Speed of a propagating electromagnetic field?

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Discussion Overview

The discussion centers around the propagation speed of magnetic fields, particularly in the context of changing magnetic fields and their relationship to electromagnetic waves. Participants explore experimental methods for measuring this speed and the theoretical implications of electromagnetic wave propagation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about experiments measuring the speed of propagating magnetic fields.
  • Another clarifies that "propagation" refers to the effects of changing a magnetic field, which generates an electric field and leads to electromagnetic waves, whose speed has been measured historically.
  • A participant notes that it is generally assumed the speed of the magnetic field changes is equivalent to the speed of light but seeks specific experimental references, such as those using synchronized atomic clocks.
  • Another participant argues that measuring the phase difference of the magnetic field using search coils at different distances from an AC current loop is sufficient, suggesting that such experiments have been conducted in various forms.
  • One participant mentions that measuring the velocity of a transverse electromagnetic wave in a coaxial cable is straightforward, providing a formula for calculation.
  • A later reply emphasizes that acceptance of the magnetic field as part of an electromagnetic wave is crucial to the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and methods of measuring the speed of magnetic field propagation, with some asserting that it is inherently linked to electromagnetic waves while others seek more direct experimental evidence. No consensus is reached regarding the specific experimental methods or the need for advanced techniques like synchronized atomic clocks.

Contextual Notes

Participants highlight various assumptions about the relationship between magnetic fields and electromagnetic waves, as well as the historical context of measuring their speed. The discussion does not resolve the complexities of measuring these phenomena or the implications of different experimental approaches.

Exiguus
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Hi, I'm wondering if there has been any actual experiments done to measure the speed at which magnetic fields propagate?
 
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When you say "propagate" you must be referring to the effect of switching on or changing a magnetic field. You must 'mark it' in some way if you want to find its speed. Once you start to change a magnetic field, an Electric field is formed and then you have an Electromagnetic Wave. The speed of that has been measured with increasing accuracy for several hundred years.
 
Yes, when turning on or altering a magnetic field created by a current. It seems to me when I read that it is more or less assumed it travels with the speed of light, but I cannot find any reference to accurately measuring that using 2 synchronised atomic clocks for example.
 
But it's just a propagating em wave, isn't it? As I said, once you change the H field, an E field results. You can't have one without the other.
You don't need atomic clocks to measure the phase difference in the magnetic field which two search coils would pick up at different distances from a loop with AC passing through it. All you need is a reasonably good measure of the frequency of your AC and look at the phase with a suitable 'scope or timing mechanism. This sort of experiment has been done implicitly or explicitly on many occasions and in many different guises.
It always boils down to measuring the speed of em waves, which always proves to be independent of their frequency. You can either use an electric pickup ( a dipole) or a magnetic pickup (a loop). You get the same answer. I think you can't find references to what you say because it's sort of taken for granted, bearing in mind the theory and past evidence.
 
It is easy to measure the velocity of a TEM (transverse electric magnetic) wave in an air-filled coaxial cable. It is [itex]v=\left[ \frac{1}{\epsilon_o \mu_o} \right]^{1/2}[/itex]
 
Once you have accepted that the magnetic field is part of an EM wave, that is.
 

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