This chapter extends the material presented in the chapter on inductors and introduces the concept of magnetic circuits. Basic applications are also illustrated. Here we shall take a closer look at the parameters of magnetic circuits such as permeability and flux density, and introduce new parameters including reluctance and magnetizing force. We shall also examine a key graphic that helps define the magnetic properties of materials, namely the \(BH\) curve. In the process we shall define new terms including coercivity and retentivity, and introduce the concept of hysteresis.
As we shall see, there are certain parallels between magnetic circuits and electrical circuits. For example, there is an “Ohm's law for magnetic circuits”, sometimes referred to as Hopkinson's law. There is also a version of Kirchhoff's voltage law dealing with the magnetic equivalents of voltage rises and drops.
Perhaps the two most common applications of magnetic circuits for the new student involve the use of transformers and relays, both of which will be examined in this chapter. Other common applications include dynamic loudspeakers and microphones, magnetic resonance imaging (i.e., medical MRI), pick-up cartridges for vinyl phonographs and the various applications made available by magnetic Hall effect sensors such as advanced anti-lock braking systems, timing control systems for internal combustion engines, and other instances requiring position and motion sensing. These magnetic devices tend to be much more reliable and capable than simple devices such as mechanical switches. This does not mean that magnetic systems are in some way “taking over”, indeed, there are some applications where they have become obsolete. One example is the use of output transformers to couple a home audio power amplifier to a loudspeaker. At one time, these were an integral part of every design, however, today they generally are not used. With this in mind, it is perhaps best to think of magnetic circuits as another item in the engineer's or technician's toolkit to help solve practical problems.