TM 9-8000
c. Some elements can lose electrons more readily
Section III. 11-9. Amperage(Current) and Voltage.
than other elements. Copper loses electrons easily, so
there are always many free electrons in a copper wire.
a. Amperes. Current flow, or electron flow, is
Other elements, such as iron, do not lose their electrons
measured in amperes. While it is normally considered
quite as easily, so there are fewer free electrons in an
that one ampere is a rather small current of electricity
iron wire (comparing it to a copper wire of the same
(approximately what a 100-watt light bulb would draw), it
size). Thus, with fewer free electrons, fewer electrons
is actually a tremendous flow of electrons. More than 6
can push through an iron wire; that is, the iron wire has
billion billion electrons a second are required to make up
more resistance than the copper wire.
one ampere.
d. A small wire (in thickness or cross-sectional
b. Voltage. Electrons are caused to flow by a
area) offers more resistance than a large wire. in the
difference in electron balance in a circuit; that is, when
small wire, there are fewer free electrons (because fewer
there are more electrons in one part of a circuit than in
atoms), and thus fewer electrons can push through.
another, the electrons move from the area where they
are concentrated to the area where they are lacking.
e. Most metals show an increase in resistance with
This difference in electron concentration is called
an increase in temperature, while most nonmetals show
potential difference, or voltage. The higher the voltage
a decrease in resistance with an increase in
goes, the greater the electron imbalance becomes. The
temperature. For example, glass (a nonmetal) is an
greater this electron imbalance, the harder the push on
excellent insulator at room temperature but is a very poor
the electrons (more electrons repelling each other) and
insulator when heated to red heat.
the greater the current of electrons in the circuit. When
there are many electrons concentrated at the negative
11-11. Ohm's Law.
terminal of a generator (with a corresponding lack of
electrons at the positive terminal), there is a much
a.
stronger repelling force on the electrons and,
The general statements about voltage,
consequently, many more electrons moving in the wire.
amperage, and ohms (para 11-9 and 11-10) can all be
This is exactly the same as saying that the higher the
related in a statement known as ohm's law, so named for
voltage, the more electric current will flow in a circuit, all
the scientist Georg Simon Ohm who first stated the
other things, such as resistance (para 11-10), being
relationship. This law says that voltage is equal to
equal.
amperage times ohms. Or, it can be stated as the
mathematical formula:
11-10. Resistance.
E=IxR
a.
Even though a copper wire will conduct
where E is volts, I is current in amperes, and R is
electricity with relative ease, it still offers resistance to
resistance in ohms.
For the purpose of solving
electron flow. This resistance is caused by the energy
problems, the ohms law formula can be expressed three
necessary to break the outer shell electrons free, and the
ways:
collisions between the atoms of the conductor and the
free electrons. It takes force (or voltage) to overcome
(1) To find voltage: E = IR
the resistance encountered by the flowing electrons.
This resistance is expressed in units called ohms. The
(2) To find amperage: l = E/R
resistance of a conductor varies with its length, cross-
sectional area, composition, and temperature.
(3) To find ohms: R = E/I
b. A long wire offers more resistance than a short
b. This formula is a valuable one to remember
wire of the same cross-sectional area. The electrons
because it makes understandable many of the things
have farther to travel.
that happen in an electric circuit. For instance, if the
voltage remains constant, the
11-10
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