Fundamental Engineering Concepts - Mathematics and Statistics for the Environment - page 297

FIGURE 11.62 Schematic symbol for an inductor.

The henry is a large unit of inductance and is used with relatively large inductors. The unit
employed with small inductors is the millihenry (mh). For still smaller inductors, the unit of induc-
tance is the microhenry (μh).

10.7.12.1 Self-Inductance
As previously explained, current flow in a conductor always produces a magnetic field surrounding,
or linking with, the conductor. When the current changes, the magnetic field changes, and an emf is
induced in the conductor. This emf is referred to as a self-induced emf because it is induced in the
conductor carrying the current.

Note: Even a perfectly straight length of conductor has some inductance.

The direction of the induced emf has a definite relation to the direction in which the field that
induces the emf varies. When the current in a circuit is increasing, the flux linking with the circuit is
increasing. This flux cuts across the conductor and induces an emf in the conductor in such a direction
as to oppose the increase in current and flux. This emf is sometimes referred to as counterelectromo-
tive force (cemf). The two terms are used synonymously throughout this manual. Likewise, when the
current is decreasing, an emf is induced in the opposite direction and opposes the decrease in current.

Note: The effects just described are summarized by L enz's law , which states that the induced emf
in any circuit is always in a direction opposed to the effect that produced it.

Shaping a conductor so the electromagnetic field around each portion of the conductor cuts across
some other portion of the same conductor increases inductance, as shown in its simplest form in Figure
11.63A. The conductor is looped so two portions of the conductor lie adjacent and parallel to one
another. These portions are labeled conductor 1 and conductor 2. When the switch is closed, electron
flow through the conductor establishes a typical concentric field around all portions of the conductor.
The field is shown in a single plane (for simplicity) that is perpendicular to both conductors. Although
the field originates simultaneously in both conductors, it is considered as originating in conductor 1,
and its effect on conductor 2 will be noted. With increasing current, the field expands outward, cut-
ting across a portion of conductor 2. The dashed arrow shows the resultant induced emf in conductor
2. Note that it is in opposition to the battery current and voltage, according to Lenz's law. In Figure
11.63B, the same section of conductor 2 is shown but with the switch open and the flux collapsing.

Note: In Figure 11.63, the important point to note is that the voltage of self-induction opposes both
changes in current. It delays the initial buildup of current by opposing the battery voltage
and delays the breakdown of current by exerting an induced voltage in the same direction in
which the battery voltage acted.

Four major factors affect the self-inductance of a conductor, or circuit:

1. Number of turns —Inductance depends on the number of wire turns. Wind more turns to
increase inductance; take turns off to decrease the inductance. Figure 11.64 compares the
inductance of two coils made with different numbers of turns.

2. Spacing between turns —Inductance depends on the spacing between turns, or the length
of the inductor. Figure 11.65 shows two inductors with the same number of turns. The
turns of the first inductor have a wide spacing. The turns of the second inductor are close