(1) Electron movement always takes place from a point of excess electrons
(negative charge) toward a point having a deficiency of electrons
(positive charge).
Since electrons are essentially negative charges,
current flow is taken to mean electron movement from a negative charge
toward a positive charge. This direction of current flow will be used
throughout this subcourse.
(2) Since current flow is based on the movements of electrons, current will
flow more easily through materials which have a large number of free
electrons. In a material such as copper there are many free electrons.
If an external force is used to make the electrons move, great numbers
of electrons will be put into motion.
Thus it is said that current
flows through copper easily, the conductivity of copper is high, or
that copper is an electrical conductor.
(3) In a material such as glass there are very few free electrons. If an
external force is used to make the electrons move, very few electrons
will be put into motion and consequently there will be very little
current flow.
It is said that substances such as glass are
nonconductors, that the conductivity of glass is low, or that
substances such as glass are good insulators.
(4) There are no perfect conductors and no perfect insulators. In general.
metals are good conductors, while rubber, mica, and glass are good
insulators.
(5) The flow of current is evidenced by one of the following effects:
(a) Heat.
Current, when flowing through a substance, produces heat.
For example, the heating element of an electric toaster becomes red
hot when electric current passes through it.
(b) Magnetic.
Current flowing through a coil of wire produces
a
magnetic effect very similar to the effects of an ordinary magnet.
(c) Chemical.
Current flowing through a liquid produces a chemical
effect.
An example of this effect is the charging of a storage
battery.
b. Voltage. Current, being electron flow, cannot exist until a force exerts
the necessary pressure to move the electrons along the conductor.
(1) In batteries, generators, and other sources of power, an excess of
electrons appears at one terminal, while the other terminal has a
deficiency of electrons.
The terminal that is deficient in electrons
exerts an attraction for free electrons, and the terminal with an
excess of electrons tends to repel electrons away from it.
This
combination establishes an electrical force to move the electrons
through a conductor. The unit of measurement is the volt.
(2) The terms used to denote this force, or electrical pressure,
are
electromotive force (EMF), voltage, and, potential difference.
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