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Chapter 9 Magnetism Objectives • After completing this chapter, you will be able to: – Identify three types of magnets – Describe the basic shapes of magnets – Describe the differences between permanent magnets and temporary magnets – Describe how the earth functions as a magnet – State the laws of magnetism Objectives (cont’d.) – Explain magnetism based on the theory of atoms and electron spin – Explain magnetism based on the domain theory – Identify flux lines and their significance – Define permeability – Describe the magnetic effects of current flowing through a conductor Objectives (cont’d.) – Describe the principle of an electromagnet – Describe how to determine the polarity of an electro-magnet using the left-hand rule – Define magnetic induction – Define retentivity and residual magnetism – Define a magnetic shield Objectives (cont’d.) – Describe how magnetism is used to generate electricity – State the basic law of electromagnetism – Describe how the left-hand rule for generators can be used to determine the polarity of induced voltage – Describe how AC and DC generators convert mechanical energy into electrical energy Objectives (cont’d.) – Describe how a relay operates as an electromechanical switch – Discuss the similarities between a doorbell and a relay – Discuss the similarities between a solenoid and a relay – Describe how a magnetic phonograph cartridge works. – Describe how a loudspeaker operates Objectives (cont’d.) – Describe how information can be stored and retrieved using magnetic recording – Describe how a DC motor operates Magnetic Fields • Magnetite – Natural magnet • Artificial magnet – Created by rubbing soft iron with another magnet • Electromagnet – Created by current flowing through a coil of wire Magnetic Fields (cont’d.) Figure 9-2. The earth’s magnetic north and south poles are situated close to the geographic North and South Poles. Figure 9-3. The domains in unmagnetized material are randomly arranged with no overall magnetic effect Figure 9-4. When material becomes magnetized, all the domains align in a common direction. Magnetic Fields (cont’d.) Figure 9-6. Magnetic lines of flux can be seen in patterns of iron filings. Electricity and Magnetism Figure 9-7. A current flowing through a conductor creates a magnetic field around the conductor. Figure 9-8. Determining the direction of the flux lines around a conductor when the direction of the current flow is known (left-hand rule for conductors). Electricity and Magnetism (cont’d.) Figure 9-9. When current flows in opposite directions through two conductors placed next to each other, the resulting magnetic fields repel each other. Electricity and Magnetism (cont’d.) Figure 9-10. When current flows in the same direction through two conductors placed next to each other, the magnetic fields combine. Figure 9-11. Determining the polarity of an electromagnet (left-hand rule for coils). Magnetic Induction Figure 9-12. Placing an iron bar in a magnetic field extends the magnetic field and magnetizes the iron bar. MOTION FLUX CURRENT Figure 9-13. The left-hand rule for generators can be used to determine the direction of the induced current flow in a generator. Figure 9-14. Induced voltage in an AC generator. Figure 9-15. Induced voltage in an DC generator. Figure 9-18. Operation of a DC motor. Summary • Electric current always produces some form of magnetism • Magnetism is the most common method for generating electricity • One theory of magnetism is based on the spin of electrons as they orbit around an atom Summary (cont’d.) • Another theory of magnetism is based on the alignment of domains • A magnetic field surrounds a wire when current flows through it • Left-hand rule: for conductors, coils, and generators • Faraday’s law: the basic law of electromagnetism