Chapter 12 Magnetism 99 Copyright Goodheart-Willcox Co., Inc. Copyright Goodheart-Willcox Co., Inc. pole, as shown in Figure 12-3. When molecules are in this orderly formation, the steel is magnetized. This theory is proven to some extent by the fact that if the magnetized steel bar were broken into smaller pieces, each piece of steel would be a magnet. You can try this experiment. Take a steel bar about two or three feet long. Hold it in a north-south direction and hit it several times with a hammer. The bar will become weakly magnetized and will attract a compass needle. Now turn the bar in an east-west direction and again hit it several times with the hammer. A test with your compass will now show that it is demagnetized. These phenomena can be better understood with the realization that the earth is one enormous magnet, and many invisible lines of magnetic force exist between the north and south pole, Figure 12-4. When you turned your bar in a north-south direction, the earth’s magnetic lines of force fl owed through your bar because the bar has greater permeability than the air surrounding it. The bar conducts magnetism better than air. When you hit the bar, the molecules were physically jarred, and they lined up in their magnetized positions. Permeability can be compared to conductivity (G) in an electrical circuit. The resistance of a material to conduct magnetism is called reluctance. It can be compared to resistance (R) in an electrical circuit. Laws of Magnetism As you learned in Chapter 2, Electrons, electrons and protons in atoms are attracted to each other because they have opposite electrical charges. In the same way, the north pole of one magnet is attracted to the south pole of another magnet, and vice versa, because they have opposing magnetic forces. Magnetic poles that are like (have the same force) are repelled from each other. Figure 12-5 is a sketch that shows how the poles react to each other. Magnetic Field Pictures It is hard to believe something that you cannot see. Let’s take an ordinary bar magnet, over which we will place a fl at sheet of paper that we sprinkle with iron fi lings. Iron fi lings are nothing more than tiny slivers of iron fi led from a larger piece of iron. By tapping the paper gently, the particles of iron will conform to magnetic lines of force similar to those illustrated in Figure 12-6. N N N N N N N N N N N N N N N N N N N N N S N Goodheart-Willcox Publisher Figure 12-3. The molecules are in line in a magnetized steel bar. Magnetic pole North pole geographical Magnetic pole South pole geographical Goodheart-Willcox Publisher Figure 12-4. The earth is a big magnet. A compass points toward the magnetic pole. Attraction Repulsion N S N S N S N S Goodheart-Willcox Publisher Figure 12-5. Unlike poles attract. Like poles repel.