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The question If I create a varying electric field and it will then create a varying magnetic field, so will it also create light? Will I see a light ray? got me thinking.

I'm pretty sure that if I could put a strong enough magnetic or electrostatic dipole on a shaft spinning sufficiently fast, I could make a low frequency radio wave that would propagate to the far field and receive it with a suitably low frequency antenna and radio receiver.

I'm curious if such a demonstration has actually been done like that.

I'm not asking for analogous demonstrations or "that's in effect what a radio transmitter does with a loop antenna" type answers, I'd like to know if such a practical demonstration has ever been successfully carried out.

There's got to be a real, mechanical rotating shaft and a real magnetostatic or electrostatic dipole, like a bar magnet or two charged spheres separated by an insulating rod for example, and an actual receiver in the far field recording propagating electromagnetic waves, not just some evanescent tail.

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    $\begingroup$ see, for example, ieeexplore.ieee.org/document/8939554 and its references, also en.wikipedia.org/wiki/Alexanderson_alternator $\endgroup$
    – hyportnex
    May 16, 2022 at 21:24
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    $\begingroup$ @hyportnex oh that's excellent! In fact I'm going to broaden my question to include rotating magnetic dipoles as well, something I should have thought about in the beginning. $\endgroup$
    – uhoh
    May 16, 2022 at 21:29
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    $\begingroup$ magnetic dipole is "better" than electric dipole for the former can have higher energy density; higher current at lower voltage is preferable than higher voltage at lower current in a high power transmitter because of arcing and some such. $\endgroup$
    – hyportnex
    May 16, 2022 at 21:31
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    $\begingroup$ @hyportnex okay, as long as it's an actual demonstration and not just a proposal; for the purposes of this question there's gotta be a real receiver in the far field picking up real, propagating EM waves. $\endgroup$
    – uhoh
    May 16, 2022 at 21:34
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    $\begingroup$ @hyportnex The Alexanderson alternator used a conventional antenna to radiate the energy. $\endgroup$
    – John Doty
    May 16, 2022 at 22:10

2 Answers 2

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It's very difficult. A "turnstile" antenna is effectively a rotating dipole, simulated electrically. For the usual half-wave turnstile, the tips of the effective spinning element are moving faster than the speed of light! If you make it smaller, thus reducing the effective linear rotation speed, its capability as a radiator declines rapidly. I don't know of a mechanical system that can rotate >1/100,000 the speed of light.

Edit: In response to comments, here's a quick and dirty engineering sketch of an experiment.

Reference Data for Engineers (E. C. Jordan Ed., 1989) tells me that the radiation resistance of a dipole antenna scales as length squared (Jackson, of course, tells me the same). The current is charge/time, so it scales as velocity for the same charge. Power is proportional to the square of the current times the resistance. So, rotating a dipole at 10^-5 c radiates ~10^-20 of the power that the turnstile radiates for the same amount of charge on the elements. RDfE tells me the natural noise on Earth at 10 kHz (unlikely to be practical as a mechanical rotation speed) is ~160 dB above the nominal thermal.

Nominal thermal is -204 dBW/Hz, thus natural noise at this frequency is -44 dBW. Let's imagine that we can transmit +30 dbW (1 kW) with our turnstile: then, our mechanical version would transmit -170 dBW. Even if our receiver could capture this all (impractical), our SNR is -126dB in a 1 Hz bandwidth. Thus, we'd need to integrate for ~10^13 seconds to detect a signal. I don't expect to live that long ツ

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    $\begingroup$ "I'm not asking for analogous demonstrations or 'that's in effect what a radio transmitter does with a loop antenna' type answers..." The whole point of the question is to focus on mechanical rotation. $\endgroup$
    – uhoh
    May 16, 2022 at 22:24
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    $\begingroup$ If it were easy demonstrations would be well known and I wouldn't have asked. If it rotates slower than the speed of light it will produce a strong evanescent field because the induced field (magnetic in the case of rotating charge dipole) will be weak, and while the propagating EM field will be weak, it won't be zero. $\endgroup$
    – uhoh
    May 16, 2022 at 22:56
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    $\begingroup$ your answer is "I don't know, but sounds hard"? "well known" by who? Let's see what others post. There's already a proposed device mentioned in a comment under my question which suggests it's doable; not all physicists keep track of what electrical engineers are doing; let's wait and see what other answers are posted. $\endgroup$
    – uhoh
    May 16, 2022 at 23:03
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    $\begingroup$ That sounds like something that could be demonstrated quantitatively using math and physics rather than just asserting in prose. Can you demonstrate that in a convincing way rather than just saying it without support? I mean, "it's practically impossible because otherwise I would have heard of it" is not really a Physics SE answer. $\endgroup$
    – uhoh
    May 16, 2022 at 23:08
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    $\begingroup$ I'm referring to the other link in the comment. "The proposed mechatronic antenna..." I don't have access to the whole paper, I can't tell if there is a conventional antenna on top of that or not. I'll check the paper later today. Anyway, I'm still looking forward to either a citation of a demonstration or a physics-based (numbers and all) answer to why this is impossible, say based on a quarter-wave receiving antenna and $k_B T$ arguments, which I'm pretty sure it is not. $\endgroup$
    – uhoh
    May 16, 2022 at 23:15
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Physically rotating the magnet on an orbital motion around a point in space is the principle used Faraday induction, to generate electric current in electric power plants. The electric current is induced by the rotating magnetic field into electric coils.

"The RPM will depend on the number of poles in the alternator. In case of 2 magnetic poles the RPM would be 3600, for 4 pole 1800 for 6 pole 1200 and so on The formula is RPM=120 x f/n Where f is the required frequency and n is the no of poles in the alternator."

https://www.quora.com/In-order-to-generate-electricity-at-a-frequency-of-60Hz-a-generator-in-a-power-plant-must-be-operated-at-how-much-RPM

If you mean in your question just spinning very fast a permanent magnet around its own N-S axis, I don't think you will accomplish anything the field will remain still static and there will be no noticeable EM radiation unless the magnet has large dimensions. The physically spinning magnet will additionally precess that would generate EM waves of $ω_{p}$/2π frequency in Ηz units:

$$ \omega_{\mathrm{p}}=\frac{m g r}{I_{\mathrm{s}} \omega_{\mathrm{s}}} $$

where $ω_{s}$ the spin angular velocity in rad/s, $I_{s}$ the moment of inertia, m the mass of the magnet and g Earth's gravitational acceleration, $r$ the cross-section diameter of the magnet pole divided by two and $ω_{p}$ the EM precession frequency.

To get a feeling of the above equation where Is=7.5x10^-10 Kg m^2 is the calculated moment of inertia of a sphere ferrite magnet, ωs=47.77Hz, m=0.3gr, g=9.81, m/s^2 and r=2.5mm the radius of the sphere magnet. The value obtained of $ω_{p}=5.2$ Hz units radiated EM waves corresponds to a value of no less than 312 rpm, Newtonian precession rotations. Imagine now try to spin on its N-S axis a large 10 cm magnet at 312 rpm! The mechanical stress would break the magnet apart.

If you mean to spin the magnet on an axis perpendicular to its N-S axis then yes it will radiate EM waves of frequency proportional to its spin rpm.

As for related experiments of what you are asking please see my comments on your question.

Here is an example of a rotating magnet ELF-ULF radio transmission antenna, and reception of the EM waves in the near field: https://www.jpier.org/PIERM/pierm72/14.18070204.pd

Reception in the far field is practically very difficult to be demonstrated remotely (i.e. receiver must be at least 2 wavelengths away from transmmiter) for such low frequency EM waves which can have wavelengths of many Km. Unless, you have access to a ULF reception antenna array station (300Hz-3KHz) (wavelengths 1Km to 100Km)! en.wikipedia.org/wiki/Project_Sanguine or maybe HAARP https://en.wikipedia.org/wiki/High-frequency_Active_Auroral_Research_Program .

Of course one could try far reception with a sensitive ULF-ELF electrical short antenna like these magnetic antennas here: aaronia-shop.com/products/antennas-sensors/magnetic-antenna

Otherwise, as far as I know, there is no demonstration or reference currently to be found of such thing you are asking thus if someone has demonstrated reception specific in the far field of ULF or ELF EM waves generated by a mechanically rotating EM charge.

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  • $\begingroup$ I've asked specifically "Has anyone ever put a magnetic or electrostatic dipole on a rotating shaft, spun it and demonstrated reception of a propagating wave in the far-field?" $\endgroup$
    – uhoh
    Aug 27, 2022 at 13:16
  • $\begingroup$ @uhoh My answer was updated with a last paragraph. $\endgroup$
    – Markoul11
    Aug 29, 2022 at 12:31
  • $\begingroup$ Your last paragraph is an invitation for each reader to go sorting through a long trail of comments (which could at any time in the future be cleaned up) and to look for several of yours. While all of this is interesting, it's not a Stack Exchange answer to the question as asked. I don't want to sound unappreciative, but the question to be answered is "Has anyone X and demonstrated Y?" Of course if it were not so challenging it would have been done a long time ago. I don't see why one would consider rotating a physical dipole "around" it's axis, one would rotate perpendicular to it to radiate. $\endgroup$
    – uhoh
    Aug 29, 2022 at 22:05
  • $\begingroup$ If links in one of your comments point to an actual demonstration of rotating a magnetic or electric dipole and far field propagating EM radiation was demonstrated, then that certainly can be the basis of an answer. Yes an electric field would be produced in the process, but the actual physical thing that was being physically rotated would be a physical dipole magnet, not a distribution of electric charges. $\endgroup$
    – uhoh
    Aug 29, 2022 at 22:09
  • $\begingroup$ @utah Magnetism of electrons is actually electromagnetism. There is really nothing else more fundamental :) $\endgroup$
    – Markoul11
    Aug 29, 2022 at 23:54

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