Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?

[SeanB]

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.

[jonovid]

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

[MrW0lf]

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.

[Zero999]

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

Also refer to The Thing.
https://en.wikipedia.org/wiki/The_Thing_(listening_device)

[IanMacdonald]

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

[hermit]

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.

[Ampera]

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,



Problem?

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.

[BrianHG]

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

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...

[T3sl4co1l]

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

[T3sl4co1l]

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

Just because you aren't aware of them, doesn't mean they don't exist.

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

[BrianHG]

This guy got a neodymium magnet to spin at over 16.6Khz/1000000 RPM.

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.

[chickenHeadKnob]

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.

[T3sl4co1l]

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.

Tim

[BrianHG]

                  
Just found a man made 600 000 000 rpm spinning object.  That's 10MHz.  Too bad it's not a magnet.

They claim the acceleration on the surface it is 1 billion times the gravity on earth...

[CD4007UB]

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.

[IanB]

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.

[T3sl4co1l]

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

[ejeffrey]

In the case of a spinning magnet in free space where does the angular momentum go?

Into the radiated electromagnetic fields.

[MrW0lf]

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

[T3sl4co1l]

Well, a(t) ~= sin(w*t) so a'(t) ~= w*cos(w*t) and so on.

Tim

[Zero999]

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.

[RoGeorge]

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.

[Zero999]

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

[RoGeorge]

I forgot to add one more thing, a nice demonstration and also some interpretation and explanations of Maxwell's equations and their consequences:



For those who don't have a full hour to spend, at minute 48:37 is stated exactly why a variation in one field is always accompanied by a variation in the other field.

[MrW0lf]

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... 

Joke time!

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!"