This document provides an introduction to magnetic circuits, including key definitions: - Magnet provides magnetic property, magnetic flux is the number of field lines passing through a closed surface. - Magnetic flux density B and field intensity H are defined, with units of tesla and ampere/meter respectively. - Magneto motive force (MMF) is the force that causes flux to flow, measured in ampere-turns. Reluctance is the opposition to flux flow, measured in ampere-turns/weber. - Permeance is the reciprocal of reluctance. Permeability is the ability of a material to allow flux flow. - Series and parallel magnetic circuits are discussed, behaving similarly to electric