Magnetic dipole magnetic dipole as the distance from the source increases. In classical physics, the magnetic field of a dipole is calculated as the limit of either a current loop or a pair of charges as the source shrinks to a point while keeping the magnetic moment constant. For the current loop, this limit is most easily derived for the vector potential. Outside of the source region, this potential is (in SI units) with 4"π r" being the surface of a sphere of radius "r"; and the magnetic flux density (strength of the B-field) in teslas is In spherical coordinates with the
Magnetic reluctance towards regions of higher flux so it is always an attractive force (pull). The reluctance of a uniform magnetic circuit can be calculated as: or where The inverse of reluctance is called "permeance". Its SI derived unit is the henry (the same as the unit of inductance, although the two concepts are distinct). Reluctance can also be applied to: Magnetic reluctance </math>, formula_1, formula_2 Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is defined as the ratio of magnetomotive force (mmf) to magnetic flux. It represents the opposition to magnetic flux, and