Feynman Lectures Simplified 2B

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Robert L. Piccioni, Ph.D.

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2B: Magnetism and

Feynman Simplified 2B covers one quarter of Volume 2, the freshman course, of The Feynman Lectures on Physics. The topics we explore include:
  • Magnetostatics
  • Dynamic Electric & Magnetic Fields
  • Filters & Transmission Lines
  • Electromagnetic Waves in Vacuum
  • Electrical Circuits & Components
  • Circuit & Cavity Resonances


What Speed Light?

When physicists began developing  the theory of electromagnetism, they did not appreciate the connection between the speed of light and the constants in the electromagnetic equations. Maxwell’s equations were not initially written with c2, as Feynman does, but rather as:

Ď•E = ρ / ε0
ĎB = 0 (j + ∂E/∂t)

In fact, many authors still write the equations this way. The two constants ε0 and 0 are called the vacuum permittivity and the vacuum permeability, respectively. Their values are determined experimentally by measuring the force between charges and the force between currents.

As we discovered in the prior section, Maxwell’s equations show that electromagnetic fields propagate at velocity v, which as the equations were once written is:

v = 1/√(ε0 0)

Feynman says that Maxwell “remarked on the mysterious coincidence that [v] was the same as the speed of light. ‘We can scarcely avoid the inference,’ said Maxwell, ‘that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena.” (We now know that the field undulates without a medium.)

The theory of electromagnetism first unified the concepts of electricity and magnetism, and ultimately unified these with light. All these are different aspects of one common natural phenomenon.