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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 $A-T=NI$ Magnetic field intensity (magneto motive force per unit length) H $\frac{NI}{length}=\frac{A-T}{length}$ Permeability  (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: 4π×10-7 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: $\mu =\frac{B}{H}={{\mu }_{o}}{{\mu }_{r}}$ Reluctance (Opposition to the establishment of magnetic flux) ℜ Ampere turns per Weber   $\Re =\frac{MMF}{\phi }=\frac{A-T}{Wb}$

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