The above methods are applicable to electromagnets with a magnetic circuit and 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 like the one shown at the top of this article. Only focusing on the force between two electromagnets (or permanent magnets) with well-defined "poles" where the field lines emerge from the core, a special analogy called a magnetic-charge model which assumes the magnetic field is produced by fictitious 'magnetic charges' on the surface of the poles. This model assumes point-like poles instead of the really existing surfaces, and thus it only yields a good approximation when the distance between the magnets is much larger than their diameter, so it is useful just for a force between them.