Geoscience Reference
In-Depth Information
Note: No general formulas are available for solving series-parallel circuits because so many differ-
ent forms of these circuits exist.
Note: The total current in the series-parallel circuit depends on the effective resistance of the par-
allel portion and on the other resistances.
11.7.9 C onduCtors
Earlier we mentioned that electric current moves easily through some materials but with greater
difficulty through others. Three good electrical conductors are copper, silver, and aluminum (gen-
erally, we can say that most metals are good conductors). Today, copper is the material of choice
for electrical conductors. Under special conditions, certain gases are also used as conductors; for
example, neon gas, mercury vapor, and sodium vapor are used in various kinds of lamps.
The function of the wire conductor is to connect a source of applied voltage to a load resistance
with a minimum IR voltage drop in the conductor so most of the applied voltage can produce cur-
rent in the load resistance. Ideally, a conductor must have a very low resistance; a typical value for
a conductor such as copper is less than 1 ohm per 10 feet.
Because all electrical circuits utilize conductors of one type or another, in this section we discuss
the basic features and electrical characteristics of the most common types of conductors. Moreover,
because conductor splices and connections (and insulation of such connections) are also an essential
part of any electric circuit, they are also discussed.
11.7.9.1 Unit Size of Conductors
A standard (or unit size) of a conductor has been established to compare the resistance and size of
one conductor with another. The unit of linear measurement used (with regard to the diameter of a
piece of wire) is the mil (0.001 of an inch). A convenient unit of wire length is the foot. Thus, the
standard unit of size in most cases is the mil-foot; that is, a wire will have unit size if it has diameter
of 1 mil and a length of 1 foot. The resistance in ohms of a unit conductor or a given substance is
called the resistivity (or specific resistance ) of the substance. As a further convenience, gauge num-
bers are also used to compare the diameter of wires. The Browne and Sharpe (B&S) gauge was used
in the past; now the most commonly used gauge is the American Wire Gauge (AWG).
11.7.9.2 Square Mil
Figure 11.51 shows a square mil , which is a convenient unit of cross-sectional area for square or
rectangular conductors. As shown in Figure 11.51, a square mil is the area of a square, the sides of
which are 1 mil. To obtain the cross-sectional area in square mils of a square conductor, square one
side measured in mils. To obtain the cross-sectional area in square mils of a rectangular conductor,
multiply the length of one side by that of the other, each length being expressed in mils.
1 mil
1 mil
1 mil
1 mil
1
1 mil
(A) Square mil
(B) Circular mil
(C) Square and circular mil
FIGURE 11.51
(A) Square mil; (B) circular mil; (C) comparison of circular to square mil.
Search WWH ::




Custom Search