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Unformatted text preview: 32 The Magnetic Field Recommended class days: 4 minimum Background Information Research has found that more than 50% of a typical class of students begins their study believing that • A positively charged rod held near the center of a pivoted bar magnet causes the magnet to rotate, with the positive charge repelling the north pole and attracting the south pole. • If one end of a bar magnet attracts a paper clip, the opposite end will repel the paper clip. In addition to thinking that electric charges and magnet poles are more-or-less equivalent, students tend to use electric and magnetic fields interchangeably. These beliefs are little changed by conventional instruction. For most students, their experience of magnetism extends little beyond the use of refrigerator magnets. Virtually all school children have used a magnet to pick up small steel objects, but the high percentage that expect one end of a magnet to repel the object suggests that these experiences have been infrequent. Informal surveys find that many students have never seen or experienced the repulsive force between two magnets. And, not surprisingly, few students are familiar with electromagnets. The presentation of magnetism usually begins by reminding students that they are familiar with refrigerator magnets and perhaps other permanent magnets, then switches immediately to the development of a theory of electro magnetism. This is a classic case of bait-and-switch, with few texts ever making any connection between electromagnetism and permanent magnets. Few, if any, conventional textbooks ever answer the most obvious question that a student might have, namely, “How does the magnet stick to the refrigerator?” This is the magnetic equivalent of “How does a charged comb pick up pieces of paper?” 32-1 32-2 Instructor’s Guid e Student Learning Objectives • To acquire familiarity with basic magnetic phenomena. • To develop a dipole model of magnetism, analogous to the charge model of electricity, that allows students to understand and reason about basic magnetic phenomena. • To learn the magnetic fields due to currents in wires, loops, and solenoids. • To study the motion of charged particles in magnetic fields. • To understand the magnetic forces and torques on wires and current loops. • To present a simple atomic-level model of ferromagnetism. • To connect the theory of electromagnetism to the phenomena of permanent magnets. Pedagogical Approach Many students are unfamiliar with the most basic of magnetic phenomena. Without such knowledge and awareness, the theory that is to be developed is not grounded in physical reality. Consequently, this chapter emphasizes: • The phenomena of magnetism. Although many basic characteristics are described in the textbook, this cannot substitute for demonstrations and hands-on experience....
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- Spring '10
- Magnetic Field