Editing Electromagnet (section)

=== Force between electromagnets ===
The previous methods are applicable to electromagnets with a [[magnetic circuit]]; however, they do not apply when a large part of the magnetic field path is outside the core. (A non-circuit example would be a magnet with a straight cylindrical core.) To determine the force between two electromagnets (or permanent magnets) in these cases, a special analogy called a ''magnetic-charge model'' can be used. In this model, it is assumed that the magnets have well-defined "poles" where the field lines emerge from the core, and that the magnetic field is produced by fictitious "magnetic charges" on the surface of the poles. This model assumes point-like poles (instead of surfaces), and thus it only yields a good approximation when the distance between the magnets is much larger than their diameter; thus, it is useful just for determining a force between them.

The magnetic pole strength ''<math>m</math>'' of an electromagnet is given by
<math display="block">m = \frac{NIA}{L}</math>
and thus the force between two poles is
<math display="block">F = \frac{\mu_0 m_1 m_2}{4\pi r^2}.</math>
Each electromagnet has two poles, so the total force on magnet 1 from magnet 2 is equal to the [[Vector (mathematics and physics)|vector]] sum of the forces of magnet 2's poles acting on each pole of magnet 1.