66020CHAPTEROUTLINE20.1Induced emf and Magnetic Flux20.2Faraday’s Law of Induction20.3Motional emf20.4Lenz’s Law Revisited (the Minus Sign in Faraday’s Law)20.5Generators20.6Self-Inductance20.7RLCircuits20.8Energy Stored in a Magnetic FieldPhotoDisc/Getty ImagesThe vibrating strings induce a voltagein pickup coils that detect and amplifythe musical sounds being produced.The details of how this phenomenonworks are discussed in this chapter.Induced Voltages and InductanceIn 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 Fnding was the Frst evidence of a link between electricity and magnetism. Becausenature is often symmetric, the discovery that electric currents produce magnetic Felds ledscientists to suspect that magnetic Felds could produce electric currents. Indeed, experimentsconducted by Michael ±araday in England and independently by Joseph Henry in the UnitedStates in 1831 showed that a changing magnetic Feld could induce an electric current in acircuit. The results of these experiments led to a basic and important law known as ±araday’slaw. In this chapter we discuss ±araday’s law and several practical applications, one of which isthe production of electrical energy in power generation plants throughout the world.20.1INDUCED EMF AND MAGNETIC FLUXAn experiment frst conducted by Faraday demonstrated that a current can be pro-duced by a changing magnetic feld. The apparatus shown in Active Figure 20.1(page 661) consists o± a coil connected to a switch and a battery. We will re±er tothis coil as the primary coiland to the corresponding circuit as the primary circuit.The coil is wrapped around an iron ring to intensi±y the magnetic feld producedby the current in the coil. A second coil, at the right, is wrapped around the ironring 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 thatthere is no battery in the secondary circuit.At frst glance, you might guess that no current would ever be detected in the secondary circuit. However, when the switch in the primary circuit in ActiveFigure 20.1 is suddenly closed, something amazing happens: the ammeter
20.1Induced EMF and Magnetic Flux661measures a current in the secondary circuit and then returns to zero! When theswitch is opened again, the ammeter reads a current in the opposite direction andagain returns to zero. Finally, whenever there is a steady current in the primarycircuit, the ammeter reads zero.From observations such as these, Faraday concluded that an electric current couldbe produced by a changingmagnetic feld. (A steady magnetic feld doesn’t produce acurrent, unless the coil is moving, as explained below.) The current produced in thesecondary circuit occurs only ±or an instant while the magnetic feld through the sec-ondary coil is changing. In e±±ect, the secondary circuit behaves as though a source o±
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