BASIC ELECTRICAL THEORY 2

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ELECTRICAL TERMINOLOGY

 

Knowledge of key electrical terminology is necessary to fully understand
principles in electrical science.

Conductors

Conductors are materials with electrons that are loosely bound to their atoms, or materials that
permit free motion of a large number of electrons. Atoms with only one valence electron, such
as copper, silver, and gold, are examples of good conductors. Most metals are good conductors.

Insulators

Insulators, or nonconductors, are materials with electrons that are tightly bound to their atoms
and require large amounts of energy to free them from the influence of the nucleus. The atoms
of good insulators have their valence shells filled with eight electrons, which means they are
more than half filled. Any energy applied to such an atom will be distributed among a relatively
large number of electrons. Examples of insulators are rubber, plastics, glass, and dry wood.

Resistors

Resistors are made of materials that conduct electricity, but offer opposition to current flow.
These types of materials are also called semiconductors because they are neither good conductors
nor good insulators. Semiconductors have more than one or two electrons in their valence shells,
but less than seven or eight. Examples of semiconductors are carbon, silicon, germanium, tin, and
lead. Each has four valence electrons.

Voltage

The basic unit of measure for potential difference is the volt (symbol V), and, because the volt
unit is used, potential difference is called voltage. An object’s electrical charge is determined
by the number of electrons that the object has gained or lost. Because such a large number of
electrons move, a unit called the “coulomb” is used to indicate the charge. One coulomb is equal
to 6.28 x 10 to the 18th power (billion, billion) electrons. For example, if an object gains one coulomb of
negative charge, it has gained 6,280,000,000,000,000,000 extra electrons. A volt is defined as
a difference of potential causing one coulomb of current to do one joule of work. A volt is also
defined as that amount of force required to force one ampere of current through one ohm of
resistance. The latter is the definition with which we will be most concerned in this module.

Current

The density of the atoms in copper wire is such that the valence orbits of the individual atoms
overlap, causing the electrons to move easily from one atom to the next. Free electrons can drift
from one orbit to another in a random direction. When a potential difference is applied, the
direction of their movement is controlled. The strength of the potential difference applied at each
end of the wire determines how many electrons change from a random motion to a more
directional path through the wire. The movement or flow of these electrons is called electron
current flow or just current.
To produce current, the electrons must be moved by a potential difference. The symbol for
current is (I). The basic measurement for current is the ampere (A). One ampere of current is
defined as the movement of one coulomb of charge past any given point of a conductor during
one second of time.
If a copper wire is placed between two charged objects that have a potential difference, all of the
negatively-charged free electrons will feel a force pushing them from the negative charge to the
positive charge. This force opposite to the conventional direction of the electrostatic lines of
force is shown in Figure 9.

electron flow

The direction of electron flow, shown in Figure 10, is from the negative (-) side of the battery,
through the wire, and back to the positive (+) side of the battery. The direction of electron flow
is from a point of negative potential to a point of positive potential. The solid arrow shown in
Figure 10 indicates the direction of electron flow. As electrons vacate their atoms during electron
current flow, positively charged atoms (holes) result. The flow of electrons in one direction
causes a flow of positive charges. The direction of the positive charges is in the opposite
direction of the electron flow. This flow of positive charges is known as conventional current
and is shown in Figure 10 as a dashed arrow. All of the electrical effects of electron flow from
negative to positive, or from a higher potential to a lower potential, are the same as those that
would be created by a flow of positive charges in the opposite direction. Therefore, it is
important to realize that both conventions are in use and that they are essentially equivalent; that
is, all effects predicted are the same. In this text, we will be using electron flow in our
discussions.

difference of potential

Generally, electric current flow can be classified as one of two general types: Direct Current
(DC) or Alternating Current (AC). A direct current flows continuously in the same direction.
An alternating current periodically reverses direction. We will be studying DC and AC current
in more detail later in this text. An example of DC current is that current obtained from a
battery. An example of AC current is common household current.

Real and Ideal Sources

An ideal source is a theoretical concept of an electric current or voltage supply (such as a
battery) that has no losses and is a perfect voltage or current supply. Ideal sources are used for
analytical purposes only since they cannot occur in nature.
A real source is a real life current or voltage supply that has some losses associated with it.

 

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