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# 31 - Faraday's Law - Chapter 31 Faradays Law CHAPTE R OUTLI...

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Faraday’s Law CHAPTE R OUTLI N E 31.1 Faraday’s Law of Induction 31.2 Motional emf 31.3 Lenz’s Law 31.4 Induced emf and Electric Fields 31.5 Generators and Motors 31.6 Eddy Currents 31.7 Maxwell’s Equations In a commercial electric power plant, large generators produce energy that is transferred out of the plant by electrical transmission. These generators use magnetic induction to generate a potential difference when coils of wire in the generator are rotated in a magnetic field. The source of energy to rotate the coils might be falling water, burning fossil fuels, or a nuclear reaction. (Michael Melford/Getty Images) Chapter 31 967

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968 T he focus of our studies in electricity and magnetism so far has been the electric fields produced by stationary charges and the magnetic fields produced by moving charges. This chapter explores the effects produced by magnetic fields that vary in time. Experiments conducted by Michael Faraday in England in 1831 and indepen- dently by Joseph Henry in the United States that same year showed that an emf can be induced in a circuit by a changing magnetic field. The results of these experi- ments led to a very basic and important law of electromagnetism known as Faraday’s law of induction . An emf (and therefore a current as well) can be induced in various processes that involve a change in a magnetic flux. With the treatment of Faraday’s law, we complete our introduction to the funda- mental laws of electromagnetism. These laws can be summarized in a set of four equa- tions called Maxwell’s equations . Together with the Lorentz force law , they represent a complete theory for describing the interaction of charged objects. Michael Faraday British Physicist and Chemist (1791–1867) Faraday is often regarded as the greatest experimental scientist of the 1800s. His many contributions to the study of electricity include the invention of the electric motor, electric generator, and transformer, as well as the discovery of electromagnetic induction and the laws of electrolysis. Greatly influenced by religion, he refused to work on the development of poison gas for the British military. (By kind permission of the President and Council of the Royal Society)
SECTION 31.1 Faraday’s Law of Induction 969 Finally, the galvanometer reads zero when there is either a steady current or no current in the primary circuit. The key to understanding what happens in this experi- ment is to note first that when the switch is closed, the current in the primary circuit produces a magnetic field that penetrates the secondary circuit. Furthermore, when Active Figure 31.1 (a) When a magnet is moved toward a loop of wire connected to a sensitive ammeter, the ammeter deflects as shown, indicating that a current is induced in the loop. (b) When the magnet is held stationary, there is no induced current in the loop, even when the magnet is inside the loop. (c) When the magnet is moved away from the loop, the ammeter deflects in the opposite direction, indicating that the induced current is opposite that shown in part (a). Changing the direction of the

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