is theohm (W).
Resistorsare devices manufactured to have a specific
resistance and are used in a circuit to limit current flow and to
reduce the voltage applied to other components. A resistor is
usually shown symbolically on an electrical drawing in one of
two ways, a zigzag line or an unfilled rectangle.
In addition to resistors, all other circuit components and the
conductors that connect components to form a circuit also have
The basic unit for resistance is 1 ohm; however, resistance is often
expressed in multiples of the larger units shown in the following
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