chapter_20 - The vibrating strings induce a voltage in...

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660 20 CHAPTER O U T L I N E 20.1 Induced emf and Magnetic Flux 20.2 Faraday’s Law of Induction 20.3 Motional emf 20.4 Lenz’s Law Revisited (the Minus Sign in Faraday’s Law) 20.5 Generators 20.6 Self-Inductance 20.7 RL Circuits 20.8 Energy Stored in a Magnetic Field PhotoDisc/Getty Images The vibrating strings induce a voltage in pickup coils that detect and amplify the musical sounds being produced. The details of how this phenomenon works are discussed in this chapter. Induced Voltages and Inductance In 1819, Hans Christian Oersted discovered that an electric current exerted a force on a mag- netic compass. Although there had long been speculation that such a relationship existed, Oersted’s finding was the first evidence of a link between electricity and magnetism. Because nature is often symmetric, the discovery that electric currents produce magnetic fields led scientists to suspect that magnetic fields could produce electric currents. Indeed, experiments conducted by Michael Faraday in England and independently by Joseph Henry in the United States in 1831 showed that a changing magnetic field could induce an electric current in a circuit. The results of these experiments led to a basic and important law known as Faraday’s law. In this chapter we discuss Faraday’s law and several practical applications, one of which is the production of electrical energy in power generation plants throughout the world. 20.1 INDUCED EMF AND MAGNETIC FLUX An experiment first conducted by Faraday demonstrated that a current can be pro- duced by a changing magnetic field. The apparatus shown in Active Figure 20.1 (page 661) consists of a coil connected to a switch and a battery. We will refer to this coil as the primary coil and to the corresponding circuit as the primary circuit. The coil is wrapped around an iron ring to intensify the magnetic field produced by the current in the coil. A second coil, at the right, is wrapped around the iron ring and is connected to an ammeter. This is called the secondary coil , and the cor- responding circuit is called the secondary circuit. It’s important to notice that there is no battery in the secondary circuit . At first glance, you might guess that no current would ever be detected in the secondary circuit. However, when the switch in the primary circuit in Active Figure 20.1 is suddenly closed, something amazing happens: the ammeter
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20.1 Induced EMF and Magnetic Flux 661 measures a current in the secondary circuit and then returns to zero! When the switch is opened again, the ammeter reads a current in the opposite direction and again returns to zero. Finally, whenever there is a steady current in the primary circuit, the ammeter reads zero. From observations such as these, Faraday concluded that an electric current could be produced by a changing magnetic field. (A steady magnetic field doesn’t produce a current, unless the coil is moving, as explained below.) The current produced in the secondary circuit occurs only for an instant while the magnetic field through the sec- ondary coil is changing. In effect, the secondary circuit behaves as though a source of
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