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

[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?

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

[T3sl4co1l]

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

[MrW0lf]

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!

[IanB]

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.

[Doc Daneeka]

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.

[Zero999]

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.

^

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.

[T3sl4co1l]

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?

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

[MrW0lf]

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!)

[MrW0lf]

and the textbooks seem to agree

...but Feynman does not:
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. "

[Red Squirrel]

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

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! 

[StillTrying]

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. 

[T3sl4co1l]

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

[BrianHG]

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. 

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.

[T3sl4co1l]

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. 

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

[BrianHG]

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.

[T3sl4co1l]

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

[BrianHG]

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.

[T3sl4co1l]

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

[CatalinaWOW]

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.

[Zero999]

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?

[BrianHG]

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.

[T3sl4co1l]

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

[IanB]

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.)

[BrianHG]

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).

[BrianHG]

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