where B
is a dummy integration variable. The above
equation and its result are valid for the media with linear
property. It also can be used to calculate the force on the
nonlinear magnetic materials (such as magnets) if the linear
media (usually air) surrounds them [14, 15]. To a system
with no electrical energy input or no coils (such as a system
with permanent magnets only), this force can be calculated
directly from the change of magnetic field energy