FL&O_section_10[1]

FL&O_section_10[1] - Chapter M. Magnetic Materials....

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1 Chapter M. Magnetic Materials. (Traugott Fischer, Stevens Tech.) APC’s Magetism. After studying this chapter, the student will be able to: 1. For important classes of applications such as transformers, door catches and magnetic data storage, define whether a soft or hard magnet is required. 2. Draw a B-H curve and identify the coercive field and, the remnant magnetization 3. Draw the B-H curves for a hard and a soft magnetic material. 4. Describe energy losses in terms of hysteresis and eddy currents. 5. Describe the origin of ferromagnetism in terms of electron spins and self- alignment of magnetic moments. 6. Relate changes in magnetic domain structure to different regions of the B-H curve. 7. Describe the salient properties and the processing of soft magnets. 8. Describe the salient properties and the processing of hard magnets. 9. Name the most important magnetic materials. 10. Select an appropriate material for a given application. 1. Uses of magnets and the requirements for the materials. We are all familiar with magnets: they are used in the home to attach notes to the refrigerator door, as magnetic catches holding the refrigerator door closed or to pick up Figure M.1. Chronology of the advances made in raising the strength (left) and the coercive field H C (right) of permanent magnets. From National Materials Advisory Board Report NMAB-426 on Magnetic Materials 1985.
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2 small metallic objects. With these magnets, we require that the magnetization be strong enough and that it be permanent; we do not wish that the magnetization be reduced or modified by contact with other magnets or by stray magnetic fields. Such devices use hard ferromagnetic materials. Advances in the magnetic strength of these materials allow for greater efficiency and miniaturization in motors, loudspeakers, tool holders. Judging by figure M1, progress in the last century has been impressive. Another class of materials is employed on a large scale in generators, transformers, inductors for the generation, transmission, storage and conversion of electric power. These important materials perform the critical function of shaping and concentrating magnetic flux in alternating current equipment, a task that must be accomplished with minimum energy loss. This requires that the magnetization be changed and reversed with a minimum expenditure of energy. These are the soft ferromagnetic materials. The almost two orders of magnitude decline in this loss (figure M2), since the beginnings of the electric power industry, has been responsible for huge energy savings. For example, annual 60Hz core energy losses in the U.S. are currently estimated to be about 10 14 watt-hours - a magnitude equal to the output of several large utility power generating plants. Figure M.2.
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FL&O_section_10[1] - Chapter M. Magnetic Materials....

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