EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: BrianHG on September 02, 2017, 07:59:54 am
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Ok, ignoring the difficulty here, and assuming the magnet's north and south pole is sufficiently wide apart to create an antenna tuned to 100hz, you can see why spinning this monster would be absurdly difficult due to length, and ignoring the hardware and location needed to spin the massive thing and G-force which would break the magnet at that length and speed, (I know how any forum member here can easily pick this scenario to pieces...) would this rotating magnet create a 100hz electromagnetic wave?
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I can see the spinning magnet helping to create the magnetic part of a wave, but where would the electro- part come from? I think there needs to be an electric field as well as a magnetic field to construct an electromagnetic wave.
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...would this rotating magnet create a 100hz electromagnetic wave?
Yes.
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And yes it is, investigate how MRI works, wobbling little proton-magnets around:
https://www.youtube.com/watch?v=Ok9ILIYzmaY (https://www.youtube.com/watch?v=Ok9ILIYzmaY)
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I think this is the same idea that if you were able to create a circuit that oscillated at 400 THz, would the wire glow red? Yes. But good luck doing that with anything similar to actual electronic components.
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I think this is the same idea that if you were able to create a circuit that oscillated at 400 THz, would the wire glow red? Yes. But good luck doing that with anything similar to actual electronic components.
Don't LEDs or laser diodes do this?
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And yes it is, investigate how MRI works, wobbling little proton-magnets around:
https://www.youtube.com/watch?v=Ok9ILIYzmaY (https://www.youtube.com/watch?v=Ok9ILIYzmaY)
Looking at the MRI, as that video is presented, it's like the MRI primary magnet is aligning the water molecules orientation, and using an added injected em RF signal to vibrate them, then listen to the signal they make as they return to alignment to the main magnet. At this scale it is still difficult to imagine a powerless magnet just spinning in space creating my described 100hz radio wave.
It's comes to me as a 100hz AC generator core, but, without an electrical loading coil, how long would this magnet spin in free space in a vacuum. Or, will the generation of the field itself slow down the rotation even though there may not be any close metal/coil load to create drag like in a generator.
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It's not the water who's aligned, it's the protons inside the Hydrogen atoms. While that video is correct, it makes no sense unless one already knows how MRI works. Also, MRI is a complex mix of physics phenomena and signal processing technologies, let's forget about MRI for the moment, it will just add confusion to the topic. Let's stay with our rotating magnet.
Yes, the rotating magnet will slow down "by itself", because the rotating magnet is generating radio waves, and radio waves carry energy.
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Amazingly enough, the pioneers of commercial radio were able to get up to 100KHz and powers of hundreds of KW by mechanically varying the magnetic field. They used a spinning slotted steel disk to vary the reluctance of the magnetic path, because materials strength issues made it impractical to spin magnets fast enough.
https://en.wikipedia.org/wiki/Alexanderson_alternator (https://en.wikipedia.org/wiki/Alexanderson_alternator)
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I can see the spinning magnet helping to create the magnetic part of a wave, but where would the electro- part come from? I think there needs to be an electric field as well as a magnetic field to construct an electromagnetic wave.
If you generate an alternating magnetic field, the electric field component of the wave emerges on its own.
Similarly, most dipole antennas generate only the electric component. The magnetic component emerges from the electric one.
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I can see the spinning magnet helping to create the magnetic part of a wave, but where would the electro- part come from? I think there needs to be an electric field as well as a magnetic field to construct an electromagnetic wave.
If you generate an alternating magnetic field, the electric field component of the wave emerges on its own.
Similarly, most dipole antennas generate only the electric component. The magnetic component emerges from the electric one.
That means if you get a magnet spinning, you can harvest free electricity from it! Infinite energy is possible, kappa.
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I can see the spinning magnet helping to create the magnetic part of a wave, but where would the electro- part come from? I think there needs to be an electric field as well as a magnetic field to construct an electromagnetic wave.
If you generate an alternating magnetic field, the electric field component of the wave emerges on its own.
Similarly, most dipole antennas generate only the electric component. The magnetic component emerges from the electric one.
That means if you get a magnet spinning, you can harvest free electricity from it! Infinite energy is possible, kappa.
To harvest electricity from the spinning magnet, you need a coil near it, which will generate an alternating magnetic field, opposing the spinning magnet, just slowing it down.
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I can see the spinning magnet helping to create the magnetic part of a wave, but where would the electro- part come from? I think there needs to be an electric field as well as a magnetic field to construct an electromagnetic wave.
If you generate an alternating magnetic field, the electric field component of the wave emerges on its own.
Similarly, most dipole antennas generate only the electric component. The magnetic component emerges from the electric one.
That means if you get a magnet spinning, you can harvest free electricity from it! Infinite energy is possible, kappa.
To harvest electricity from the spinning magnet, you need a coil near it, which will generate an alternating magnetic field, opposing the spinning magnet, just slowing it down.
then all you need to do is put the energy from the coil and place it into a motor that will spin the magnet. b00m, problem solved. take that facts and actuality.
kappa
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I went to the woods and while harvesting mushrooms thought occurred: Antennas are impedance matched to "free space". This is what gets stuff out. Now for magnet to "slow down" it must also impedance match to something. Simple with loaded coil but what about original case of floating in "free space". Perhaps magnet should be subjected to some additional accelerations besides steady spinning to match with "free space"...
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Perhaps magnet should be subjected to some additional accelerations besides steady spinning to match with "free space"...
Except anything spinning is already accelerating. An object spinning at constant speed is under constant acceleration. This is why a spinning magnet can emit radiation, whereas a magnet moving at constant speed in a straight line cannot.
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Constant acceleration is puppy compared to jerk 8)
https://en.wikipedia.org/wiki/Jerk_(physics) (https://en.wikipedia.org/wiki/Jerk_(physics))
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In the case of a spinning magnet in free space where does the angular momentum go?
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Adding a question, what is the optimal size of a magnet that can be bolted to a 6000rpm motor for this? (I hope 1/2 or 1/4 wavelength can work...) What about one bolted to a 15000rpm SCSI hard drive spindle motor?
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In the case of spinning magnet in free space where does the angular momentum go?
If a spinning magnet emits radiation then the radiation carries energy away with it, which depletes the kinetic energy of the spinning magnet.
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Adding a question, what is the optimal size of a magnet that can be bolted to a 6000rpm motor for this? (I hope 1/2 or 1/4 wavelength can work...) What about one bolted to a 15000rpm SCSI hard drive spindle motor?
Guys on this forum are technicians, engineers and electronics hobbiests. Shouldn't be afraid of numbers. So do some calculating.
Go to a freshman physics book (perhaps yours) or google and find the centripetal acceleration involved with rotary motion.
a=v^2/r=w*r^2. (Sorry, I am not a web guru or Latex maven and am not going to make the equations pretty. Read w as omega, which is 2*pi*f).
You see that the acceleration goes as the square of the radius, and at a radius of 1/4 wavelength will be truly impressive.
You need to do an integral to get the stress in the magnet, but some simple bounding expressions will tell you the neighborhood you are playing in. Try assuming that all of the mass of the magnet is at half radius and at the full radius. Or a couple of minutes of google will get you an exact (sort of) answer. You can get the mass from the density and google will give you answers on the strength of materials.
This stuff takes minutes now in the age of the internet. There is no excuse for not applying real numbers to simple problems like these.
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then all you need to do is put the energy from the coil and place it into a motor that will spin the magnet. b00m, problem solved. take that facts and actuality.
kappa
You can do "/s" or an emote, instead of a Twitch meme.
Or you can use the classic memes,
(https://pics.me.me/problem-solved-no-more-need-for-petrol-rc-troll-physics-5450822.png)
Problem? ;D
Tim
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I went to the woods and while harvesting mushrooms thought occurred: Antennas are impedance matched to "free space". This is what gets stuff out. Now for magnet to "slow down" it must also impedance match to something. Simple with loaded coil but what about original case of floating in "free space". Perhaps magnet should be subjected to some additional accelerations besides steady spinning to match with "free space"...
Well, what does impedance mean?
It's a ratio of voltage to current.
It's also a ratio of electric to magnetic fields (E in V/m divided by H in A/m is still ohms). Hmm, that sounds useful!
We get magnetization directly off the magnet itself, so, wham, halfway there.
Where do we get volts? Aha, from spinning it, Faraday's law!
So as you spin up the magnet, the impedance in its near field increases (from zero, when stationary -- no E, all H). As it gets closer to electrical resonance, E/H approaches Zo, and radiation hits a maxima.
Tim
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Only small problem. Magnets spinning in free space are not known to slow down. Might even include some planets! Even worse, planets have more like grown and seemingly acquired energy :P
Of course experiment is king but in practice quite nightmare to set up:
- vacuum
- magnetic suspension with non-conducting magnets
- no conductors nearby
Fist couple of years would go into dealing with losses only :-DD
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It's important on what axis the magnet spins. If you spin it around N-S axis it won't generate radio waves, and it won't slow down by itself. Spinning it on an axis perpendicular to N-S it will generate radio waves, and will lose energy, so it will slow down.
The planets do slow down, at least the Earth is slowly slowing down for sure, that's what astronomers think.
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The planets do slow down, at least the Earth is slowly slowing down for sure, that's what astronomers think.
Some persons are even said to kill themselves because of depressing theories about heat death of universe, no need to take them too seriously.
As for spinning magnet case I will stick to opinion that we are missing some subtle details unless there is some experimental proof of contrary.
I will put this on todo experiments list with some modifications. Maybe someday...
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The extreme cases of spinning magnets, neutron stars and the like, are well known to slow down, the period does increase by a few nanoseconds per year as energy is dissipated in the massive magnetic field of what was once a slowly spinning large star. The magnetic fields are very large there, and the rotation rate is such that some, the size of the whole planet earth, are spinning at 1kHz, dragging this field around with them. They are spinning fast enough that you can measure relativistic effects on the surface, and surface gravity is also incredibly high, enough to keep the surface compressed into a near monocrystalline layer over the single atom ( effectively as it is compressed to the point where there are only neutrons touching each other) inside that is holding that magnetic field.
Now, interesting thing is what happens to the magnetic field when a star over the Chandrasekhar limit collapses such that surface escape velocity exceeds C.
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thought this might be of interest but a bit off topic.
Eavesdropping using microwaves - addendum | EE Times
https://eesage.com/pages/90973912-eavesdropping-using-microwaves-addendum-ee-times (https://eesage.com/pages/90973912-eavesdropping-using-microwaves-addendum-ee-times)
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The extreme cases of spinning magnets, neutron stars and the like, are well known to slow down,
Slow down maybe, but might be million other reasons. Speed can be affected by internal mass redistribution for example.
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Even if the mass is not charged or magnetic, then it will emit gravitational waves, if it's spherically asymmetric.
https://en.wikipedia.org/wiki/Gravitational_wave
thought this might be of interest but a bit off topic.
Eavesdropping using microwaves - addendum | EE Times
https://eesage.com/pages/90973912-eavesdropping-using-microwaves-addendum-ee-times (https://eesage.com/pages/90973912-eavesdropping-using-microwaves-addendum-ee-times)
Also refer to The Thing.
https://en.wikipedia.org/wiki/The_Thing_(listening_device)
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OP: The answer is that if would IF there is metal in the vicinity, into which it can induce a current.
Even so, the signal would be vanishingly small. Reason is that the length of antenna for efficient transmission is inversely proportional to the frequency. You can overcome this to some extent by 'loading' the antenna with a series tuned circuit, but this will be so far short of optimum length that it won't make much odds.
http://www.csgnetwork.com/antennagenericfreqlencalc.html (http://www.csgnetwork.com/antennagenericfreqlencalc.html)
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The planets do slow down, at least the Earth is slowly slowing down for sure, that's what astronomers think.
They slow down to one revolution per cycle. Just like the moon. Mercury is getting there. It just takes a while.
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then all you need to do is put the energy from the coil and place it into a motor that will spin the magnet. b00m, problem solved. take that facts and actuality.
kappa
You can do "/s" or an emote, instead of a Twitch meme.
Or you can use the classic memes,
(https://pics.me.me/problem-solved-no-more-need-for-petrol-rc-troll-physics-5450822.png)
Problem? ;D
Tim
The reason I chose to use Kappa is that it would confuse more people unfamiliar with the meme. Kappa is less well known, and less obvious. It was designed to be better bait.
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OP: The answer is that if would IF there is metal in the vicinity, into which it can induce a current.
Even so, the signal would be vanishingly small. Reason is that the length of antenna for efficient transmission is inversely proportional to the frequency. You can overcome this to some extent by 'loading' the antenna with a series tuned circuit, but this will be so far short of optimum length that it won't make much odds.
http://www.csgnetwork.com/antennagenericfreqlencalc.html (http://www.csgnetwork.com/antennagenericfreqlencalc.html)
A 1/4 wavelength 100hz magnet would need to be 234000 feet long. I guess we could simulate one by creating a coil wrapped single steel bar feeding it DC to create such a magnet...
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A 1/4 wavelength 100hz magnet would need to be 234000 feet long. I guess we could simulate one by creating a coil wrapped single steel bar feeding it DC to create such a magnet...
Not only that, but it needs to be laminated (0.8mm or less thickness?), and a few hundred feet around. Because the permeability is only a few thousand, so you only get a length ratio of about as much!
Tim
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Only small problem. Magnets spinning in free space are not known to slow down. Might even include some planets! Even worse, planets have more like grown and seemingly acquired energy :P
Just because you aren't aware of them, doesn't mean they don't exist. :P
Also, planets are astonishingly electrically neutral, so there is very little radiation produced from gross physical motion like we're talking about here.
Some planets do have a magnetic field, usually produced by a conductive molten core; but this does not vary much over a rotation (that is, the magnetic poles are mostly aligned to the axis of rotation -- a magnet spinning on the N-S axis radiates none). And orbits are much slower still, so you wouldn't expect any sensible radiation produced by that.
As mentioned, neutron stars spin fast enough that radiation is sensible: not just electromagnetic but gravitational as well.
Tim
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This guy got a neodymium magnet to spin at over 16.6Khz/1000000 RPM.
https://www.youtube.com/watch?v=RB7zzs9x5Og (https://www.youtube.com/watch?v=RB7zzs9x5Og)
We would need to do this with a sphere magnet with a diameter of 13000 feet to have an appreciable 16.6KHz radio wave.
The outer surface would be moving with such speed generating such centrifugal force, I don't think we have a material which could hold together at that speed let alone a neodymium magnet.
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It's important on what axis the magnet spins. If you spin it around N-S axis it won't generate radio waves, and it won't slow down by itself. Spinning it on an axis perpendicular to N-S it will generate radio waves, and will lose energy, so it will slow down.
The planets do slow down, at least the Earth is slowly slowing down for sure, that's what astronomers think.
well the earth has a relatively massive companion which is gaining kinetic energy (orbit is expanding) through tidal coupling- momentum transfer. The interesting thing about the tidal bulge deformation on earth's surface is that it is not pointed directly at luna or sol.
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well the earth has a relatively massive companion which is gaining kinetic energy (orbit is expanding) through tidal coupling- momentum transfer. The interesting thing about the tidal bulge deformation on earth's surface is that it is not pointed directly at luna or sol.
Interesting because, anywhere you see a phase shift that's not a multiple of 90 degrees, you know there's loss going on. Which... is exactly your point. ;D
Tim
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:scared: :scared: :scared: :scared: :scared: :scared: :scared: :scared: :scared: :scared:
Just found a man made 600 000 000 rpm spinning object. That's 10MHz. Too bad it's not a magnet.
https://www.youtube.com/watch?v=ySZNPUu3v58 (https://www.youtube.com/watch?v=ySZNPUu3v58)
They claim the acceleration on the surface it is 1 billion times the gravity on earth...
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It's a fun question: does a spinning magnet radiate? Sadly, I think the answer is "no", or at least not as envisaged here.
Why not? Because the magnetization of the magnet is fixed, so it effectively has a fixed surface current (thinking of a bar magnet viewed as a solenoid). To produce radiation, you need to accelerate charge. The spinning magnet can no more radiate than if you set up a DC current in a circuit with a battery and then wave the whole thing around - the current in the circuit stays fixed, and it doesn't radiate.
Invoking pulsars is bit of a red herring. They radiate because the rotating B field creates a strong E field that accelerates charged particles, which then radiate. It's not the rotating dipole moment of the pulsar that directly produces the radiation. The gravitational radiation that was mentioned is for binary pulsars (one orbiting very rapidly around the other).
So, if you want your spinning magnet to radiate, you may need to embed it in an ionized gas to get something of the 'pulsar' effect. But that is not quite what the OP was envisaging. An alternative might be to heat and cool the magnet above and below its Curie temperature, so that its magnetization varied with time - probably not a very good radiator though.
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To produce radiation, you need to accelerate charge.
But as mentioned in an earlier post, a spinning body is under constant acceleration. So if acceleration is required, it is present.
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Invoking pulsars is bit of a red herring. They radiate because the rotating B field creates a strong E field that accelerates charged particles, which then radiate. It's not the rotating dipole moment of the pulsar that directly produces the radiation. The gravitational radiation that was mentioned is for binary pulsars (one orbiting very rapidly around the other).
The above is good to note, by the way. We know pulsars radiate tremendous amounts of energy, because they precess, and we observe the rate of the precession decreasing. What are we measuring to find this? Radio waves, usually, which are radiated by the stuff being pushed around by the pulsar.
Rotating masses exhibit frame dragging, but that's a momentum transfer phenomenon, not necessarily a radiating one, while binaries do radiate, yes.
Tim
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In the case of a spinning magnet in free space where does the angular momentum go?
Into the radiated electromagnetic fields.
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To produce radiation, you need to accelerate charge.
Exactly. What some dudes here do not realize is that:
- rate of change of acceleration in spinning magnet for each point (domain) = 0
- in actual antenna charges are under time varying acceleration
Sadly cannot explain further someone may invent something that I plan to :P
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Well, a(t) ~= sin(w*t) so a'(t) ~= w*cos(w*t) and so on.
Tim
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It's a fun question: does a spinning magnet radiate? Sadly, I think the answer is "no", or at least not as envisaged here.
Why not? Because the magnetization of the magnet is fixed, so it effectively has a fixed surface current (thinking of a bar magnet viewed as a solenoid). To produce radiation, you need to accelerate charge. The spinning magnet can no more radiate than if you set up a DC current in a circuit with a battery and then wave the whole thing around - the current in the circuit stays fixed, and it doesn't radiate.
It depends on which way it is spinning. If the axis of rotation is in alignment with the poles, then it won't radiate, but if it's across the poles, i.e. the north and south are continuously flipping over, then it will radiate. As mentioned above, the radiation will be minimal, unless the size of the magnet is significant, compared to the wavelength, which is impractical for 100Hz, due to the forces involved.
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Some people will refuse to admit that:
- a variable magnetic field, booom, instantly produces a variable electric field
- a variable electric field, bammm, instantly produces a variable magnetic field
Once a field is variable, the other one appears there by itself, no matter what.
It's impossible to have a variable electric field without an accompanying variable magnetic field.
It's impossible to have a variable magnetic field without an accompanying variable electric field.
They will both vary together, always. There is no need for charges to move or to be present. Since a field varies, it's about photons now, electrical charges are not a must any more.
Magnetic and electric fields can exist "alone" only if they are static fields, and nothing varies. Once a field starts to vary, sorry, no more "loneliness". The other field's variations will appear out of nowhere and accompany the variation of the other field. Pretty crazy.
Anyway, no matter how much people like or hate those ideas, this is how the world is.
Of course, all these can be totally wrong interpretations of some other much deeper truth that we don't yet understand. Sure, but so far there is not one evidence that the theory above is wrong. If anybody have a better theory that can explain all electromagnetism better, then just start explaining it.
Just saying no to the best we have so far, without coming with another viable explanation, is not accepted in science.
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Some people will refuse to admit that:
- a variable magnetic field, booom, instantly produces a variable electric field
- a variable electric field, bammm, instantly produces a variable magnetic field
Once a field is variable, the other one appears there by itself, no matter what.
It's impossible to have a variable electric field without an accompanying variable magnetic field.
It's impossible to have a variable magnetic field without an accompanying variable electric field.
They will both vary together, always. There is no need for charges to move or to be present. Since a field varies, it's about photons now, electrical charges are not a must any more.
Magnetic and electric fields can exist "alone" only if they are static fields, and nothing varies. Once a field starts to vary, sorry, no more "loneliness". The other field's variations will appear out of nowhere and accompany the variation of the other field. Pretty crazy.
Anyway, no matter how much people like or hate those ideas, this is how the world is.
Of course, all these can be totally wrong interpretations of some other much deeper truth that we don't yet understand. Sure, but so far there is not one evidence that the theory above is wrong. If anybody have a better theory that can explain all electromagnetism better, then just start explaining it.
Just saying no to the best we have so far, without coming with another viable explanation, is not accepted in science.
You are absolutely right. I couldn't have put it better myself.
Whether or not the magnetic field dominates, depends on where the observer is. Both the electric and magnetic fields exist. Right next to the spinning magnet (near field region) the magnetic field will dominate, but as one travels further away, the magnetic field will, at first, decay faster, than the electric field, until they start balance (far field region) and from then, onwards decay with inverse square law.
The above is true for a magnet, but if it were a spinning capacitor, with one plate charge to positive and the other negative, then the situation will be the inverse. In the near field region, the electric field will dominate and decay more than the magnetic field, until the far field region, where they decay with inverse square law.
https://en.wikipedia.org/wiki/Near_and_far_field
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I forgot to add one more thing, a nice demonstration and also some interpretation and explanations of Maxwell's equations and their consequences:
https://www.youtube.com/watch?v=Yn-MEMaiA0Y&t=48m37s (https://www.youtube.com/watch?v=Yn-MEMaiA0Y&t=48m37s)
For those who don't have a full hour to spend, at minute 48:37 (https://youtu.be/Yn-MEMaiA0Y?t=48m37s) is stated exactly why a variation in one field is always accompanied by a variation in the other field.
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Whether or not the magnetic field dominates, depends on where the observer is.
Aha.... so apple can be either apple or mushroom, depending where the observer is? ::)
And can "fixed stars" observe themselves... !? hmm... :o
Joke time! >:D
Professor in the university to depressed audience:
"You know Earth is basically dead cooling rock and universe as whole is slowly dying heat death".
Demons down in hell keeping the fire up:
"Thats why I boil those academic types for extra long - they have no respect for our work!"
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I can't put right all of the misconceptions in this thread, but I can try to clarify the issue of radiation.
Many of you may believe that the spinning magnet must radiate because the rotating magnetic field creates an electric field that can be used in a generator etc. Indeed, it creates a time varying electric field, and you might then leap to the conclusion that it radiates. But that is not correct. Electromagnetic induction (as used in a transformer) is not the same thing as radiation - there's no propagation of energy away from a source of induction as there is with a source of radiation (e.g., an antenna).
In fact, as some of you may know, antennas create both a near field (induction) and a far field (the radiation). For the radiation, the electric field varies as 1/r, and power falls off as 1/r^2. In the near field, the induced E field falls off as ~1/r^3 (the field from a dipole), and there is no flow of energy associated with it. The extent of the induced field is ~ the wavelength of the radiation - at 100Hz that is ~3000km. In principle, you could detect the rotating magnet thousands of km away, but you'd be using the induced field, not a radiated field, and it would be a very weak field because of the 1/r^3 dependence.
In fact, some of the earliest 'radio' broadcasts in London were made at quite low frequencies. So, technically, the coil in the receiver was picking up the induced field from an oscillating current a mile or two away. It wasn't radio in the sense developed by Marconi, as you can't 'broadcast' using the short-range induced field.
Of course, electromagnetic theory rests upon Maxwell's equations and the subsequent work by Heaviside and others. The mathematical treatment of radiation is beyond most laypersons, but if you're interested you can consult Ch.9 of 'Classical Electromagnetism' by J.D. Jackson. At a more accessible level, you'll find that some treatments of antennas discuss both the near field and the far field.
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Many of you may believe that the spinning magnet must radiate because the rotating magnetic field creates an electric field that can be used in a generator etc. Indeed, it creates a time varying electric field, and you might then leap to the conclusion that it radiates. But that is not correct.
So, if what you call an "antenna" produces the variable electric field, then the variable electric field will radiate.
If the variable electric field is produced by a variable magnetic field, then the variable electric field will not radiate.
How does the variable electric field knows who produced it, so it can take the decision to radiate, or not to radiate?
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Damn I must have mixed up mushroom species... another joke did occur, about why one should not trust their gear:
After long and exhausting maneuvers great space experimentator Spacey (who thought Maxwell was his dog) finally did halt rocket engines - at absolute rest compared to fixed stars. With some very expensive robotic arm he placed single charged ball bearing in absolute rest near ship and observed zero magnetic field around it with even more expensive gear. Satisfied with result he went to the forum for clues with what new gear to pat himself on the back.
Suddenly space punk Punkey flies by bearing at completely illegal near light speeds and destroys his ANENG multimeter, that got immense induction in leads. Spacey got "patted" real well with robotic arm - but is unsure to this day was there magnetic field after all or not!? Only the back sure did hurt... :'(
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So, if what you call an "antenna" produces the variable electric field, then the variable electric field will radiate.
If the variable electric field is produced by a variable magnetic field, then the variable electric field will not radiate.
How does the variable electric field knows who produced it, so it can take the decision to radiate, or not to radiate?
EM fields don't 'know' anything and they don't make decisions.
You may know that electromagnetic waves involve both electric and magntic fields. Their relative phases are important in determining whether we get an electromagnetic wave (radiation) or not. This can also be treated using the Poynting vector (E x H in mathematical notation), which points in the direction of energy flow for a wave. So, you need to look at both the electric field E and the magnetic field H to analyse the energy flow. However, I won't go into the explicit maths for that here.
In this context, the key principle (which cuts out a lot of unnecessarily complicated analysis) is to recognise that radiation is produced by accelerating electric charges. The simplest case is a single oscillating charge. The oscillating non-uniform current in an antenna acts in much the same way because it produces an oscillating charge distribution of the surface of the antenna.
With the rotating magnet, there are no accelerating charges. (This treatment is non-relativistic, which is appropriate for this problem.)
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So, what is the difference between a variable electric field produced in the 2 different ways. Why one does radiate and the other does not? How are they different?
You keep telling about moving charges and antennas. It doesn't matter. But let's suppose you are right, let's suppose it does matter. Now, what if you are on the other side of a thin paper wall, and you don't know what is on the other side, a rotating magnet or an antenna? All you can detect is a variable electric field. Of course, you will also detect a variable magnetic field, but you don't know who produced it. The magnetic variations might be produced by the moving charges in an antenna, or it might be produced by a rotating magnet.
Will it radiate or not?
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Improve experiment: Fix magnet on carousel at the center, axis parallel to horizontal plane. Put detector on the edge so it would rotate with magnet. Observe reading. Now replace magnet with antenna.
If confused may read this and get even more confused:
http://www.mathpages.com/home/kmath528/kmath528.htm (http://www.mathpages.com/home/kmath528/kmath528.htm)
"There are subtle issues of interpretation when trying to equate the energy of radiation with the work done on a particle, not to mention the difficulty of isolating the inertial mass m from the electromagnetic mass."
..."subtle issues" indeed :-DD
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I can't put right all of the misconceptions in this thread, but I can try to clarify the issue of radiation.
Many of you may believe that the spinning magnet must radiate because the rotating magnetic field creates an electric field that can be used in a generator etc. Indeed, it creates a time varying electric field, and you might then leap to the conclusion that it radiates. But that is not correct. Electromagnetic induction (as used in a transformer) is not the same thing as radiation - there's no propagation of energy away from a source of induction as there is with a source of radiation (e.g., an antenna).
The thing it, it does radiate but it's not the same principle as a generator: electromagnetic radiation induction is a near field phenomenon.
Perfect transformers and generators don't radiate at all, because the magnetic field is tightly confined to within the device. In reality, all generators and transformers leak some magnetic field and will theoretically radiate but the amount of power will be tiny, because the device is so small, compared to the wavelength of the mains frequency, even the higher harmonics.
In fact, as some of you may know, antennas create both a near field (induction) and a far field (the radiation). For the radiation, the electric field varies as 1/r, and power falls off as 1/r^2. In the near field, the induced E field falls off as ~1/r^3 (the field from a dipole), and there is no flow of energy associated with it. The extent of the induced field is ~ the wavelength of the radiation - at 100Hz that is ~3000km. In principle, you could detect the rotating magnet thousands of km away, but you'd be using the induced field, not a radiated field, and it would be a very weak field because of the 1/r^3 dependence.
If you're several wavelengths away, then you won't be able to tell whether the radiation is from a spinning magnet or an antenna. The magnetic field would have decayed to the same level as the electric field. All you'd see is EM radiation, i.e. photons.
In fact, some of the earliest 'radio' broadcasts in London were made at quite low frequencies. So, technically, the coil in the receiver was picking up the induced field from an oscillating current a mile or two away. It wasn't radio in the sense developed by Marconi, as you can't 'broadcast' using the short-range induced field.
Yes, near field. Also, if the coil and tuning capacitor, were taken out of the radio and connected to an oscillator, tuned to the resonant frequency, it would radiate EM radiation, with magnetic field dominating the near field region.
Of course, electromagnetic theory rests upon Maxwell's equations and the subsequent work by Heaviside and others. The mathematical treatment of radiation is beyond most laypersons, but if you're interested you can consult Ch.9 of 'Classical Electromagnetism' by J.D. Jackson. At a more accessible level, you'll find that some treatments of antennas discuss both the near field and the far field.
Don't forget it's quite likely there are smarter and more educated people, than both you and me, who'll be reading this. :P
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Regarding the "Ch.9 of 'Classical Electromagnetism' by J.D. Jackson", I couldn't find it, but I found instead "Classical Electrodynamics" by the same author, and it has a chapter 9 called "Radiating systems, Multipole Fields and Radiation". I assume this is the book: http://www.fisica.unlp.edu.ar/materias/electromagnetismo-licenciatura-en-fisica-medica/electromagnetismo-material-adicional/Jackson%20-%20Classical%20Electrodynamics%203rd%20edition.pdf/view (http://www.fisica.unlp.edu.ar/materias/electromagnetismo-licenciatura-en-fisica-medica/electromagnetismo-material-adicional/Jackson%20-%20Classical%20Electrodynamics%203rd%20edition.pdf/view)
I don't pretend I understood everything that is written in that chapter at the first look, but I couldn't find any clue to the question "Why a rotating magnet (or a variable magnetic field) can not radiate radio waves?". So, CD4007UB, could you please point me to the right page, or explanation, or formula that should answer that question. Looking over that chapter 9, there is even a place where it says in words that the radiation patterns are the same for a magnetic dipole, or an electrical one:
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I think the question has been answered in this thread, and the textbooks seem to agree, that a magnet spinning on an axis perpendicular to its poles will radiate.
So I think trying to find an explanation for why it will not radiate will be very difficult.
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In fact, as some of you may know, antennas create both a near field (induction) and a far field (the radiation). For the radiation, the electric field varies as 1/r, and power falls off as 1/r^2. In the near field, the induced E field falls off as ~1/r^3 (the field from a dipole), and there is no flow of energy associated with it. The extent of the induced field is ~ the wavelength of the radiation - at 100Hz that is ~3000km. In principle, you could detect the rotating magnet thousands of km away, but you'd be using the induced field, not a radiated field, and it would be a very weak field because of the 1/r^3 dependence.
Near/far field is a useful distinction, but it's not actually important to the semantics in this case: if the near field is not wholly contained inside an ideal shield, then there will be nonzero radiation. The radiation might indeed be very small (the magnet spins for a very long time indeed, or slows predominantly due to other forces), but it will be nonzero nonetheless. And spinning it faster will yield more and more radiation, up to the first resonance (or the magnet exploding).
And even if it's contained within a shield, we don't know of any ideal AC conductors (type I superconductors are good -- Q factor in the 10^7 range, better than quartz crystal resonators -- but still not actually zero loss), so there will still be drag on the magnet, losses.
At the first resonance, radiation is maximal, which, actually, for an NdFeB magnet, assuming you could spin it that fast without it exploding, would probably be pretty significant. Comparable to the torque on a motor using the same magnet as a rotor.
At lower speeds, the magnet is simply a "short dipole". The coupling to free space is poor, but remains nonzero. Probably, the torque on such a magnet at F_res / 100 would still be significant (spinning down noticeably).
Tim
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Whether or not the magnetic field dominates, depends on where the observer is.
Aha.... so apple can be either apple or mushroom, depending where the observer is? ::)
In fact, it is exactly this insight which led to the development of special, and then general, relativity. Maxwell's equations do not obey Galilean relativity (everything looks the same, independent of position and velocity), but instead, Lorentz discovered a different rule. Einstein eventually developed this into a theory of spacetime itself.
To this day, E&M, Relativity and QED are the most stupendously accurate theoretical models of their subjects, ever known, so you might want to check what kind of mushrooms those are. ;)
Tim
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Wait a moment... if its all relative... ::) I'm just going to cut hole in the middle of carousel and put magnet on the ground, stationery. Then merry-go-round with antenna around it - shall receive loads of free radiation! :scared:
Of course there will be counter-torque on magnet because it's transmitting energy to me - but its bolted to earth so no prob - earth will just slow down a little not to scare conservation of energy alarmists too much. Main thing - not to mess up with magnet orientation - otherwise earth may speed up and alarmists go all crazy :rant:
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I think the question has been answered in this thread, and the textbooks seem to agree, that a magnet spinning on an axis perpendicular to its poles will radiate.
So I think trying to find an explanation for why it will not radiate will be very difficult.
Yes, I don't see how anyone with a formal education on the subject, can say a spinning magnet, won't produce electromagnetic waves. The science has been settled and proven with countless experiments, over a hundred years ago.
Wait a moment... if its all relative... ::) I'm just going to cut hole in the middle of carousel and put magnet on the ground, stationery. Then merry-go-round with antenna around it - shall receive loads of free radiation! :scared:
Of course there will be counter-torque on magnet because it's transmitting energy to me - but its bolted to earth so no prob - earth will just slow down a little not to scare conservation of energy alarmists too much. Main thing - not to mess up with magnet orientation - otherwise earth may speed up and alarmists go all crazy :rant:
Yes, that's true. an observer circling a stationary magnet, pole to pole, should experience electromagnetic radiation, but the magnetic field would be overwhelming, because they would be in the near field region.
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Yes, that's true. an observer circling a stationary magnet, pole to pole, should experience electromagnetic radiation, but the magnetic field would be overwhelming, because they would be in the near field region.
But suppose it's really big merry-go-round...
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I'm feeling outnumbered here, but I'll to try clarify the basic physics if I can.
Someone thinks they can hide the spinning magnet behind a wall and that they couldn't distinguish that from an antenna. Well, you can, in fact, tell the difference, but you need to measure both the electric and the magnetic fields - we are, after all, discussing an 'electro-magnetic' problem, and the two fields are linked.
In a radiated field, like that in a radio broadcast, the perpendicular components of E and B oscillate in phase. This means that the time average of the Poynting vector (which measures energy flow) is non-zero, and the energy propagates away from the antenna (in the direction of the Poynting vector). As you may (or possibly may not) know, that is not the case with induction: Faraday's law tells us that the emf is proportional to -dB/dt, so the E and B fields are 90degrees out of phase. By measuring the phase between the perpendicular components of E and B you can therefore distinguish between radiated fields and induced fields.
The 90degree phase difference between E and B for the induced fields is important because it means that the Poynting vector averages to zero, and there is no energy flow away from the inductive source. More detailed analysis (e.g., that in Jackson) shows that the induced fields also decrease much more rapidly with distance than 1/r - that's another way of seeing that they don't contribute to the radiation, without having to invoke Poynting's vector.
So, if you want to prove that your spinning magnet radiates, you'll need to show that it produces orthogonal E and B components that oscillate in phase. Otherwise, the great John Henry Poynting may start spinning in his grave.
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^
And what you'll find is, the components are not quite 90 degrees, so that the radiation field mostly cancels out at a distance -- but not entirely, and there will be some residual.
Tim
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^
And what you'll find is, the components are not quite 90 degrees, so that the radiation field mostly cancels out at a distance -- but not entirely, and there will be some residual.
Tim
An interesting assertion, but it raises some obvious, simple questions:
1. How do you produce a 1/r radiation field from rotating a permanent magnet whose B field falls off as 1/r^3?
2. What do you mean by "the radiation field mostly cancels out at a distance"?
3. If your proposed 'residual' radiation field is meant to fall off as, maybe, 1/r^3, how do you satisfy the inverse-square law? (As I expect you know, energy conservation requires that the radiated field falls off as 1/r - you could make a lot of money if you have a way round that.)
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^
And what you'll find is, the components are not quite 90 degrees, so that the radiation field mostly cancels out at a distance -- but not entirely, and there will be some residual.
Tim
An interesting assertion, but it raises some obvious, simple questions:
1. How do you produce a 1/r radiation field from rotating a permanent magnet whose B field falls off as 1/r^3?
2. What do you mean by "the radiation field mostly cancels out at a distance"?
3. If your proposed 'residual' radiation field is meant to fall off as, maybe, 1/r^3, how do you satisfy the inverse-square law? (As I expect you know, energy conservation requires that the radiated field falls off as 1/r - you could make a lot of money if you have a way round that.)
The same way you make a radiating field from a dipole whose [near] field goes as 1/r^3. :P
Or if you prefer, more precisely: a phased, perpendicular dipole pair, so you get the same rotating field as a magnet, without the inconvenience of having to spin it up.
I don't know the math, myself. Sounds like something good to look up. :)
Tim
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with the previous E-to-M dipole exchange? http://www.physicspages.com/2015/01/20/radiation-from-a-rotating-dipole/ (http://www.physicspages.com/2015/01/20/radiation-from-a-rotating-dipole/)
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@CD4007UB, I gather from what you are saying, the magnet wont radiate any photons/EM field by itself and never will. But, if I place a wire anywhere near the magnet, that wire will radiate the 100hz all by it self from the alternating 100hz magnetic field. Though, the magnet still radiates nothing at all, no photons/EM, ever. So, you are saying an oscillating magnetic force has nothing to do in creating or modulating photons. It's exclusively the varying electric signal in that near wire (part receiving voltage generator, part antenna) which now generates the photons/EM, but the wire now does not generate any magnetic field.
I am having trouble swallowing that.
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If you put your hand in water and swish it around you are only moving water molecules around. You are creating waves, but nothing new. Just waves in the water. Your hand is analogous to the magnet in this case. It's fields 'stir' the stuff around it. I think that is what he is saying.
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@CD4007UB, I gather from what you are saying, the magnet wont radiate any photons/EM field by itself and never will. But, if I place a wire anywhere near the magnet, that wire will radiate the 100hz all by it self from the alternating 100hz magnetic field. Though, the magnet still radiates nothing at all, no photons/EM, ever. So, you are saying an oscillating magnetic force has nothing to do in creating or modulating photons. It's exclusively the varying electric signal in that near wire (part receiving voltage generator, part antenna) which now generates the photons/EM, but the wire now does not generate any magnetic field.
I am having trouble swallowing that.
What's fantastic about electromagnetism is, it literally does not matter how big your test mass is, or indeed, if there is any at all; the electromagnetic field is there, independent of the observer*!
*Metaphysics aside. But, ah, that lies outside of physics, so I'm pretty safe with this one.
Tim
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If you put your hand in water and swish it around you are only moving water molecules around. You are creating waves, but nothing new. Just waves in the water. Your hand is analogous to the magnet in this case. It's fields 'stir' the stuff around it. I think that is what he is saying.
Yes, isn't the rotating magnetic field generated by the spinning magnet stir/wave up space, making the photon waves?
I mean isn't varying the electric charge in an antenna, back and forth creating the same effect?
Or, must we flip the polarity of the magnet without motion, say by magic, to replicate the way an areal generates the EM field?
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What's fantastic about electromagnetism is, it literally does not matter how big your test mass is, or indeed, if there is any at all; the electromagnetic field is there, independent of the observer*!
Ok, suppose charge tinfoil hat to some voltage. It's moving thru space with immense speed along with solar system / earth and entire galaxy in matter of fact. It should work quite well as magnet then?
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What's fantastic about electromagnetism is, it literally does not matter how big your test mass is, or indeed, if there is any at all; the electromagnetic field is there, independent of the observer*!
Ok, suppose charge tinfoil hat to some voltage. It's moving thru space with immense speed along with solar system / earth and entire galaxy in matter of fact. It should work quite well as magnet then?
What do you mean "along with"?
Is there relative velocity between the charge and the observer? If so, it is a current, and consequently generates a magnetic field.
Tim
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What's fantastic about electromagnetism is, it literally does not matter how big your test mass is, or indeed, if there is any at all; the electromagnetic field is there, independent of the observer*!
Ok, suppose charge tinfoil hat to some voltage. It's moving thru space with immense speed along with solar system / earth and entire galaxy in matter of fact. It should work quite well as magnet then?
What do you mean "along with"?
Is there relative velocity between the charge and the observer? If so, it is a current, and consequently generates a magnetic field.
Tim
But you just said it's always there, independent of observer? :( And how it can generate unless there is power source inside (puzzled experimentators head doesnt count).
Edit: even worse, suppose 2 observer are in same point of space in relation to charged object, but move in opposite directions, one toward charge, other away. How does charge know for who which polarity field to generate?
Edit2: And if generate 2 they would either require 2x more power to generate or cancel alltogether :scared:
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The field is always there; how it looks, depends on how fast, and in what direction, you're moving relative to the source.
It's not the source making an electric or magnetic field, it's the source making an electromagnetic field that is observer dependent.
Tim
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Damn good field source, imagine power and computing reserve it has. Must handle even case when number of observers moving near light speed approaches infinity.
Wish had PSU like this!
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Put another way, the observer observes a magnetic field depending on the relative motion with the charged object. It's all about the observer, not about the charged object.
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The simplest 'proof' I can think of is point the magnet at a test volume in space - there is now magnetic field in that volume, and energy stored in that field. you could calculate it. Now turn the magnet a bit to point another way - the field in the test volume has changed, maybe it got weaker, maybe stronger, but it changed so the energy in that test volune has changed, so power has flowed somewhere. The only reasonable explaination is it was carried by an em wave.
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I'm feeling outnumbered here, but I'll to try clarify the basic physics if I can.
Someone thinks they can hide the spinning magnet behind a wall and that they couldn't distinguish that from an antenna. Well, you can, in fact, tell the difference, but you need to measure both the electric and the magnetic fields - we are, after all, discussing an 'electro-magnetic' problem, and the two fields are linked.
You've not specified the antenna? Loop (magnetic field) or rod (electric field)?
If the loop antenna has a magnetic material inside, emits the same strength field and has similar dimensions to the spinning magnet, then the two will be virtually indistinguishable.
If the antenna is a rod, then it will be easier to tell the difference, between a spinning magnet and antenna, behind a brick wall, but only if you're close enough. If your many wavelengths away, then it will be virtually impossible.
In a radiated field, like that in a radio broadcast, the perpendicular components of E and B oscillate in phase. This means that the time average of the Poynting vector (which measures energy flow) is non-zero, and the energy propagates away from the antenna (in the direction of the Poynting vector). As you may (or possibly may not) know, that is not the case with induction: Faraday's law tells us that the emf is proportional to -dB/dt, so the E and B fields are 90degrees out of phase. By measuring the phase between the perpendicular components of E and B you can therefore distinguish between radiated fields and induced fields.
The 90degree phase difference between E and B for the induced fields is important because it means that the Poynting vector averages to zero, and there is no energy flow away from the inductive source. More detailed analysis (e.g., that in Jackson) shows that the induced fields also decrease much more rapidly with distance than 1/r - that's another way of seeing that they don't contribute to the radiation, without having to invoke Poynting's vector.
How about a spinning capacitor? Two plates, separated by a dielectric and charged to a high voltage. Does that just radiate an oscillating electric field and no magnetic field?
So, if you want to prove that your spinning magnet radiates, you'll need to show that it produces orthogonal E and B components that oscillate in phase. Otherwise, the great John Henry Poynting may start spinning in his grave.
I think Tim answered that.
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And what you'll find is, the components are not quite 90 degrees, so that the radiation field mostly cancels out at a distance -- but not entirely, and there will be some residual.
Tim
Or perhaps CD4007UB thinks the movement of charged particles are necessary to generate electromagnetic waves? Well yes that's true, because charged particles are responsible for the magnetic field in the first place.
https://en.wikipedia.org/wiki/Magnetic_moment#Examples_of_magnetic_moments
If you put your hand in water and swish it around you are only moving water molecules around. You are creating waves, but nothing new. Just waves in the water. Your hand is analogous to the magnet in this case. It's fields 'stir' the stuff around it. I think that is what he is saying.
Yes, isn't the rotating magnetic field generated by the spinning magnet stir/wave up space, making the photon waves?
I mean isn't varying the electric charge in an antenna, back and forth creating the same effect?
Or, must we flip the polarity of the magnet without motion, say by magic, to replicate the way an areal generates the EM field?
Yes, and if the antenna is a loop antenna, it will have a strong near H-field and will be very similar to the spinning magnet.
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If the loop antenna has a magnetic material inside, emits the same strength field and has similar dimensions to the spinning magnet, then the two will be virtually indistinguishable.
If the antenna is a rod, then it will be easier to tell the difference, between a spinning magnet and antenna, behind a brick wall, but only if you're close enough. If your many wavelengths away, then it will be virtually impossible.
Indeed, one can always construct a static equivalent that works out the same. Give or take mechanical tolerances, because that's always a challenge, but designing, say, an intersecting pair of magnetic solenoids or loops, to recreate the field (B and E within so-and-so tolerance, beyond a modest super-near-field distance*) from a spinning magnet, isn't at all impossible.
*Corresponding to a multiple of the wire diameter, or winding pitch, or solenoid diameter, say.
How about a spinning capacitor? Two plates, separated by a dielectric and charged to a high voltage. Does that just radiate an oscillating electric field and no magnetic field?
For a really pathological case, what about a capacitor with a coil wrapped around it, such that the field lines happen to be perpendicular, yet both fields are "near field" as such? ;D
There are a number of "woo" antenna designs out there, which try to use this gimmick; unfortunately for them, it's still not possible to violate the bandwidth-gain-size law.
Tim
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Put another way, the observer observes a magnetic field depending on the relative motion with the charged object. It's all about the observer, not about the charged object.
Aha, so charged object has only information around it, about being charged. And observer invests energy into creating disturbance that makes sense from his viewpoint? Otherwise charged objects field would have to contain all energy and orientation info for all possible observers.
Much like say volume of air with little rockets in it. Each rocket invests energy into creating shockwave in relation to still air.
Edit: Note that there is hierarchy in this situation. Suppose freeze all observers still. There is no way for charged object to move so it could mimic original situation when it's stationary instead and observers move (all in different directions!)
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and the textbooks seem to agree
...but Feynman does not:
http://www.applet-magic.com/feynmanEM.htm (http://www.applet-magic.com/feynmanEM.htm)
"There is evidently some trouble here, since we have inherited a prejudice that an accelerating charge should radiate, whereas we do not expect a charge lying in a gravitational field to radiate. "
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then all you need to do is put the energy from the coil and place it into a motor that will spin the magnet. b00m, problem solved. take that facts and actuality.
kappa
You can do "/s" or an emote, instead of a Twitch meme.
Or you can use the classic memes,
(https://pics.me.me/problem-solved-no-more-need-for-petrol-rc-troll-physics-5450822.png)
Problem? ;D
Tim
If you were to drop the magnet in front of the car and the magnet has better grip, then the car would be attracted to it. So you just need to pick it up and drop it a little bit ahead and repeat this very fast and this would work! >:D
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So does this mean if you made a neat DC electromagnet and spun it you could detect the slight increase in current because it's radiating. What if it was you that was spinning instead of the electromagnet. >:D
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No, but you would detect power consumed on the thing that's spinning it (i.e., it draws torque from the motor).
However, if you modulated it, AC power would be drawn, in much the same way that a plate-modulated AM transmitter consumes modulation power. Hmm, maybe.
Tim
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So does this mean if you made a neat DC electromagnet and spun it you could detect the slight increase in current because it's radiating. What if it was you that was spinning instead of the electromagnet. >:D
Yes, (I must go against T3sl4co1l on this one) I actually think you would. The same way as if you had a tuned antenna, or metal object near this DC powered spinning magnet, you could register the current as well as you DC powered magnet will react to the load...
Now, when I say you would, in free open space, this figure would be astronomically small.
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So does this mean if you made a neat DC electromagnet and spun it you could detect the slight increase in current because it's radiating. What if it was you that was spinning instead of the electromagnet. >:D
Yes, (I must go against T3sl4co1l on this one) I actually think you would. The same way as if you had a tuned antenna, or metal object near this DC powered spinning magnet, you could register the current as well as you DC powered magnet will react to the load...
Now, when I say you would, in free open space, this figure would be astronomically small.
I assumed he was referring to the DC bias current, which obviously cannot be increased by rotation: that would be equivalent to the bar magnet spontaneously losing magnetization, which doesn't make any sense. There would be induced currents, sure, but that's just ordinary changing fields. :)
Tim
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Yes, there is an AC component with rotation, but, there should also be a slight additional DC current component as well. The spinning magnetic force, with the poles at the edges is in a sense stirring up EM waves. This should exhibit an additional DC current on the magnet's power source as the rapid rotation in effect loads the magnetic poles and does produce a drag on the entire magnetic structure.
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What should the DC current be proportional to, then?
Is it absolute? If we reverse the terminal polarity, does the current continue in the same direction, thus generating power from the EM field instead?
What about the permanent magnet?
Tim
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Either direction is an identical load. The faster the magnet spins, the larger the DC load. Example, if we spun the magnet fast enough to emit visible light, the additional DC load would be significant compared to the magnet being stationary.
One test might be to make an electric magnet and drag it across a stationary flat sheet of iron to amplify the effect we want to measure down to a size and speeds we can handle with a large enough effect we could measure. Though the direction wont matter, during a fixed motion VS having the electromagnet stationary, same distance from the magnet, does the DC current load change? I know if you have a neodymium magnet suspended just above said iron sheet, it resists horizontal movement in any direction and generates heat illustrating a load.
Yes, spinning a permanent magnet would load it similar to dragging the magnet across a linear sheet of metal. You can forcefully degause magnets affecting their strength which show they can be affected.
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Suppose we use a superconducting coil, and charge it up to some steady DC current. Then we spin it in this test.
Does its loop current:
a. Decrease
b. Remain constant
c. Increase
If a or c, at what rate?
Tim
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Time to do an experiment. Magnet. Air turbine to avoid confusion with electric drive motor. Measure field fall off vs distance. If pure 1/R^3 then pure induction. If asymptote at distance is 1/R^2 then radiation is occurring. Could also use antenna design to help sort the answers.
Then it is time to see who interpreted the math wrong. Or applied it incorrectly.
Note that the experiment may have to be done very carefully to be definitive. It took generations of measurement improvement to prove that Newton was close, but not quite right.
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Suppose we use a superconducting coil, and charge it up to some steady DC current. Then we spin it in this test.
Does its loop current:
a. Decrease
b. Remain constant
c. Increase
If a or c, at what rate?
Tim
If there are surrounding magnetic fields/reflective objects, then won't the current fluctuate, even if its average value remains constant?
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Suppose we use a superconducting coil, and charge it up to some steady DC current. Then we spin it in this test.
Does its loop current:
a. Decrease
b. Remain constant
c. Increase
If a or c, at what rate?
Tim
a. Decrease.
Spin it fast enough, and it's generated EM field will slowly bleed off charge.
Now as for CatalinaWOW's comment, is this because of induction, or radiation.
At human speeds, normal motor RPMs, and human size, if there is any loss, then it is mainly due to induction.
If you have no other matter to interact with and your spin and magnet size is fast enough to create EM waves, then the power of these waves also would draw from the magnet source.
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If the superconducting current decreases also, then where does it go?
That is, how does the short-circuit loop open up in order to drop that flux? Where does it go? Did it momentarily cease to superconduct? (delta I = integral V dt / L)
Tim
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a. Decrease.
Spin it fast enough, and it's generated EM field will slowly bleed off charge.
This is not logical. If a permanent magnet would not lose its magnetism (it wouldn't), why would a superconducting magnet lose its magnetizing current? (Because in some sense, a permanent magnet is an assembly of atomic scale superconducting magnets with all their poles aligned.)
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a. Decrease.
Spin it fast enough, and it's generated EM field will slowly bleed off charge.
This is not logical. If a permanent magnet would not lose its magnetism (it wouldn't), why would a superconducting magnet lose its magnetizing current? (Because in some sense, a permanent magnet is an assembly of atomic scale superconducting magnets with all their poles aligned.)
You are correct in saying that magnets do not loose their magnetism so long as the magnet is kept below it's curing temperature point and no introduction of external mechanical force like a sudden physical impact, or, another magnetic signal with enough strength to alter the magnet's internal atomic aligned poles. IE if you keep the magnet cool and stationary, it should last up until the matter within begins to fall apart at the end of the universe.
So, by this logic, when spinning a permanent magnet, the em-field generated will take a load on the rotational energy and the magnet will be loaded, but, it would not weaken the permanent magnet unless it's temperature is beyond the curing point.
Now, for 'T3sl4co1l's electrical powered magnet, the situation is different. His magnetic field is generated by circular electrical current flow, not permanently aligned atomic poles fixed in a material. This means that when spinning, which induces an additional load at the magnetic poles, direction doesn't matter, either (A) the current goes up spin speed, or, (B) the strength at the poles of the magnet is weakened with spin speed, but the current stays the same, or finally (C), a mix of (A) & (B).
My vote is on (C).
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Simple quote:
Do rare earth magnets lose strength over time?
Very little. Neodymium magnets are the strongest and most permanent magnets known to man. If they are not overheated or physically damaged, neodymium magnets will lose less than 1% of their strength over 10 years - not enough for you to notice unless you have very sensitive measuring equipment.
K&J Magnetics - FAQ
www.kjmagnetics.com/faq.asp (http://www.kjmagnetics.com/faq.asp)
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Now, for 'T3sl4co1l's electrical powered magnet, the situation is different. His magnetic field is generated by circular electrical current flow, not permanently aligned atomic poles fixed in a material. This means that when spinning, which induces an additional load at the magnetic poles, direction doesn't matter, either (A) the current goes up spin speed, or, (B) the strength at the poles of the magnet is weakened with spin speed, but the current stays the same, or finally (C), a mix of (A) & (B).
My vote is on (C).
So now you're saying it goes up (my C from earlier)?
B is a non-sequitur, because the back EMF from radiation will lower the measured field by superposition. You're pushing on space, and space is pushing back. This is, however, not a change of the magnet's magnetization, nor the electromagnet's current flow (whether powered or superconducting), neither momentary nor permanent. It's just the superposition of fields: when you force two magnets together, their fields cancel and you get zero field in the middle, and less field within each magnet. But the magnetization remains the same. Indeed, the force required to push the magnets together, is the same force that is acting on the rotor, causing drag: this is where the radiant power comes from.
The correct answer must be consistent for all three cases (electromagnet, superconductor and permanent magnet). If you agree that a permanent magnet will not loose magnetization, then there is only one correct answer for the other cases. :)
Tim
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Now, for 'T3sl4co1l's electrical powered magnet, the situation is different. His magnetic field is generated by circular electrical current flow, not permanently aligned atomic poles fixed in a material. This means that when spinning, which induces an additional load at the magnetic poles, direction doesn't matter, either (A) the current goes up spin speed, or, (B) the strength at the poles of the magnet is weakened with spin speed, but the current stays the same, or finally (C), a mix of (A) & (B).
My vote is on (C).
So now you're saying it goes up (my C from earlier)?
Not your 'C', my example of (C), whee I say there is a load on the magnet stirring space +, if your magnetic field is created through electric current, the load action of stirring space would be measured in a change of current since any load on the electric magnet would register.
B is a non-sequitur, because the back EMF from radiation will lower the measured field by superposition. You're pushing on space, and space is pushing back. This is, however, not a change of the magnet's magnetization, nor the electromagnet's current flow (whether powered or superconducting), neither momentary nor permanent. It's just the superposition of fields: when you force two magnets together, their fields cancel and you get zero field in the middle, and less field within each magnet. But the magnetization remains the same. Indeed, the force required to push the magnets together, is the same force that is acting on the rotor, causing drag: this is where the radiant power comes from.
The correct answer must be consistent for all three cases (electromagnet, superconductor and permanent magnet). If you agree that a permanent magnet will not loose magnetization, then there is only one correct answer for the other cases. :)
Tim
Ok, there is an issue here with a true superconductor. Any voltage applied would create an infinite current. The question is 'Infinity + an added small # any larger'? Ok, you win this one, Infinity is Infinity according to math, you cant go higher (unless you enter the complex number plane and other mathematical tricks). I know you can add amps to your electromagnet, but will it be infinite? Having a DC superconducting electromagnet with say 100amp going through it, taking a second such magnet, bringing it into contact with the first, would that effect the charge in both charge superconducting magnets?
:scared: Arrrrrggggg, my head is buzzing, since now, I'm beginning to think that the speed of approach of the 2 charged superconducting electromagnets, which cannot heat heat on their own, but we know such an action does affect electrical current in such a circuit.
( |O Don't ask why now I'm thinking this...) Now you got me thinking that a spinning a magnet at 6000rpm WONT create a 100hz the way I imagined. Maybe vibrating it back and forth 6000 times a second would be a much better candidate, all be it a weak one, generate EM waves, or like with a transmitting antenna, rapid oscillating the artificial magnet polarity back and forth along a fixed axis.
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Not your 'C', my example of (C), whee I say there is a load on the magnet stirring space +, if your magnetic field is created through electric current, the load action of stirring space would be measured in a change of current since any load on the electric magnet would register.
I got your (C) of course, but as your (B) seemed to be a roundabout way of saying my (b), given my explanation why that's in fact a normal observation, and (C) is "(A) and/or (B)", that leaves your (A), which seems to be saying my (c).
...Eh? ;)
Ok, there is an issue here with a true superconductor. Any voltage applied would create an infinite current. The question is 'Infinity + an added small # any larger'? Ok, you win this one, Infinity is Infinity according to math, you cant go higher (unless you enter the complex number plane and other mathematical tricks). I know you can add amps to your electromagnet, but will it be infinite? Having a DC superconducting electromagnet with say 100amp going through it, taking a second such magnet, bringing it into contact with the first, would that effect the charge in both charge superconducting magnets?
Oh dear! If you're stuck in DC terms, no wonder this mess is confusing. :-\
Yes, the total DC voltage drop is zero, on a superconductor. The voltage and current are always finite.
If you transiently apply a voltage, the current ramps up. The coil has inductance! The instantaneous voltage can be nonzero, as long as you can supply the current demanded by the change.
Once charged, the current just sits there happily going around. It doesn't do any work. It does exert a magnetic field, but a static magnetic field does no work (for example, causing charged particles to move in an arc: momentum is exchanged, but not energy).
Or, you could even go so far as to say it's not a DC current at all. In the sense that, there is no resistance to define an L/R time constant, and therefore no cutoff frequency below which everything is "DC" and V = I*R, and above which is AC where we have to worry about reactance and waveforms.
And yes, there's always the good old nihilistic "no true DC" as long as the universe is finite, but for superconductors, that perhaps has a more practical side: the superconductor isn't thermodynamically stable in its environment, and will eventually be quenched somehow. (Well, here on Earth, where it's rather warm. In outer space, superconductors could persist for quite a while; but then, your concerns shift to as-yet-poorly understood loss mechanisms; cosmic ray damage eventually causing the superconductor to fall apart; or, back to nihilism, the heat death of the universe where everything ends up swallowed by black holes.)
In any case, when a superconductor is quenched, or brought down to zero current how ever -- it's done by applying the reverse voltage it was charged with. If this happens suddenly due to temperature, it's dissipated in the material (usually, superconducting wire is an intermetallic or ceramic phase embedded in copper, and the copper has both DC and AC losses -- which also limits charging speed, because the copper is a resistor in parallel with the superconductor, dissipating power into the cryostat until it's done), or in a huge arc if the circuit is broken*, or in a "flyback pulse" if it's discharged into a proper electrical load.
*I wonder if there's a video of this anywhere, hmm. Seems it's something they've thought of, at least:
https://www.hindawi.com/journals/mse/2008/359210/ (https://www.hindawi.com/journals/mse/2008/359210/)
Unclear if it's been tested though. That would be a somewhat damaging test, seeing as they have MJ of energy in those coils!
:scared: Arrrrrggggg, my head is buzzing, since now, I'm beginning to think that the speed of approach of the 2 charged superconducting electromagnets, which cannot heat heat on their own, but we know such an action does affect electrical current in such a circuit.
Hmmm, interesting.
Well, let's see.
Suppose we approach a copper loop with a magnet. The loop has a gap cut into it, so we can measure the V and I there. The voltage is the EMF due to the change in flux in the center of the loop.
When the magnet approaches, the voltage spikes, then subsides (or, if the magnet continues through the center, it reverses and goes negative). The time integral of that spike is exactly the change in flux due to the magnet.
Suppose we short-circuit the gap, and measure the current. As the magnet approaches, it pushes against the loop, and a current is induced. If we hold the magnet inside the loop for a moment (some milliseconds, say), the current subsides -- it's dissipated by the copper's resistance.
Suppose, then, we cool the plate so that its resistance is very small. The same remains exactly true, but the time constant is merely longer: seconds instead.
If we replace copper with superconductor, then the current never dissipates, because there is no resistance to do it. (Uh, we'll suppose we're not using a conventional shunt resistor type current meter here. :) )
So, suppose we have a superconducting ring that was charged with some current, then bring a magnet near it. The situation is identical: we've merely got a superposition, where there's an initial current, plus the induced current from pushing the magnet up to the loop.
So the current will go up.
( |O Don't ask why now I'm thinking this...) Now you got me thinking that a spinning a magnet at 6000rpm WONT create a 100hz the way I imagined. Maybe vibrating it back and forth 6000 times a second would be a much better candidate, all be it a weak one, generate EM waves, or like with a transmitting antenna, rapid oscillating the artificial magnet polarity back and forth along a fixed axis.
Sure, that works too. Or torsional vibration, say a magnet at the tip of a rigid shaft, twisting back and forth through a small angle. These give straight dipole (rather than circular omnidirectional) radiation patterns. :)
Tim
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Ok, perhaps I got the situation wrong, I think. So, I'll start a different thread (slightly different topic) asking about replacing the permanent magnets in a permanent magnet generator with a charged superconducting coil as a DC magnet, everything else normal metal and copper wire in the generator. Will spinning this generator have any effect in the charge of the superconducting coil core.
For now, don't answer this question here...
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@T3sl4co1l, you win... The charge in the superconducting coil which generating a magnetic field will not be altered by being exposed or interacting with a new external magnetic field.
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@T3sl4co1l, you win... The charge in the superconducting coil which generating a magnetic field will not be altered by being exposed or interacting with a new external magnetic field.
Do you know why though?
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Any induced field which may temporarily alter the flow of the current will always return in the opposite way then back to the same speed once that load vanishes. This is my guess... Like an antenna can only broadcast and receive AC em signals, the superconductor's overall DC charge is immune since it will never has any internal current load to speak of. The charge flows without any resistance whatsoever.
Accelerating a spinning superconductor coil will give a temporary illusion of change in the strength of the charge flowing, but stopping the spin will put everything back as it was. The shape and orientation of this coil would determine the behavior of the ripples in the charge going through the coil as it spins. My guess once again.
Through with alternating magnetic waves, we might be able to create matching ripples in the superconductor's charge flow, these being balanced, going above and below the average, but the magnetic strength as a whole remains constant and the only way to change this is to introduce resistance in the superconductor loop.
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Any induced field which may temporarily alter the flow of the current will always return in the opposite way then back to the same speed once that load vanishes. This is my guess... Like an antenna can only broadcast and receive AC em signals, the superconductor's overall DC charge is immune since it will never has any internal current load to speak of. The charge flows without any resistance whatsoever.
Accelerating a spinning superconductor coil will give a temporary illusion of change in the strength of the charge flowing, but stopping the spin will put everything back as it was. The shape and orientation of this coil would determine the behavior of the ripples in the charge going through the coil as it spins. My guess once again.
Through with alternating magnetic waves, we might be able to create matching ripples in the superconductor's charge flow, these being balanced, going above and below the average, but the magnetic strength as a whole remains constant and the only way to change this is to introduce resistance in the superconductor loop.
That's correct. You've got it.