[Electro Magnetic Field]

[robobang]

Magnetic field (green) of a typical electromagnet, with the iron core C forming a closed loop with two air gaps G in it. Most of the magnetic field B is concentrated in the core. However some of the field lines B_L, called the "leakage flux", do not follow the full core circuit and so do not contribute to the force exerted by the electromagnet. In the gaps G the field lines spread out beyond the boundaries of the core in "fringing fields" B_F. This increases the "resistance" (reluctance) of the magnetic circuit, decreasing the total magnetic flux in the core. Both the leakage flux and the fringing fields get larger as the gaps are increased, reducing the force exerted by the magnet. Line L shows the average length of the magnetic circuit, used in equation (1) below. It is the sum of the length L_core in the iron core and the length L_gap in the air gaps