There are four important properties that materials can have with respect to magnetic flux. These properties are ferromagnetism, diamagnetism, permeability, and retentivity.
Some substances cause magnetic lines of flux to bunch closer together than they would in the medium of air or a vacuum. This property is called ferromagnetism, and materials that exhibit it are called ferromagnetic. You’ve already learned something about this!
Another property is known as diamagnetism, and materials that exhibit it are called diamagnetic. This type of substance decreases the magnetic flux density by causing the magnetic flux lines to diverge. Wax, dry wood, bismuth, and silver are examples. No diamagnetic material reduces the strength of a magnetic field by anywhere near the factor that ferromagnetic substances can increase it. Diamagnetic materials are generally used to keep magnetic objects apart, while minimizing the interaction between them. In recent years, they have also found some application in magnetic levitation devices.
Permeability is a quantitative indicator of the extent to which a ferromagnetic material concentrates magnetic lines of flux. It is measured on a scale relative to a vacuum, or free space. Free space is assigned permeability 1. If you have a coil of wire with an air core, and a current is forced through the wire, then the flux in the coil core is at a certain density, just about the same as it would be in a vacuum. Therefore, the permeability of pure air is about equal to 1. If you place an iron core in the coil, the flux density increases by a large factor. The permeability of iron can range from 60 (impure) to as much as 8000 (highly refined).
If you use certain ferromagnetic alloys as the core material in electromagnets, you can increase the flux density, and therefore the local strength of the field, by as much as a million times. Such substances thus have permeability as great as 1,000,000 (106). following table gives permeability values for some common materials.
When a substance, such as iron, is subjected to a magnetic field as intense as it can handle, say by enclosing it in a wire coil carrying a massive current, there will be some residual magnetism left.
|Air, dry, at sea level||1|
|Aluminum||Slightly more than 1|
|Bismuth||Slightly less than 1|
|Iron, powdered and pressed||100–3000|
|Iron, solid, refined||3000–8000|
|Iron, solid, unrefined||60–100|
|Silver||Slightly less than 1|
|Wax||Slightly less than 1|
|Wood, dry||Slightly less than 1|
when the current stops flowing in the coil. Retentivity, also sometimes called remanence, is a measure of how well the substance “memorizes” the magnetism and thereby becomes a permanent magnet.
Retentivity is expressed as a percentage, and is symbolized Br. If the flux density in the material is x tesla or gauss when it is subjected to the greatest possible magnetomotive force, and then goes down to y tesla or gauss when the current is removed, the retentivity is equal to 100( y/x)%.
Suppose that a metal rod can be magnetized to 135 G when it is enclosed by a coil carrying an electric current. Imagine that this is the maximum possible flux density that the rod can be forced to have. (For any substance, there is always such a maximum.) Now suppose that the current is shut off, and 19 G remain in the rod. Then the retentivity, Br, is calculated as follows:
Br = 100(19/135)% = (100 × 0.14)% = 14%
Some ferromagnetic substances have high retentivity. These materials are excellent for making permanent magnets. Other substances have low retentivity. They work well as electromagnets, but not as permanent magnets.
If a ferromagnetic substance has poor retentivity, it is especially well-suited for use as the core material for an ac electromagnet, because the polarity of the magnetic flux can reverse within the material at a rapid rate. Materials with high retentivity do not work well for ac electromagnets, because they resist the polarity reversal that takes place with ac.