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Glossary of Terms and Formulae
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z All

AL Value (nH/N2)

The inductance rating of a core in nanohenries (100 Henries) per turn squared based on a peak AC flux density of 10 gauss (1 millitesla) at a frequency of 10 KHz. Note: 35.0 nH/N2 =350µH for 100 turns =35.0 mH for 1000 turns.

Ampere's law

Defines the relationship between magnetizing force and current. It is commonly written as

        H = (.4pNI)/ML

H = magnetizing force in Oersteds
N = Number of Turns
I = Current through N turn
ML = Magnetic path length of core.

Related Links
Ampere's law in detail...


The gapping of E Cores by equally spacing all three legs of the cores rather than introducing a gap in the center-leg only. Twice as much center-leg gap is required to electrically duplicate a given butt-gap.


Another term for an inductor which is intended to filter or choke out signals.

Coercive Force (Hc)

That value of magnetizing force required to reduce the flux density to zero.

Common-Mode Noise

Electrical interference that is common to both lines in relation to earth ground.

Conversion Factors

oersteds * 0.795 = amp-turns/cm
gauss * 0.0001 = tesla
mH/1000 turns * 10 = uH/100 turns
in2 * 6.425 = cm2
circ mils * 5.07 x 10-6 = cm2
watts/lb * 17.62 = mW/cm3

Copper Loss (Watts)

The power loss (PR) or heat generated by current (I) flowing in a winding with resistance (R)

Core Loss (Watts)

The power loss or heat generated by a magnetic material subjected to an alternating magnetic field.

Cross-Sectional Area (A)

The effective cross-sectional area (cm2) of a core available for magnetic flux. The cross-sectional area listed for toroidal cores is based on bare core dimensions with a 5% radius correction.

Differential-Mode Noise

Electrical interference that is not common to both lines but is present between both lines. This is also known as normal-mode noise.


Electricity is a property of matter that results from the presence or movement of electric charge. Together with magnetism, it constitutes the fundamental interaction known as electromagnetism. Electricity is responsible for many well-known physical phenomena such as lightning, electric fields and electric currents, and is put to use in industrial applications such as electronics and electric power.

Electromotive force

Electromotive force (emf), often denoted by Epsilon(lower-case epsilon), is a measure of the strength of a source of electrical energy and is measured in volts.

Energy Storage (½LI2)

The amount of energy stored, in microjoules (10-6 joules), is the product of one-half the inductance (L) in microhenries ( 10-6 Henries), times the current (I) squared in amperes.

Faraday's Law

Defines the relationship of voltage and flux as:

E = N [df/dt] x 10-8

For sinusoidal voltage conditions, it is written:

E = 2.22 ft FN X 10-8 or E = 4.44 BmAc FN X 10-8


E = Voltage desired
Bm = Flux density of material in gausses.
t = Total flux capacity of core
Ac = Effective core cross-sectional area
F = Design frequency
N = Number of turns
ft = 2 Bm Ac


The term "ferrite" is derived from the Latin word "ferrum", meaning iron. Ferrites are homogeneous ceramic materials composed of various oxides containing iron oxide as their main constituent. Ferrite has a cubic crystalline structure with the chemical formula MO·Fe2O3 where Fe2O3 is iron oxide and MO refers to a combination of two or more divalent metal oxides such as zinc, nickel, manganese or copper. The addition of such metal oxides in various amounts allows the creation of many different materials whose properties can be tailored for a variety of uses.

Being a ceramic material, ferrites are hard, inert, and free of organic substances. Ferrite is rigid and brittle. Like other ceramics, ferrite can easily chip and break if handled roughly. Often such chips and cracks are only cosmetic and do not affect the materials magnetic properties. Ferrite range in color from silver gray to black. The electromagnetic properties of ferrite materials can be affected by environmental and operating conditions such as temperature, pressure, field strength, frequency and time.

Full Winding

-A winding for toroidal cores which will result in 45% of the core’s inside diameter remaining.
-A winding for E Cores which will result in a full bobbin. The type of insulation, tightness of winding, and coil winding equipment limitations will all introduce variations.


The unit of magnetic induction in the cgs electromagnetic system. The gauss is equal to 1 Maxwell per square centimeter.

Henry (H)

The henry (symbol: H) is the SI unit of inductance. It is named after the American scientist Joseph Henry.  Definition:  If the rate of change of current in a circuit is one ampere per second and the resulting electromotive force is one volt, then the inductance of the circuit is one henry.

1 H = Wb/A = 1 m2·kg·s–2·A–2 = 1 V·s/A


Electrical impedance or simply impedance is a measure of opposition to a sinusoidal electric current. The concept of electrical impedance generalizes Ohm's law to AC circuit analysis. Unlike electrical resistance, the impedance of an electric circuit can be a complex number. Oliver Heaviside coined the term impedance in July of 1886.


Inductance is a measure of the amount of magnetic flux produced for a given electric current.



L is the inductance in henries,
i is the current in amperes,
Φ is the magnetic flux in webers

Compare the above definition with that for capacitance.

The symbol L is used for inductance in honour of the physicist Heinrich Lenz. The term inductance was coined by Oliver Heaviside in February 1886. The SI unit of inductance is the henry (symbol: H).

Strictly speaking, the quantity just defined is called self-inductance, because the magnetic field is created solely by the conductor that carries the current.

When a conductor is coiled upon itself N number of times around the same axis, the current required to produce a given amount of flux is reduced by a factor of N compared to a single turn of wire. Thus, the inductance of a coil of wire of N turns is given by:


where λ is the total 'flux linkage'.

Such a coiled conductor is an example of an Inductor.

Related Links
Inductance in detail...

Inductive Coupling

In electronics, inductive coupling refers to the transfer of energy from one circuit component to another through a shared magnetic field. A change in current flow through one device induces current flow in the other device. The two devices may be physically contained in a single unit, as in the primary and secondary sides of a transformer, or may be separated as in the antennae on a transmitter and a receiver. Coupling may be intentional or unintentional. Unintentional coupling is called cross-talk, and is a form of interference. Inductive coupling favors low frequency energy sources. High frequency energy sources generally use capacitive coupling.

Initial Permeability (µ0)

That value of permeability at a peak AC flux density of 10 gauss (1 millitesla).

Magnetic flux

The product of the magnetic induction, B, and the cross-sectional area, when the magnetic induction B is uniformly distributed and normal to the plane of cross-section.

Magnetic flux, is a measure of quantity of magnetism, taking account of the strength and the extent of a magnetic field. The flux through an element of area perpendicular to the direction of magnetic field is given by the product of the magnetic field density and the area element. More generally, magnetic flux is defined by a scalar product of the magnetic field density and the area element vector. Gauss's law for magnetism, which is one of the four Maxwell's equations, states that the total magnetic flux through a closed surface is zero. This law is a consequence of the empirical observation that magnetic monopoles do not exist or are not measureable. The SI unit of magnetic flux is the weber, and the unit of magnetic flux density is the weber per square meter.

Related Links
Magnetic Flux

Magnetizing Force (H)

The magnetic-field-strength which produces magnetic flux. 1 Oersted = 79.58 A/m=.7958 A/cm

In cgs units:

H =         .4P NI

In SI units:

H =      NI


H = Oersteds (Oe)
N = Number of turns
I = Current (amperes)
/ = Mean Magnetic Path (cm)

Magnetomotive force

Magnetomotive force is any physical cause that produces magnetic flux. It is analogous to electromotive force or voltage in electricity.


The unit of magnetic flux in cgs units. One Maxwell equals 10-8 Webers.

Mean Magnetic Path Length (/)

The effective magnetic path length of a core structure (cm).

MLT (cm)

The mean-length-per-turn of wire for a core.


The unit of magnetizing force in cgs units. One Oersted equals a magneto-motive force of one Gilbert per centimeter of path length.

Peak AC Flux Density (B)

The number of lines of flux per unit of cross-sectional area generated by an alternating magnetic field (from zero or a net DC). In general:

In cgs units

Bpk = Eavg108

Bpk = Gauss (G),(1 gauss = 10-4 Telsa)
Eavg = Average voltage per half-cycle (volts)
A  = Cross-sectional area (cm2)
N  = Number of turns
f   = Frequency (Hertz)

Peak to Peak Flux Density (B)

In an alternating magnetic field, it is assumed that the peak to peak flux density is twice the value of peak AC flux density. B = 2 Bpk.

Percent Initial Permeability (%µ0)

Represents the percent change in permeability from the initial value.

Percent Ripple

The percentage of ripple or AC flux to total flux; or in an inductor, the percentage of alternating current to average current.

Percent Saturation

This is equal to 100% minus Percent Initial Permeability. ie: 20% saturation = 80% of initial permeability.

Permeability (µ)

In general, the ratio of the changes in magnetic induction to changes in magnetizing force (B to H) is called the permeability.

Q factor

The Q factor or quality factor is a measure of the "quality" of a resonant system. Resonant systems respond to frequencies close to their natural frequency much more strongly than they respond to other frequencies. The Q factor indicates the amount of resistance to resonance in a system. Systems with a high Q factor resonate with a greater amplitude (at the resonant frequency) than systems with a low Q factor. Damping decreases the Q factor.

Related Links
Q in detail...

Residual Flux

That value of magnetic induction that remains in a magnetic circuit when the magneto-motive force is reduced to zero.

Simple Winding

A winding for toroidal cores which will result in 78% of the core’s inside diameter remaining. Often times this will produce a single-layer winding.

Single-Layer Winding

A winding for a toroidal core which will result in the full utilization of the inside circumference of the core without the overlapping of turns. The thickness of insulation and tightness of winding will affect results.

Squareness Ratio

The ratio of residual flux density to the maximum (saturation) flux density.

Surface Area (cm2)

The effective surface area of a typical wound core available to dissipate heat.


A term used to describe how inductance responds to changes in current. Example: A 2:1 swing corresponds to an inductor which exhibits 2 times more inductance at very low current than it does at its maximum rated current. This would also correspond to the core operating at 50% of initial permeability (also 50% saturation) at maximum current t.

Temperature Rise (DT)

The increase in surface temperature of a component in free-standing air due to the total power dissipation (both copper and core loss).

The following formula has been used to approximate temperature rise:
DT (°C) = [ Total Power Dissipation (milliwatts) ].833
                                   surface Area (cm2)     


The volt (symbol: V) is the SI derived unit of electric potential difference. The number of volts is a measure of the strength of an electrical source in the sense of how much power is produced for a given current level. It is named in honor of the Italian physicist Alessandro Volta (1745–1827), who invented the voltaic pile, the first chemical battery.

The volt is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power. Hence, it is the base SI representation m2 · kg · s-3 · A-1, which can be equally represented as one joule of energy per coulomb of charge, J/C.

    1 V = 1 W/A = 1 m2·kg·s–3·A–1

Winding Factor (K)

The ration of the total area of copper wire in the center hole of a toroid to the window area of the toroid.

Window Area (Wa)

The area of the hole of a toroidal core, or the window within a square or EI core.


Copyright © 2008 RAF Tabtronics LLC.
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