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

[CD4007UB]

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.

[RoGeorge]

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?

[MrW0lf]

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...

[CD4007UB]

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.)

[RoGeorge]

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?

[MrW0lf]

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
"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

[Zero999]

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. 

[RoGeorge]

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

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:

[IanB]

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.

[T3sl4co1l]

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

[T3sl4co1l]

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

[MrW0lf]

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!
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

[Zero999]

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!
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

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.

[MrW0lf]

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...

[CD4007UB]

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.

[T3sl4co1l]

^

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

[CD4007UB]

^

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.)

[T3sl4co1l]

^

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.

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

[f5r5e5d]

with the previous E-to-M dipole exchange? http://www.physicspages.com/2015/01/20/radiation-from-a-rotating-dipole/

[BrianHG]

@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.

[hermit]

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.

[T3sl4co1l]

@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

[BrianHG]

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?

[MrW0lf]

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?

[T3sl4co1l]

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