# Magnetic energy

Magnetic energy and electric energy are related by Maxwell's equations. The potential energy of a magnet of magnetic moment m in a magnetic field B is defined as the work of magnetic force (actually of magnetic torque) on re-alignment of the vector of the magnetic dipole moment, and is equal:

$E_{\rm p,m} = -m\cdot B$

while the energy stored in an inductor (of inductance L) when current I is passing via it is

$E_{\rm p,m} = {1\over 2}LI^2$.

This second expression forms the basis for superconducting magnetic energy storage.

Energy is also stored in a magnetic field. The energy per unit volume in a region of space of permeability $\mu$ containing magnetic field B is:

$u={1\over 2}{B^2 \over \mu}$