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..The idea of “using up” the magnetic field surrounding a permanent magnet has al ready been alluded to. Consider, for example, the levitation experiment depicted in FIG. 5. A common-sense approach to this situation might suggest that the price exacted for resisting gravitational force on the upper disk magnet would be the dissipation of magnetism in one or both of the magnets. But, such is not the case. The upper magnet will remain levitated indefinitely. Any diminution of magnetic strength in the two magnets appears to be no more than what would occur if they were stored on a shelf. This simple situation is truly food for thought—students in beginning physics often present quite logical arguments suggesting that the magnets must become demagnetized in order to satisfy energy conservation laws!
FIG. 5 Demonstration of magnetic levitation. Nonmagnetic rod; Free to
move on rod; Ferrite; Fixed
A somewhat more sophisticated experiment is illustrated in FIG. 6. Here, the magnet provides the field for a basic alternating-current generator. Not shown is some kind of a prime mover that rotates the armature loop and slip-ring assembly, which are on a common shaft. (A small gasoline engine would serve this purpose.) The electrical load is a light bulb; its glow provides visual evidence of the production of electrical energy. Again, the question might be asked whether the magnet is thereby “used up”; that is, whether it is demagnetized in exchange for energy extracted from it. Now that you are familiar with these matters, you might correctly suspect that such is not the case. But, if you were contemplating the original design of such a device, you would likely feel concern over such a possibility. In actuality, the energy exchange occurs between the mechanical energy supplied by the prime mover and the electrical energy delivered to the load.
FIG. 6 Elementary ac generator.