SUMMARY
The important points of this chapter are summarized below. Study this information
before continuing, as this information will lay the foundation for later chapters.

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____INDUCTANCE__ - The characteristic of an electrical circuit that opposes a change in
current. The reaction (opposition) is caused by the creation or destruction of a magnetic
field. When current starts to flow, magnetic lines of force are created. These lines of
force cut the conductor inducing a counter emf in a __direction that opposes current__.

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____SELF-INDUCTANCE__ - The process by which a circuit induces an emf into itself by its
own moving magnetic field. All electrical circuits possess self-inductance. This
opposition (inductance), however, only takes place when there is a change in current.
Inductance does NOT oppose current, only a CHANGE in current. The property of inductance
can be increased by forming the conductor into a loop. In a loop, the magnetic lines of
force affect more of the conductor at one time. This increases the self-induced
emf.

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____INDUCTANCE OF A COIL__ - The property of inductance can be further increased if the
conductor is formed into a coil. Because a coil contains more loops, more of the conductor
can be affected by the magnetic field. Inductors (coils) are classified according to core
type. The core material is normally either air or soft iron.

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____FACTORS AFFECTING COIL INDUCTANCE__ - The inductance of a coil is entirely dependent
upon its physical construction. Some of the factors affecting the inductance are:

The number of turns in the coil. Increasing the number of turns will increase the
inductance.

The coil diameter. The inductance increases directly as the cross-sectional area of the
coil increases.

The length of the coil. When the length of the coil is increased while keeping the
number of turns the same, the turn-spacing is increased. This decreases the inductance of
the coil.

The type of core material. Increasing the permeability of the core results in
increasing the inductance of the coil.

Winding the coil in layers. The more layers used to form a coil, the greater effect the
magnetic field has on the conductor. By layering a coil, you can increase the inductance.

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____UNIT OF INDUCTANCE__ - Inductance (L) is measured in henrys (H). An inductor has an
inductance of one henry (H) if an emf of one volt is induced in the inductor when the
current through the inductor is changing at the rate of 1 ampere per second. Common units
of inductance are henry (H), millihenry (mH) and the microhenry (μH).

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____GROWTH AND DECAY OF CURRENT IN AN LR CIRCUIT__ - The required for the current in an
inductor to increase to 63.2 percent of the maximum current or to decrease to 36.8 percent
of the maximum current is known as the time constant. The letter symbol for an LR time
constant is L/R. As a formula:

The time constant of an LR circuit may also be defined as the time required for the
current in the inductor to grow or decay to its final value if it continued to grow or
decay at its initial rate. For all practical purposes, the current in the inductor reaches
a maximum value in 5 "Time Constants" and decreases to zero in 5 "Time
Constants".

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____POWER LOSSES IN AN INDUCTOR__ - Since an inductor (coil) contains a number turns of
wire, and all wire has some resistance, the inductor has a certain amount of resistance.
This resistance is usually very small and has a negligible effect on current. However,
there are power losses in an inductor. The main power losses in an inductor are copper
loss, hysteresis loss, and eddy-current loss. Copper loss can be calculated by multiplying
the square of the current by the resistance of the wire in the coil (I_{2}R).
Hysteresis loss is due to power that is consumed in reversing the magnetic field of the
core each time the current direction changes. Eddy-current loss is due to core heating
caused by circulating currents induced in an iron core by the magnetic field of the coil.

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____MUTUAL INDUCTANCE__ - When two coils are located so that the flux from one coil cuts
the turns of the other coil, the coils have mutual inductance. The amount of mutual
inductance depends upon several factors: the relative position of the axes of the two
coils; the permeability of the cores; the physical dimensions of the two coils; the number
of turns in each coil, and the distance between the coils. The coefficient of coupling K
specifies the amount of coupling between the coils. If all of the flux from one coil cuts
all of the turns of the other coil, the coefficient of coupling K is 1 or unity. If none
of the flux from one coil cuts the turns of the other coil, the coefficient of coupling is
zero. The mutual inductance between two coils (L_{1} and L_{2}) may be
expressed mathematically as:

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____COMPUTING THE INDUCTANCE OF A CIRCUIT__ - When the total inductance of a circuit is
computed, the individual inductive values are treated the same as resistance values. The
inductances of inductors in series are added like the resistances of resistors in series.
That is,

The inductances of inductors in parallel are combined mathematically like the
resistances of resistors in parallel. That is,

Both of the above formulas are accurate, providing there is no mutual inductance
between the inductors.