1. “A magnet (from Greek μαγνήτις λίθος magnḗtis líthos, "Magnesian stone") is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.
A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic). These include iron, nickel, cobalt, some alloys of rare earth metals, and some naturally occurring minerals such as lodestone. Although ferromagnetic (and ferrimagnetic) materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism.
Ferromagnetic materials can be divided into magnetically "soft" materials like annealed iron, which can be magnetized but do not tend to stay magnetized, and magnetically "hard" materials, which do. Permanent magnets are made from "hard" ferromagnetic materials such as alnico and ferrite that are subjected to special processing in a powerful magnetic field during manufacture, to align their internal microcrystalline structure, making them very hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied, and this threshold depends on coercivity of the respective material. "Hard" materials have high coercivity, whereas "soft" materials have low coercivity.
An electromagnet is made from a coil of wire that acts as a magnet when an electric current passes through it but stops being a magnet when the current stops. Often, the coil is wrapped around a core of ferromagnetic material like steel, which enhances the magnetic field produced by the coil.
The overall strength of a magnet is measured by its magnetic moment or, alternatively, the total magnetic flux it produces. The local strength of magnetism in a material is measured by its magnetization.
(Wikipedia, Magnets, 8/16/2012)
2. “Magnetorheological materials are typically comprised of magnetizable particles suspended in a carrier material. A magnetorheological material exhibits rapid and reversible changes that are controllable by an applied magnetic field. The shear stress and viscosity of such a material is related to whether the material is in the presence of a magnetic field, termed the on-state, or the absence of a magnetic field, termed the off-state. In the on-state, the magnetizable particles align with the magnetic field and increase the shear yield stress and viscosity of the material over its off-state value.
Typical state-of-the-art magnetorheological (MR) fluids are multiphase materials consisting of magnetizable particles suspended in a liquid carrier fluid. These MR fluids exhibit properties typical of a viscoelastic material. In addition to the magnetizable particles, the carrier fluid serves as a continuous insulating material. Some of the carrier fluids typically utilized are silicone, hydrocarbon, and synthetic oils. An additional component that is often present in MR fluids is a stabilizer, which serves to keep the particles suspended in the fluid. MR fluids demonstrate non-Newtonian fluid behavior when exposed to a magnetic field.
[Fuchs et al, US Patent 8,241517 (8/14/2012)]
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Roger D. Corneliussen
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Copyright 2012 by Roger D. Corneliussen.
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* Date of latest addition; date of first entry is 8/16/2012.