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Most of the time, I read articles or watch videos saying that Hertz experiments validated Maxwell's theory of electromagnetism. But Hertz only confirmed the existence of waves (that are perhaps electromagnetic) and that they travel at the speed of light.

He did not for example confirm that these waves have both an electric and magnetic components. Although we know that the spark generated at the receiver of his apparatus is due to electromagnetic induction (changing magnetic field that produces current), we don't have evidence about the electric component.

Is there any experiment that confirms there is an electric component of these waves?

Quade
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  • Please explain why downvote when you do so ! It's been a while I am trying to understand this but have not answers to this question. And this is a place to learn... – Quade Jul 12 '20 at 16:42
  • Maxwells theory is a collection of severals theories which has been validated before maxwell put them together. He predictet the speed of light. Without the electrical component the wave wouldnt propagate. The evidence for his theory is - that all predictions and calculations work. I also think, if you downvote something you could leave a comment – nuemlouno Jul 12 '20 at 16:58
  • I think you got the downvote (I am only guessing but am not sure) because your question shows no effort that you tried to find the answer yourself. For example, people on this forum do not answer homework problems unless you show what and how you have tried and why you are stuck. – hyportnex Jul 12 '20 at 17:06
  • Hertz died in 1894, so we’ve had another 126 years to validate Maxwell’s equations. – G. Smith Jul 12 '20 at 18:04
  • @hyportnex if you put your best effort to find the anser of the last question, I think you could discover that it is not a trivial question, in particular if the issue is to have a direct evidence of an electric and magnetic components beyond the range of radio waves. – GiorgioP-DoomsdayClockIsAt-90 Jul 12 '20 at 21:06
  • @GiorgioP I think you are referring to this "Is there any experiment that confirms there is an electric component of these waves?" You may notice that I took it seriously and below I answered it, at least I tried. If your comment is regarding my comment above then you misunderstand it for it was answering Sarmon's question why he was downvoted by somebody. I was not the one to do so, on the contrary, but he is a new contributor and I tried to explain to him the prevailing "etiquette" here that might have induced somebody to downvote his question. – hyportnex Jul 12 '20 at 21:24

5 Answers5

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A linear dipole antenna (thickness << length AND thickness << wavelength) is sensitive to the electric field of the EM wave, a circular loop antenna (thickness << diameter AND thickness << wavelength) is sensitive to the magnetic field of the EM wave.

hyportnex
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  • thank's for the answer. I'll put this into consideration. But do you have any further explanation about why is the linear antenna sensitive to the electric field and not the magnetic field ? – Quade Jul 12 '20 at 19:27
  • because in a thin linear antenna you cannot have anything but an oscillating linear current from one end to the other and back without forming a loop – hyportnex Jul 12 '20 at 19:34
  • Technicians know how antenna booster works:fed up with DC current (yes indeed).Does it mean that electric field pull the waves? – elias2010 Jul 13 '20 at 07:33
  • @elias2010 what is an "antenna booster" and what does it have to do with the electric field? – hyportnex Jul 13 '20 at 16:27
  • It is a TV sighal amplifier.Supplying DC to the antenna creates a static electric field on it. – elias2010 Jul 13 '20 at 17:50
  • @elias2010 an amplifier supplies no DC electricity to any antenna – hyportnex Jul 13 '20 at 18:03
  • @hyportnex No it is n't.Check the circuit diagram http://www.circuitdiagram.org/uhf-antenna-amplifier-booster.html – elias2010 Jul 13 '20 at 20:55
  • @elias2010 This is a receiver amplifier and there is a blocking capacitor between the antenna and the amplifier input; there is no bias voltage on the antenna. – hyportnex Jul 13 '20 at 21:43
  • @hyportnex You have right there is not current but there is direct connection to negative -12 V which creates a static electric field. – elias2010 Jul 14 '20 at 06:54
  • @elias2010 maybe https://electronics.stackexchange.com/ can help you better with your concerns – hyportnex Jul 14 '20 at 09:22
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Is there any experiment that confirms there is an electric component of these waves?

How electromagnetic radiation takes place

The source of EM radiation are the subatomic particles. They absorb and emit photons. Another method does not exist. Hence radiation consists of photons.

A simple method to stimulate EM radiation is to accelerate and decelerate electrons. This was done in electric bulbs. The emitted radiation was over a wide spectra form infrared to visible light.

How to generate EM waves

Accelerating electrons periodically forth and back EM radiation in the form of an EM wave will be emitted. Installing an second antenna rod, best in the same direction as the emitting rod, the photons will induce an electric current (an alternating current) in the rod. This current is measurable.

Installing a loop antenna, the photons magnetic field will induce a current in the loop.

The orientation of the magnetic to the electric field

It‘s obvious that the acceleration of photons, all in the same direction, induces the electric field component of the emitted photons all in the same direction. Otherwise it would be impossible to get a current on the receiving antenna. And the same holds for the magnetic field component.

Holding your thumb up - representing the current in the antenna rod -, your second finger of the right hand shows the direction of the magnetic field component. Or should you take the left hand?

That does not matter as long, as you do not choose, should your finger show the north south direction or the S N direction. Once defined, the empirical facts show, that both field components from the electrons radiation always follows the same chirality. Only this allows as to have magnetic loop antennas.

Long story short. EM radiation consists of photons. The electric field and the magnetic field of these photons is nit measurable due to the chaotic emission. In the special case of synchronous radiation we get an EM wave (radio wave) and the macroscopic field components are measurable.

HolgerFiedler
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  • Really nice answer. Here is an interesting one: https://physics.stackexchange.com/questions/566335/does-the-observation-instrument-need-to-be-conscious-in-order-to-change-the-inte – Árpád Szendrei Jul 18 '20 at 01:45
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Maxwell's theory has been validated by the entire body of physics since publication.

Specifically, nearly all light absorption is due to electric dipole interaction, while paramagnetic resonance such as EPR, MRI and the astronomical 21 cm line are magnetic dipole transitions.

my2cts
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    Well, I would be less sharp: i) there are many phenomena beyond Maxwell's theory which can be accounted for only by QED. QED starts with the classical field obeying Maxwell's equations, but there are phenomena which cannot be explained at the classical level; ii) the body of the question explains that the validation in the title means something more than the indirect evidence of consistence of Maxwell's theory. It is addressing the problem if there a direct experimental evidence about the electric and magnetic nature of the waves which are supposed to be described by Maxwell's equation. – GiorgioP-DoomsdayClockIsAt-90 Jul 12 '20 at 21:17
  • Although I have no doubt about the validity of Maxwell's theory based on a huge amount of indirect evidence, I feel interesting the question and I think it would deserve a direct answer. – GiorgioP-DoomsdayClockIsAt-90 Jul 12 '20 at 21:19
  • @GiorgioP Related: https://physics.stackexchange.com/questions/328655/does-a-dc-supplied-superconductive-coil-gives-off-radiation – elias2010 Jul 13 '20 at 11:30
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It is easy to show, using the Lorentz force, that the work done per unit of volume and time on free charges by EM plane waves is $W = \mathbf {E.J}$, where $\mathbf E$ is the electric field of the wave and $\mathbf J$ is the density of current. The magnetic field contribution for the Lorentz force cancels out, and doesn't contribute for the work.

Every actual receptor circuit has resistance and it is necessary power to generate currents on them. As far I as understand we could not have that dissipated power in the circuit without the electric component of the wave.

Claudio Saspinski
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I think the issue here is in understanding what does it mean "electric" and "magnetic"; what actually are these fields? Strange they are in the space and time but they are not spacetime. Is it strange for you? For me it is absolutely outstanding!

I think that with his equations Maxwell wanted to say that even if these two fields are different in a way, they are strictly interlaced. To try to answer your question, consider that applying some logic passages starting from the Maxwell equations, you obtain the Poyting theorem that states that the change in energy density in a point of space and time, depends on the presence of charges and the Poynting vector.

If you are in empty space without net physical charges, you remain with the Poynting vector, in which electric and magnetic field cannot be everywere simultaneously null, if you have a process with energy exchange; radiation is this kind of process so that explains why the two fields are both present and interlaced.

This is just the theory, talking about an experiment, consider that Maxwell arrived to these equations trough a lot of experiments, but I don't have any of them in mind, someone can surely argument them very well.

Rob Tan
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