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Chapter22[1]

Chapter22[1] - Chapter 22 What is magnetism It was named...

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Chapter 22 What is magnetism? It was named after the ancient city of Magnesia in Asia Minor. Some 2000 years ago, the Greeks observed the properties of “lodestone” (it’s also found in Lancaster, PA!) By the 12 th century, the Chinese were using it as a compass. What causes it? Why? Many of us have played with bar magnets as children and noticed that if we aligned the ends together a certain way, they would pull toward each other, but if we reversed one of the magnets, it would take all of our strength to push them together. Just as we designated charges as positive (protons) and negative (electrons), we say that a magnet has two poles: north and south . As with charges, like poles of a magnet will repel each other and unlike poles attract each other. Now what would happen if we sawed a bar magnet in half? Would we have a separate north-pole magnet and a separate south-pole magnet? NO!! What we would have in two smaller bar magnets, each of which would have both a north pole and a south pole. We could continue this process, each time making smaller and smaller bar magnets and never* be able to separate the north pole from the south pole. In "physics language", we say that THERE ARE NO MAGNETIC MONOPOLES ! Remember that "mono" means one. Recount Blas Cabrera episode. This is very different from charges which, as we know, exist as separate types. Some materials, called "ferromagnets" can easily be magnetized by placing them near a magnet. When some of these materials are moved away from the magnet, their magnetism disappears. Other ferromagnetic materials will become magnetized and remain so indefinitely. These are called "permanent magnets". MAGNETISM IS CAUSED BY CHARGES IN MOTION:
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A proton or electron that is sitting still does not induce (cause) a magnetic field. However, once the charge begins to move, a magnetic field will be present. Called electrodynamics as opposed to electrostatics. Like the electric field, B can cause “action at a distance.” Unlike E, the lines of B form closed loops. The why of this requires an understanding of relativity that we will come to later in the course. For now, it is enough to understand that magnetism is produced by moving charges. What about the bar magnets that we discussed above? They're just sitting on the table, minding their own business. Yet, if we bring a paper clip or a nail nearby, the magnets will reach out and "grab" them! The answer involves a basic understanding of quantum mechanics (electrons come spin up or spin down), which, once again, we will come to later. Remember that patience is a virtue. For now, let's just say that deep inside of the magnets, at the atomic level, the electrons are most definitely NOT standing still! THE MAGNETIC FIELD We denote as B , and its direction is that of a compass needle. The unit of measure is the Tesla (T). 1 T = 1 N/(A·m) The Tesla is BIG! 1 Gauss (G) = 1 x 10 -4 T Like the electric field we discussed in the previous lecture, we imagine the "magnetic field" surrounding a magnet or charge in motion as lines of potential force on a tiny compass needle.
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