Magnetostatic forces computation can be done numerically through Finite
Element Analysis by applying the virtual works method on either the energy
or the co-energy. They can also be computed analytically using Maxwell's
tensor. Different results have been obtained using these two procedures. In
this work we will introduce a general way of obtaining magnetostatic stress
tensors (Maxwell's tensors) from either the energy or the co-energy. Both
tensors are equivalent in induced magnetisation media (media whose
magnetisation depends on an external magnetic field), but they are different
in permanent magnets (media whose magnetisation does not depend on an
external magnetic field). In these media, normal components of the surface
forces derived from either the energy or the co-energy are the same, but
tangential components have different modules, keeping the same direction as
the tangential components of magnetic field H or magnetic induction
B respectively. Force density is also different. Forces computed
from co-energy do not have the same conservative characteristic as forces
computed from energy. The application of Maxwell tensors in the calculation
of forces over the real surface of magnetic media must take into account the
discontinuity of the forces from one medium (particularly a vacuum) to
another. Generally, normal stresses in all media, obtained from the energy
and the co-energy, are discontinuous, while tangential stresses are only
discontinuous in processes derived from the co-energy in permanent magnets.