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Natural Science - Year II

Unit 52: Maxwell and Electromagnetic Radiation

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History Weblecture for Unit 52


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History Lecture for Unit 52: Maxwell and Electromagnetic Radiation

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Outline/Summary

Maxwell's synthesis: Electricity and Magnetism

Faraday's and Henry's work made it possible to control current flow in circuits, which opened up opportunities for more experimentation. The German Gustav Kirchhoff identified rules which allowed him to predict how different circuit configurations would affect voltage and current. He realized that electrical signals in wire propagate at the speed of light, even though individual electrons move at much slower speeds.

James Clerk Maxwell

These new observations, along with the information Faraday had already collected on polarized light in magnetic fields, gave James Clerk Maxwell some ideas about the relationship between electromagnetic phenomena and light.

Maxwell was a mathematician, trained at Edinburgh University in Scotland. He used mathematics to create models of physical systems and describe their limitations. One of his first analyses showed that Saturn's rings had to be made of small particles capable of individual movement at different speeds, because a solid object of the ring's size and shape would be torn apart by Saturn's gravitational field.

Aided by his wife, Maxwell performed a number of experiments on gas, color, and electromagnetic forces. He proposed that magnetism was the result of spinning molecules, which he called molecular vortices. The direction and speed of rotation of a vortex determined the intensity of its magnetic force. Using calculus to sum up the contributions of individual particles to an overall field, he was finally able to explain electromagnetic phenomena in precise mathematical terms .

Maxwell's Equations

Today, Maxwell's equations are considered the prime example of elegance, the physicist's term for a supremely simple mathematical relationship between basic natural concepts. All electromagnetic phenomena can be explained using one or more his his four equations, which are generalizations of the discoveries of his predecessors. These are

Notice that the last two laws reinforce each other: a changing magnetic field generates an electrical field, a changing electrical field generates a magnetic field, so the size or amplitude of the fields will oscillate (get bigger and smaller in a wave-like manner). A medium (the material through which the wave travels) can support the induction of magnetic and electrical fields in a specific amount, called the permittivity of the medium for electricity and the permeability of the medium for magnetism. The quantities of permittivity and permeability had been determined earlier for free space or vacuum. Maxwell realized that if he inserted them into his last two equations, he could predict the speed of the electromagnetic wave as 3.0 * 108 meters/sec. This coincided exactly for the independently—determined speed of light. Maxwell proclaimed in his Treatise on Electricity and Magnetism, published in 1873, that light was the result of electromagnetic wave propagation.

Read about James Maxwell at the St. Andrews Mathematics site.

  • What were Maxwell's achievements in mathematics? in physics? in astronomy?
  • What work did Maxwell do on the nature of gases? Who else worked on this subject?

Study/Discussion Questions:

Further Study/On Your Own