practice - Back Copyright Copyright by Holt Rinehart and Winston All rights reserved Back CHAPTER 22 Induction and Alternating Current PHYSICS IN ACTION

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Copyright © by Holt, Rinehart and Winston. All rights reserved. PHYSICS IN ACTION In an electric guitar,the vibrations of the strings are converted to an electric signal, which is then amplified outside the guitar and heard as sound from a loudspeaker. Yet the electric guitar is not plugged directly into an electric source.Instead,it generates electric current by a process called induction. By changing the magnetic field near a coil of wire,an electric current can be induced in the coil by the vibrations of the guitar strings. This chapter explores how induction produces and changes alternating currents. • Why does the generated current last only as long as the string vibrates? • Why must the strings be ferromagnetic in order for an electric guitar to work? CONCEPT REVIEW Resistance (Section 19–2) emf (Section 20–1) Magnetic force (Section 21–3) CHAPTER 22 Induction and Alternating Current Induction and Alternating Current 793

Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 22 794 MAGNETIC FIELDS AND INDUCED EMFS Recall that in Chapter 20 you were asked if it was possible to produce an elec- tric current using only wires and no battery. So far, all electric circuits that you have studied have used a battery or an electrical power supply to create a potential difference within a circuit. In both of these cases, an emf increases the electrical potential energy of charges in the circuit, causing them to move through the circuit and create a current. It is also possible to induce a current in a circuit without the use of a battery or an electrical power supply. Just as a magnetic field can be formed by a cur- rent in a circuit, a current can be formed by moving a portion of a closed elec- tric circuit through an external magnetic field, as indicated in Figure 22-1. The process of inducing a current in a circuit with a changing magnetic field is called electromagnetic induction. Consider a circuit consisting of only a resistor in the vicinity of a magnet. There is no battery to supply a current. If neither the magnet nor the circuit is moving with respect to the other, no current will be present in the circuit. But if the circuit moves toward or away from the magnet or the magnet moves toward or away from the circuit, a current is induced. As long as there is rela- tive motion between the two, a current can form in the circuit. The separation of charges by the magnetic force induces an emf It may seem strange that there can be an induced emf and a corresponding induced current without a battery or similar source of electrical energy. Recall that in the previous chapter a moving charge can be deflected by a magnetic field. This deflection can be used to explain how an emf occurs in a wire that moves through a magnetic field.