Magnetic Circuits

Magnetism plays an integral part in almost every electrical device used today in industry, research, or the home. Generators, motors, transformers, circuit breakers, televisions, computers, tape recorders, and telephones all employ magnetic effects to perform a variety of important tasks. The compass, used by Chinese sailors as early as the second century A.D., relies on a permanent magnet for indicating direction.

What is a permanent magnet?

The permanent magnet is made of a material, such as steel or iron, that will remain magnetized for long periods of time without the need for an external source of energy. In 1820, the Danish physicist Hans Christian Oersted discovered that the needle of a compass would deflect if brought near a current-carrying conductor. For the first time it was demonstrated that electricity and magnetism were related, and in the same year the French physicist Andre-Marie Ampere performed experiments in this area and developed what is presently known as Ampere's circuital law. In subsequent years, men such as Michael Faraday, Carl Friedrich Gauss, and James Clerk Maxwell continued to experiment in this area and developed many of the basic concepts of electromagnetism—magnetic effects induced by the flow of charge, or current. There is a great deal of similarity between the analyses of electric circuits and magnetic circuits. This will be demonstrated later in this chapter when we compare the basic equations and methods used to solve magnetic circuits with those used for electric circuits. Difficulty in understanding methods used with magnetic circuits will often arise in simply learning to use the proper set of units, not because of the equations themselves. The problem exists because three different systems of units are still used in the industry. To the extent practical, SI will be used throughout this chapter.