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CHAPTER 10 TIME-VARYING FIELDS AND MAXWELL'S EQUATIONS The basic relationships of the electrostatic and the steady magnetic field were obtained in the previous nine chapters, and we are now ready to discuss time- varying fields. The discussion will be short, for vector analysis and vector calcu- lus should now be more familiar tools; some of the relationships are unchanged, and most of the relationships are changed only slightly. Two new concepts will be introduced: the electric field produced by a changing magnetic field and the magnetic field produced by a changing electric field. The first of these concepts resulted from experimental research by Michael Faraday, and the second from the theoretical efforts of James Clerk Maxwell. Maxwell actually was inspired by Faraday's experimental work and by the mental picture provided through the ``lines of force'' that Faraday introduced in developing his theory of electricity and magnetism. He was 40 years younger than Faraday, but they knew each other during the 5 years Maxwell spent in London as a young professor, a few years after Faraday had retired. Maxwell's theory was developed subsequent to his holding this university position, while he was working alone at his home in Scotland. It occupied him for 5 years between the ages of 35 and 40. The four basic equations of electromagnetic theory presented in this chap- ter bear his name. 322
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10.1 FARADAY'S LAW After Oersted 1 demonstrated in 1820 that an electric current affected a compass needle, Faraday professed his belief that if a current could produce a magnetic field, then a magnetic field should be able to produce a current. The concept of the ``field'' was not available at that time, and Faraday's goal was to show that a current could be produced by ``magnetism.'' He worked on this problem intermittently over a period of ten years, until he was finally successful in 1831. 2 He wound two separate windings on an iron toroid and placed a galvanometer in one circuit and a battery in the other. Upon closing the battery circuit, he noted a momentary deflection of the galvanometer; a similar deflection in the opposite direction occurred when the battery was disconnected. This, of course, was the first experiment he made involving a changing magnetic field, and he followed it with a demonstration that either a moving magnetic field or a moving coil could also produce a galvanometer deflection. In terms of fields, we now say that a time-varying magnetic field produces an electromotive force (emf) which may establish a current in a suitable closed circuit. An electromotive force is merely a voltage that arises from conductors moving in a magnetic field or from changing magnetic fields, and we shall define it below. Faraday's law is customarily stated as emf ±² d ± dt V ³ 1 ´ Equation (1) implies a closed path, although not necessarily a closed conducting path; the closed path, for example, might include a capacitor, or it might be a purely imaginary line in space. The magnetic flux is that flux which passes
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