The purpose of this chapter is to review some of the basic concepts associated with magnetic circuits and to develop an understanding of inductors and transformers needed in power electronics.
7.1 AMPERE-TURNS AND FLUX
Let us consider a simple magnetic structure of Figure 7.1 consisting of an N-turn coil with a current , on a magnetic core made of iron. This coil applies ampere-turns to the core. We will assume the magnetic field intensity in the core to be uniform along the mean path length . The magnetic field intensity in the air gap is denoted as . From Ampere’s law, the closed line integral of the magnetic field intensity along the mean path within the core and in the air gap is equal to the applied ampere-turns:
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, on a magnetic core made of iron. This coil applies 
ampere-turns to the core. We will assume the magnetic field intensity 
in the core to be uniform along the mean path length 
. The magnetic field intensity in the air gap is denoted as 
. From Ampere’s law, the closed line integral of the magnetic field intensity along the mean path within the core and in the air gap is equal to the applied ampere-turns: