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Chapter 5:
Fourth Equation of Electrodynamics

5.1 Ampère's Law

One of the giants in the development of the modern understanding of electricity and magnetism was the French scientist André-Marie Ampère (1775-1836). He was one of the first to demonstrate conclusively that electrical current generates magnetic fields. For a straight wire, Ampère demonstrated that (1) the magnetic field's effects were centered on the wire carrying the current, (2) were perpendicular to the wire, and (3) were symmetric about the wire. Though illustrated in an earlier chapter, this is illustrated again in Fig. 5.1.

This figure is strangely reminiscent of Fig. 4.7, reproduced here as Fig. 5.2, which depicts water circulating around a drain in a counterclockwise fashion. But now, let's put a paddle wheel in the drain, with its axis sticking in the drain as shown in Fig. 5.3. We see that the paddle wheel will rotate about an axis that is perpendicular to the plane of flow of the water. Rotate our water-and-paddle-wheel figure by 90 deg counterclockwise, and you have an exact analogy to Fig. 5.1.

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