Magnetic fields and inductance
Whenever electrons flow through a conductor,
a magnetic field will develop around that conductor. This
effect is called electromagnetism. Magnetic fields
effect the alignment of electrons in an atom, and can cause
physical force to develop between atoms across space just as
with electric fields developing force between electrically
charged particles. Like electric fields, magnetic fields can
occupy completely empty space, and affect matter at a
distance.
Fields have two measures: a field force
and a field flux. The field force is the
amount of "push" that a field exerts over a certain
distance. The field flux is the total quantity, or
effect, of the field through space. Field force and flux are
roughly analogous to voltage ("push") and current (flow)
through a conductor, respectively, although field flux can
exist in totally empty space (without the motion of
particles such as electrons) whereas current can only take
place where there are free electrons to move. Field flux can
be opposed in space, just as the flow of electrons can be
opposed by resistance. The amount of field flux that will
develop in space is proportional to the amount of field
force applied, divided by the amount of opposition to flux.
Just as the type of conducting material dictates that
conductor's specific resistance to electric current, the
type of material occupying the space through which a
magnetic field force is impressed dictates the specific
opposition to magnetic field flux.
Whereas an electric field flux between two
conductors allows for an accumulation of free electron
charge within those conductors, an electromagnetic field
flux allows for a certain "inertia" to accumulate in the
flow of electrons through the conductor producing the field.
Inductors are components designed to
take advantage of this phenomenon by shaping the length of
conductive wire in the form of a coil. This shape creates a
stronger magnetic field than what would be produced by a
straight wire. Some inductors are formed with wire wound in
a self-supporting coil. Others wrap the wire around a solid
core material of some type. Sometimes the core of an
inductor will be straight, and other times it will be joined
in a loop (square, rectangular, or circular) to fully
contain the magnetic flux. These design options all have
effect on the performance and characteristics of inductors.
The schematic symbol for an inductor, like
the capacitor, is quite simple, being little more than a
coil symbol representing the coiled wire. Although a simple
coil shape is the generic symbol for any inductor, inductors
with cores are sometimes distinguished by the addition of
parallel lines to the axis of the coil. A newer version of
the inductor symbol dispenses with the coil shape in favor
of several "humps" in a row:
As the electric current produces a
concentrated magnetic field around the coil, this field flux
equates to a storage of energy representing the kinetic
motion of the electrons through the coil. The more current
in the coil, the stronger the magnetic field will be, and
the more energy the inductor will store.
Because inductors store the kinetic energy
of moving electrons in the form of a magnetic field, they
behave quite differently than resistors (which simply
dissipate energy in the form of heat) in a circuit. Energy
storage in an inductor is a function of the amount of
current through it. An inductor's ability to store energy as
a function of current results in a tendency to try to
maintain current at a constant level. In other words,
inductors tend to resist changes in current. When
current through an inductor is increased or decreased, the
inductor "resists" the change by producing a voltage
between its leads in opposing polarity to the change.
To store more energy in an inductor, the
current through it must be increased. This means that its
magnetic field must increase in strength, and that change in
field strength produces the corresponding voltage according
to the principle of electromagnetic self-induction.
Conversely, to release energy from an inductor, the current
through it must be decreased. This means that the inductor's
magnetic field must decrease in strength, and that change in
field strength self-induces a voltage drop of just the
opposite polarity.
Just as Isaac Newton's first Law of Motion
("an object in motion tends to stay in motion; an object at
rest tends to stay at rest") describes the tendency of a
mass to oppose changes in velocity, we can state an
inductor's tendency to oppose changes in current as such:
"Electrons moving through an inductor tend to stay in
motion; electrons at rest in an inductor tend to stay at
rest." Hypothetically, an inductor left short-circuited will
maintain a constant rate of current through it with no
external assistance:
Practically speaking, however, the ability
for an inductor to self-sustain current is realized only
with superconductive wire, as the wire resistance in any
normal inductor is enough to cause current to decay very
quickly with no external source of power.
When the current through an inductor is
increased, it drops a voltage opposing the direction of
electron flow, acting as a power load. In this condition the
inductor is said to be charging, because there is an
increasing amount of energy being stored in its magnetic
field. Note the polarity of the voltage with regard to the
direction of current:
Conversely, when the current through the
inductor is decreased, it drops a voltage aiding the
direction of electron flow, acting as a power source. In
this condition the inductor is said to be discharging,
because its store of energy is decreasing as it releases
energy from its magnetic field to the rest of the circuit.
Note the polarity of the voltage with regard to the
direction of current.
If a source of electric power is suddenly
applied to an unmagnetized inductor, the inductor will
initially resist the flow of electrons by dropping the full
voltage of the source. As current begins to increase, a
stronger and stronger magnetic field will be created,
absorbing energy from the source. Eventually the current
reaches a maximum level, and stops increasing. At this
point, the inductor stops absorbing energy from the source,
and is dropping minimum voltage across its leads, while the
current remains at a maximum level. As an inductor stores
more energy, its current level increases, while its voltage
drop decreases. Note that this is precisely the opposite of
capacitor behavior, where the storage of energy results in
an increased voltage across the component! Whereas
capacitors store their energy charge by maintaining a static
voltage, inductors maintain their energy "charge" by
maintaining a steady current through the coil.
The type of material the wire is coiled
around greatly impacts the strength of the magnetic field
flux (and therefore how much stored energy) generated for
any given amount of current through the coil. Coil cores
made of ferromagnetic materials (such as soft iron) will
encourage stronger field fluxes to develop with a given
field force than nonmagnetic substances such as aluminum or
air.
The measure of an inductor's ability to
store energy for a given amount of current flow is called
inductance. Not surprisingly, inductance is also a
measure of the intensity of opposition to changes in current
(exactly how much self-induced voltage will be produced for
a given rate of change of current). Inductance is
symbolically denoted with a capital "L," and is measured in
the unit of the Henry, abbreviated as "H."
An obsolete name for an inductor is choke,
so called for its common usage to block ("choke")
high-frequency AC signals in radio circuits. Another name
for an inductor, still used in modern times, is reactor,
especially when used in large power applications. Both of
these names will make more sense after you've studied
alternating current (AC) circuit theory, and especially a
principle known as inductive reactance.
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REVIEW:
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Inductors react against changes in current
by dropping voltage in the polarity necessary to oppose
the change.
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When an inductor is faced with an
increasing current, it acts as a load: dropping voltage as
it absorbs energy (negative on the current entry side and
positive on the current exit side, like a resistor).
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When an inductor is faced with a
decreasing current, it acts as a source: creating voltage
as it releases stored energy (positive on the current
entry side and negative on the current exit side, like a
battery).
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The ability of an inductor to store energy
in the form of a magnetic field (and consequently to
oppose changes in current) is called inductance. It
is measured in the unit of the Henry (H).
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Inductors used to be commonly known by
another term: choke. In large power applications,
they are sometimes referred to as reactors.
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