MadSci Network: Physics |
Greetings, Em: As you probably know, if you saw a magnet in two, each piece is still a magnet. If you could grind it into powder (carefully, so as not to overheat it), each speck of magnet-dust will still be a magnet. The fundamental source of magnetism in solid substances is an unbalanced pair of electrons in orbit about some atom or molecule. When such atoms or molecules are piled together, there is a tendency for their magnetic fields to cause the particles to pair up in such a way that the 'north' end of each is as near as possible to the 'south' end of the other. Thus a pile of magnet- powder is not likely to exhibit very much overall magnetism; attractions and repulsions basically cancel each other out. In a way, that situation is not much different from a magnet that HAS been heated. Heat is always associated with random motion of molecules -- and random motion results in random orientations of the particles being heated. It should be obvious that in a pile of randomly oriented magnet-particles, there are equal numbers in pointing all directions, and again there is no overall magnetism. Any magnet at room temperature can be heated until it beomes a non-magnet, due to the increased random motion of its molecules. For each magnetic substance, there is a particular temperature at which its magnetic ability suddenly drops to zero. That temperature is known as the "Curie Point" for the substance (named after the physicist who first studied the effect). To manufacture a magnet, it is necessary to force some overall orientation upon the individual particles in a magnetic substance. An external force of some sort is always necessary. Planet Earth is conveniently equipped with electric currents flowing in its iron core (their cause has been debated for many decades), and the whole world is a giant magnet as a result. With respect to any flow of molten rock, the Earth's magnetic field is an "external force" that permeates it, and encourages magnetic molecules in the molten rock to become aligned. Naturally occuring magnets are known as "lodestones". They are relatively feeble, compared to the magnets that we can manufacture today (using very powerful electromagnetic fields), but lodestones were quite strong enough to mystify people for thousands of years. In some materials, simply striking it sharply can cause some of their magnetic molecules to become aligned! Those unbalanced pairs of electrons are somehow literally associated with a kind of off-center heaviness. A mechanical shock can cause the molecules to become jostled, and since this is a DIRECTIONAL mechanical shock, there is a tendency for all the 'heavy' sides of the particles to become a little more aligned in a particular direction. That tiny change from randomness is enough to yield a noticeable magnetic field. Several shocks, in the same direction of course, can enhance that resultant field -- and as you might expect, shocks in other directions can diminish it. And now, on to the effects of cooling. In this situation you must first realize that your magnet is NOT a perfectly-aligned collection of particles. There is, at the very least, plain old room temperature working to jostle molecules. Since you do have a solid substance, the molecules don't jostle very much, but it is always enough to make the magnet less perfect than it otherwise might be. (There are some magnetic materials that have a Curie Point BELOW room temperature! So we never notice their magnetism, until after they are cooled.) Within your cooling magnet, the molecules begin to jostle a little less vigorously. Now think about the fact that the magnet is permeated quite thoroughly by its own magnetic field. Its own field helps the lesser-jostling molecules to come into greater magnetic alignment! And, as has been pointed out, the more the particles are aligned, the stronger the magnet. That fact should qualify as the Answer to your Question....
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