E-M fields mutual induction in E-M radiation

[nomadreid]

In electromagnetic radiation, the electric field and the magnetic field mutually induce each other: but my impression is that it would be better to say that they are two aspects of the same wave, so that there is no time between them. However, an alternative would be that there would be that there would be a (finite or infinitesimal) interval in which the electric field induced the magnetic field, and this in turn induced the next section of the electric field, which induced the following section of the magnetic field, and so forth. Which is correct?

It is clear that the mutual induction of electric and magnetic fields in electromagnetic radiation and in electromagnetic induction are related, both derived from Maxwell's equations. But I am not sure how much closely than that they are tied. For example, in electromagnetic induction, a changing electric field induces a magnetic field, and a changing magnetic field induces an electric field. Are these the same changes as the fluctuations in the electric and magnetic fields in electromagnetic radiation?

Or am I missing the whole point?

Thanks for any help?

 

[vanhees71]

This is an utterly wrong picture and you better forget about it. The sources of the em. field are charge and current distributions but not one type of field components wrt. the other. The causal solutions of Maxwell's equations are the retarded potentials or (in the classical case) equivalently the Jefimenko equations following from them.

 

[nomadreid]

Thank you, vanhees71. I shall indeed abandon this picture

This confusion was brought about by statements such as the following:
"electromagnetic radiation. Electromagnetic influences (in the language of physics: electric and magnetic field) which, even with no electric charges present, are locked in a state of mutual excitation so that they form a waves that propagate through space."
(https://www.einstein-online.info/en/explandict/electromagnetic-radiation/)

But since I know that popularizations in fields which I know something about are often way off the mark, I can easily accept that this picture is as well. I will try to go back and get a better picture as you indicated.

 

[Dale]

Which is correct?

What experiment could be done to determine the answer?

 

[nomadreid]

Good question, Dale. In posting the question, I was thinking that the theory would be based on one of those two possibilities; since I did not know whether there would be anything in the theory that would have distinguish between them (if the general picture was correct, which vanhees71 says is not the case.), I also could not determine what sort of experiment could distinguish between them.

 

[Dale]

I also could not determine what sort of experiment could distinguish between them

That is usually a good hint that the two propositions are not actually different scientifically (or are not scientifically clear)

 

[nomadreid]

Thanks, Dale.

That is usually a good hint that the two propositions are not actually different scientifically (or are not scientifically clear)

I think I poorly phrased or at least poorly emphasized my remark that a distinguishing experiment was not known to me, which is different to a statement that a distinguishing experiment was not known (by those who are in the field). I am not a physicist, and the fact that I don't know of an experiment is not a sign that no such experiment exists or could exist.

 

[Dale]

the fact that I don't know of an experiment is not a sign that no such experiment exists or could exist.

Yes, that is why I added the parenthetical comment “(or is not scientifically clear)”. As a general rule, if you cannot think of an experiment then it means, at a minimum, that your question is not yet scientifically clear in your own mind.

 

[nomadreid]

Dale , indeed,

your question is not yet scientifically clear in your own mind

I plead guilty. I need more understanding of Maxwell's equations.
There is such a large gap between the details of the proper scientific theory and the explanations available for the lay person.

 

[tech99]

Regarding the distinction between induction and radiated fields, Hertz carried out an experiment which was really a race between the radiation from a dipole and a wave traveling on a wire. As far as I can understand the English translation, the two waves generally traveled along at the same speed. But close to the dipole, the dipole's magnetic field did not appear to have a delay. I imagine this is because there is a strong induction field near the antenna, and in that region we have mainly stored energy, which is flowing both to and from the source. This would seem to make speed indefinable.
I have the Hertz paper, but not at home, but I can send details.

 

[nomadreid]

Thanks, tech99. Yes, I would like the details, and I read German, so either the original or a translation is fine. No hurry.

 

[anorlunda]

As a general rule, if you cannot think of an experiment then it means, at a minimum, that your question is not yet scientifically clear in your own mind.

@nomadreid , think carefully about that excellent advice Dale gave you. It is one thing to lack the education of experts, but it is a different thing (and unnecessary) to hold yourself back with unclear thinking about what you do know, and unclear questions. Poor questions often produce confusing answers. Dale offered a way for you to test your own questions before asking.

 

[vanhees71]

Maybe it's the distinction between the "near-field" and the "far-field" characteristics of the electromagnetic radiation from compact sources? The former are "quasi-static" in nature, while the latter are "radiative", i.e., more wave-like, and that's why the latter allow for a clear and simple definition of wave-propagation speed.

I'd also be interested in the paper by Hertz. I guess it's about the waves moving along two parallel wires. Sommerfeld writes in his excellent book on electromagnetism (Sommerfeld, Lectures on Theoretical Physics, vol. 3) that Hertz couldn't solve the mathematical problem, because he made the ansatz of "thin wires", i.e., modelling them as arbitrarily thin lines, which prevented him from using the correct boundary conditions. Sommerfeld gives a very elegant solution in terms of "bipolar cylinder coordinates". It's one of many gems in his theoretical-physics text-book series (in my opinion still the best ever written as far as classical physics is concerned).

 

[nomadreid]

think carefully about

anorlunda: lesson humbly learned.

vanhees71: could you briefly and roughly explain what the terms

"near-field" and the "far-field"

refer to? Thanks.

 

[vanhees71]

Take a compact source of charges and currents. Then the near-field region is the region of free space close to these sources. Formally it's at distances from the sources small compared to the typical wave lengths of the radiation considered (particularly if you assume a harmonic time dependence, where the wave length is sharply defined as $\lambda=2 \pi c /\omega$). In the far-field region is at distances from the sources very large compared to their extension. Here the multipole expansion tells you that you have fields that in leading order go like $1/r$ and in any small region around a point in this region it looks like a plane wave with the dispersion relation of free em. waves, i.e., $k=2 \pi/\lambda=\omega/c$.

 

[nomadreid]

Take a compact source of charges and

Thank you, vanhees71. Understood