Author Topic: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?  (Read 13736 times)

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Offline SeanB

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #25 on: September 03, 2017, 08:27:22 am »
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.
 

Offline jonovid

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #26 on: September 03, 2017, 08:43:10 am »
thought this might be of interest but a bit off topic.
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Offline MrW0lf

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #27 on: September 03, 2017, 08:47:58 am »
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.


 

Online Zero999

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #28 on: September 03, 2017, 09:59:55 am »
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
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Offline IanMacdonald

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #29 on: September 03, 2017, 11:02:27 am »
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
 

Offline hermit

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #30 on: September 03, 2017, 04:05:19 pm »
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.
 

Offline Ampera

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #31 on: September 03, 2017, 04:12:31 pm »
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.

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Online BrianHG

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #32 on: September 03, 2017, 11:03:25 pm »
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...
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Offline T3sl4co1l

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #33 on: September 04, 2017, 12:48:58 am »
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!

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Offline T3sl4co1l

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #34 on: September 04, 2017, 01:05:42 am »
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.

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Online BrianHG

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #35 on: September 04, 2017, 01:24:03 am »
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.
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Offline chickenHeadKnob

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #36 on: September 04, 2017, 01:26:58 am »
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.
 

Offline T3sl4co1l

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #37 on: September 04, 2017, 01:30:26 am »
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

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Online BrianHG

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #38 on: September 04, 2017, 01:35:36 am »
 :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.

They claim the acceleration on the surface it is 1 billion times the gravity on earth...
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Offline CD4007UB

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #39 on: September 04, 2017, 02:00:14 am »
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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Offline IanB

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #40 on: September 04, 2017, 02:12:50 am »
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.
I'm not an EE--what am I doing here?
 
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Offline T3sl4co1l

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #41 on: September 04, 2017, 02:32:20 am »
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.

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Online ejeffrey

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #42 on: September 04, 2017, 04:00:44 am »
In the case of a spinning magnet in free space where does the angular momentum go?

Into the radiated electromagnetic fields.
 

Offline MrW0lf

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #43 on: September 04, 2017, 07:12:05 am »
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



 

Offline T3sl4co1l

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #44 on: September 04, 2017, 07:25:23 am »
Well, a(t) ~= sin(w*t) so a'(t) ~= w*cos(w*t) and so on.

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Online Zero999

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #45 on: September 04, 2017, 08:25:32 am »
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.
 

Offline RoGeorge

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #46 on: September 04, 2017, 09:30:29 am »
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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Online Zero999

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #47 on: September 04, 2017, 11:09:36 am »
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
« Last Edit: September 04, 2017, 11:17:53 am by Hero999 »
 

Offline RoGeorge

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #48 on: September 04, 2017, 11:33:17 am »
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.
« Last Edit: September 04, 2017, 11:41:10 am by RoGeorge »
 

Offline MrW0lf

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Re: Will spinning a magnet at 6000rpm create a 100hz radio broadcast wave?
« Reply #49 on: September 04, 2017, 02:04:35 pm »
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

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