Feynman Lectures Simplified 2C

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Robert L. Piccioni, Ph.D.

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2C: Electromagnetism
in Relativity & in Dense Matter

Feynman Simplified 2C covers one quarter of Volume 2, the freshman course, of The Feynman Lectures on Physics. The topics we explore include:
  • Relativistic Maxwell’s Equations
  • Lorentz Transform for Potentials & Fields
  • Field Energy, Momentum & Mass
  • Relativistic Particles in Fields
  • Crystals
  • Refraction & Reflection in Dense Matter
  • Waveguides


Energy of Fields

We want now to quantitatively analyze the energy of electromagnetic fields in various circumstances. To do this, we need to quantify energy density and energy flow. We define u to be the electromagnetic field energy density per unit volume, and S to be the energy flow vector, the amount of energy per unit time per unit area passing through a surface perpendicular to S.

In V2p27-2, Feynman says: “in perfect analogy with the conservation of charge, we can write the ‘local’ law of [field] energy conservation” as:

∂u/∂t + Ď•S = 0

This equation is valid when the energy of the electromagnetic field is conserved, which it is not in general. Total energy of all types is locally conserved, but energy freely changes from one type to another. Feynman gives the example of walking into a dark room and switching on the lights; field energy suddenly increases, while the energy of the power grid decreases.