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Magnetic Fields and Circuits

Magnetic Materials

All materials may be broadly classified as being in one of two groups. They may be magnetic or non-magnetic, depending upon the degree to which they exhibit magnetic effects. The vast majority of materials fall into the latter group, which may be further classified into diamagnetic and paramagnetic materials.

The magnetic properties of these materials are very slight, and extremely difficult even to detect. Thus, for practical purposes, we can say that they are totally non-magnetic. The magnetic materials (based on iron, cobalt, and ferrites) are ferromagnetic materials, all of which exhibit very strong magnetic effects. It is with these materials that we will be principally concerned.

The Magnetic Circuit

A magnetic circuit is all of the space occupied by the magnetic flux shows an iron-cored solenoid, supplied with direct current, and the resulting flux pattern. This is what is known as a composite magnetic circuit since the flux exists both in the iron core and in the surrounding air space. In addition, it can be seen that the spacing of the lines within the iron core is uniform, whereas it varies in the air space. Thus there is a uniform magnetic field in the core and a nonuniform field in the rest of the magnetic circuit. In order to make the design and analysis of a magnetic circuit easier, it is more convenient

Magnetic Flux and Flux Density

The magnetic flux is what causes the observable magnetic effects such as attraction, repulsion, etc. The unit of magnetic flux is the weber (Wb). This was the name of a German scientist so it is pronounced as ‘ cyber ’ . The number of webers of flux per square meter of a cross-section of the field is defined as the magnetic flux density ( B ), which is measured in tesla (T).

This sometimes causes some confusion at first, since the logical unit would appear to be weber/meter 2 . Indeed, this is the way in which it is calculated: the value of flux must be divided by the appropriate area. Tesla was the name of another scientist, whose name is thus commemorated. On reflection, it should not be particularly confusing, since the logical unit for electrical current would be coulomb/second; but it seems quite natural to use the term ampere.

Magnetomotive Force (mmf)

In an electric circuit, any current that flows is due to the existence of an emf. Similarly, in a magnetic circuit, the magnetic flux is due to the existence of an MMF. The concept of an MMF for permanent magnets is a difficult one. Fortunately, it is simple when we consider the flux being produced by the current flowing through a coil. This is the case for most practical magnetic circuits.

In section 4.2 we saw that each turn of the coil made a contribution to the total flux produced, so the flux must be directly proportional to the number of turns on the coil. The flux is also directly proportional to the value of current passed through the coil.

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