My Virginia Tech Talk: Energy Flow in Reactive Fields

Smith Chart of the near-field impedance for a magnetic dipole (top) and electric dipole (bottom).

I will visit Virginia Tech on Monday March 13th to give an EM seminar on “Energy Flow in Reactive Fields” at 1:00PM in the Whittemore 654 conference room.

  • Slides:

Abstract:  Energy Flow in Reactive Fields

Conventional wisdom treats radiation as a transverse kink in a field line and classifies the “near field” by where it appears: “near” a source. A closer examination of electromagnetic fundamentals reveals a different picture.

Electromagnetic waves propagate at the speed of light and comprise a balance of electric and magnetic energy. The free space impedance, 377 ohms, relates the electric and magnetic field intensities. When that balance is disturbed, there is an excess of electric or magnetic energy, and the energy slows down. In the limit where fields are purely static, the energy is at rest. By characterizing fields according to their impedance, we discover that “near,” or more properly, “reactive” fields exist all around us, and electromagnetic energy curves, bends, and behaves in counterintuitive ways.

These insights are crucial to understanding how radiation works – the transformation of bound electromagnetic energy into radiant energy. This talk considers radiation from an accelerating charge and from a dipole source to illustrate the fundamental point that the propagation of fields and the flow of energy behave differently, and each offers unique insights to how electromagnetic systems behave.

Finally this talk considers the application of reactive energy flow to understand not only the behavior of electrically-small antennas, but also the puzzle of two-slit interference in the context of quantum mechanics.

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