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PHYSICAL SCIENCE Magnetism Part 1: Magnets and Magnetic Fields 14.1 Magnets and Magnetic Fields Objectives • Recognize that like magnetic poles repel and unlike poles attract. • Describe the magnetic field around a permanent magnet. • Explain how compasses work. • Describe the orientation of Earth’s magnetic field. Magnets • Naturally occurring magnetic rocks, called loadstones, were first found in Magnesia (part of modern-day Greece) almost 3000 years ago. • A loadstone is composed of an iron-rich mineral called magnetite. • Permanent magnets are magnetic all of the time. • The magnetism of a permanent magnet can be weakened or removed by hammering or heat. Lodestone and Ancient Compass Magnets • Some materials retain their magnetism longer than others do. • Soft magnets are easier to magnetize than hard magnets, but they also loose their magnetism easier that hard magnets. • A magnetic pole is an area of a magnet where the magnetic force appears to be the strongest. • Two like poles of magnets repel each other; two opposite poles attract one another. • If a magnet is cut, each piece will still have two poles. Magnetic Forces Magnetic Fields • A magnetic field is a region where a magnetic force can be detected. • All magnets produce a magnetic field; some are stronger than others. • The strength of a magnetic field depends on the material from which the magnet is made and the degree to which it has been magnetized. • Magnetic field lines are used to represent magnetic fields. • A magnetic field gets weaker as the distance from the magnet increases. Bar and Horseshoe Magnets Magnetic Fields • Magnetic field lines that are close together indicate a strong magnetic field; lines that are farther apart indicate a weaker field. • A compass is a magnet suspended on top of a pivot so that the magnet can rotate freely. • A compass aligns with the Earth’s magnetic field. • The first compasses were made by placing a lodestone on a small plank of wood floated in calm water. Map and Compass Magnetic Fields • A compass can be used to determine direction because of Earth’s magnetic field. • The source of the Earth’s magnetism is under debate. • The Earth’s magnetic field has changed directions throughout its geologic history – more than 20 time in the past 5 million years alone. • Evidence for magnetic reversals can be seen in the alignment of the minerals in the seafloor rocks as they progress outward from a spreading center. • The Earth’s magnetic poles are NOT in the same place as the Earth’s geographic poles. The Earth’s Magnetic Field Magnetic Fields • The magnetic pole in Antarctica is actually a magnetic N pole and the magnetic pole near the Boothia Peninsula in Canada is actually a magnetic S pole. • The poles of magnets are named for the geographic pole they point toward for historical reasons; thus, the end of the magnet labeled N is a “north-seeking” pole, and the end of the magnet labeled S is a “south-seeking” pole.