Page "Electromagnet" Paragraph 33

Since most of the magnetic field is confined within the outlines of the core loop, this allows a simplification of the mathematical analysis.

See the drawing at right.

A common simplifying assumption satisfied by many electromagnets, which will be used in this section, is that the magnetic field strength B is constant around the magnetic circuit and zero outside it.

Most of the magnetic field will be concentrated in the core material ( C ).

Within the core the magnetic field ( B ) will be approximately uniform across any cross section, so if in addition the core has roughly constant area throughout its length, the field in the core will be constant.

This just leaves the air gaps ( G ), if any, between core sections.

In the gaps the magnetic field lines are no longer confined by the core, so they ' bulge ' out beyond the outlines of the core before curving back to enter the next piece of core material, reducing the field strength in the gap.

The bulges ( B < sub > F </ sub >) are called fringing fields.

However, as long as the length of the gap is smaller than the cross section dimensions of the core, the field in the gap will be approximately the same as in the core.

In addition, some of the magnetic field lines ( B < sub > L </ sub >) will take ' short cuts ' and not pass through the entire core circuit, and thus will not contribute to the force exerted by the magnet.

This also includes field lines that encircle the wire windings but do not enter the core.

This is called leakage flux.

Therefore the equations in this section are valid for electromagnets for which: