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Electromagnetic Induction

Scary title, huh? But it’s something that you use and experience every single day of your life

 

By Steve Humphrey

 

It was first discovered by Michael Faraday in the 1830’s while doing experiments with electricity and magnetism, both of which were familiar to physicists, but regarded as separate phenomena. What Faraday showed was that they are closely related. If you wrap a coil of conducting wire around a magnet and then move the magnet, an electrical current is generated in the wire. The effect is particularly strong when the magnet is spinning. And this is how most of the electricity in the world is generated. 

Examples: hydroelectric power, wind power, steam power, atomic energy, etc. Something, falling water, steam, wind, causes a turbine to rotate. A turbine consists of a magnet which is wrapped in a coil of wire. Through electromagnetic induction, this generates an electrical current (a stream of electrons) moving through the wire, which we use to power our homes. And this process is symmetrical. That is, if you run a current through the coil of wire, the magnet will rotate, and you have an electric motor. All electric motors operate on this principle. The challenge is to find a way to translate the spinning motion into useful motion.

Faraday was a brilliant experimentalist, with amazing physical insights, but he was untrained, and not mathematically sophisticated. His work was largely ignored by the Physics community because he could not provide a firm mathematical basis for his theories. It wasn’t until some 30 years later that James Clerk Maxwell, a Scottish physicist, was able to show rigorously that electricity and magnetism were different sides of the same basic phenomenon. The Maxwell Equations constituted the Second Great Unification in Physics, whereby two formerly distinct phenomena were proven to be different aspects of the same thing. 

His equations, however, were difficult to interpret. In fact, in order to provide a physical interpretation, he had to reintroduce that old chestnut, due to Aristotle, of the luminiferous aether, an invisible substance which pervades all of space and allows the propagation of waves and the transmission of forces. At that point in the history of Physics, mathematical rigor and empirical success were ascendent, but one still had to explain how the phenomena worked, and this explanation had to be given in terms of familiar concepts. For example, at that point, it was believed that forces could only be transmitted through some sort of direct contact. The bat hits the ball and causes it to fly off. The ball hits the window and causes it to break. The homeowner chases the miscreant and gives him a stern talking-to, causing him to reform his ways. “Spooky action-at-a-distance” was deeply distrusted. But one of the greatest contributions made by Faraday and Maxwell was the concept of a “field of force”, wherein one thing can affect another even though they are not in direct contact. 

The concept of a field was revolutionary. An invisible force can be transmitted through space over large distances. How odd. In the late 1600’s, Isaac Newton developed his theory of Universal Gravitation. One of his most significant insights inspired the proverbial story of the apple. In the First Great Unification in Physics, he realized that the same force that caused the apple to fall from the tree also was responsible for keeping the planets in orbit around the Sun and keeping the Moon in orbit around the Earth. His physical reasoning was straightforward. Suppose you could jump off a mountaintop. Where would you land? It would depend upon how fast you could jump. The greater your velocity, the farther you would go. And as the Earth curved away below you, you would fall farther and farther away from the mountain. Now, if you could jump fast enough, falling toward the ground but never hitting it, you would go around and around, and become an orbiting body. Thus, the Moon is falling toward the Earth, but because of its velocity, it keeps missing, and just stays in its orbit around the Earth.

But why does it fall? Why does the apple fall from the tree? Why doesn’t the Moon just sail off into space? There must be a force holding it in orbit, but what kind of force is it? Contemporaries of Newton, such as Descartes and Leibniz, tried to explain it in terms they understood, direct contact, and some quite imaginative theories were proposed. But Newton’s was the only one on firm mathematical footing that allowed precise predictions. He invented the Calculus for just this reason. But he had no mechanical explanation for gravity. This generated a tension in the Physics community. Good math, bad explanation vs. bad math, good explanation. Eventually, the strength of his mathematical theory outweighed the lack of an intuitive mechanical explanation. It wasn’t until the work of Faraday and Maxwell that the idea of fields was finally accepted, and Newton’s Gravitational Field was accepted.

Steve Humphrey has a Ph.D. in the history and philosophy of science, with a specialty in the philosophy of physics. Questions? Comments? Suggestions? Email him at Steve@thevoicelouisville.com