Electromagnetic plane waves in vacuo

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Discussion Overview

The discussion revolves around the mathematical treatment of electromagnetic plane waves in a vacuum, specifically examining the implications of applying Maxwell's equations to the wave equation. Participants explore the conditions under which the wave vector and electric field vector are related, and the conceptual understanding of transverse waves.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that applying Maxwell's equation (divE=0) leads to the conclusion that k must equal 0, which they find nonsensical.
  • Another participant provides a mathematical perspective, indicating that the divergence of the electric field leads to either k = 0 or sin(kz - wt) = 0 for all z and t.
  • A third participant emphasizes that electromagnetic waves are transverse and points out that the wave vector k must be perpendicular to the electric field E, suggesting that the initial approach oversimplifies the situation.
  • A later reply acknowledges a misunderstanding regarding the relationship between the coordinate system and the directions of k and E, indicating a realization of the conceptual error made.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the mathematical results, with some agreeing on the necessity of considering the transverse nature of electromagnetic waves, while others remain uncertain about the initial conclusions drawn from the equations.

Contextual Notes

The discussion highlights limitations in the initial assumptions about the relationship between the wave vector and the electric field, as well as the potential for misinterpretation when simplifying the coordinate system.

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Consider a plane wave f = cos(kz - wt). Applying Maxwell's equation (divE=0 in vacuo) gives
kcos(kz - wt) = 0 which means that k = 0. This surely doesn't make sense?
 
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Well, mathematically,

[tex]\bigtriangledown \cdot E_0 \cos(kz - wt) = E_0 \sin(kz - wt) = 0[/tex]

[tex]\Longrightarrow k = 0[/tex] or [tex]sin(kz - wt) = 0[/tex] [tex]\forall{ z, t }[/tex]

EDIT: But yes, addressed below is the conceptual issue here...
 
Last edited:
Well the reason you get that is that EM waves are transverse waves, and you have oversimplified the situation. The more general result is:
Consider a plane wave
[tex]\vec{E} = \vec{E_0}cos(\vec{k}\cdot\vec{r} - \omega t})[/tex]
Then,
[tex]\nabla\cdot\vec{E} = \vec{k}\cdot\vec{E} = 0[/tex]

Which shows that the wave vector is perpendicular to the field.
 
I think I see my mistake. I thought all i was doing was choosing my coordinates such that my z axis coincided with the wave vector k. The way i did it missed out the fact that k and E point in different directions.
 

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