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Important Magnetic Units, Terms, Symbols, and Formulas

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The following table presents important Magnetic Units, Symbols, and Their Formulas as a reference, or source of information. These formulas play a key role while dealing with magnetic circuits such as Transformers, Inductors.

Term or quantity Symbol or abbreviation SI Unit and Formula
Flux (lines) ϕ \[Weber\text{ }\left( Wb \right)=\frac{number\text{ }of\text{ }lines}{{{10}^{8}}}\]
Flux density

(magnetic flux per unit cross-sectional area at right angles to the flux lines)

B $B=\frac{\phi }{A}=Tesla\text{ }(T)$
Magneto motive Force

(that which forces magnetic lines of force through the magnetic circuit)

MMF Ampere-Turn or


Magnetic field intensity

(magneto motive force per unit length)

H $\frac{NI}{length}=\frac{A-T}{length}$

 (Ability of a material to pass, conduct, or concentrate magnetic flux; analogous to conductance in electrical circuits), i.e., the ease of establishing magnetic flux through the material.

μ Webers per ampere-turn per meter


$\mu =\frac{l}{\Re A}$

 where length (l) is length in meters reluctance (ℜ) is ampere-turns per weber

area (A) is cross-sectional area in square meters

Note: Free space, or vacuum permeability (μo) is considered to be:


Relative Permeability

(Not constant because it varies with the degree of magnetization)

μr Relative permeability of a material is a ratio. Thus,


${{\mu }_{r}}=\frac{flux\text{ }density\text{ }with\text{ }core\text{ }material}{flux\text{ }density\text{ }with\text{ }vaccum\text{ }core}$


 Where flux density in the core material is:

$B={{\mu }_{o}}{{\mu }_{r}}H$ teslas,

and absolute permeability of core materials is:

$\mu =\frac{B}{H}={{\mu }_{o}}{{\mu }_{r}}$


(Opposition to the establishment of magnetic flux)

Ampere turns per Weber


$\Re =\frac{MMF}{\phi }=\frac{A-T}{Wb}$




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About Ahmad Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.