Re: Is electromagnetic field theory unified?
From: JM Albuquerque (jm.aREM.OVE_at_sapo.pt)
Date: 02/28/05
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Date: Mon, 28 Feb 2005 01:21:06 -0000
"Timo Nieminen" <timo@physics.uq.edu.au> escreveu na mensagem
news:Pine.LNX.4.50.0502280822150.20442-100000@localhost...
> On Sun, 27 Feb 2005, JM Albuquerque wrote:
>
> > 1 - If a spinning magnet radiates and radiation carries energy
> > that could be picked up at long distance, the question is:
> > What is the source for such energy? (Radiation)
> > So far all that you both could say was a single word, namely
> > "kinetic" energy.
> >
> > At first both of you discard the said energy requirements but
> > I've insisted and you both spell "kinetic energy".
> > I've tried to explain that kinetic energy is nothing but a potential.
> > All that matters is torque, or kinetic energy change.
> > (Maybe I will try to address the problem again later).
>
> The reaction torque exerted by the radiated field on an antenna (and I say
> antenna, not magnet, since the reaction torque acting on an antenna has
> been experimentally measured) is such that, if a freely spinning magnet
> produces the same field as a 1/4-wave-out-of-phase crossed-loop magnetic
> dipole antenna, the work done by the reaction torque reduces the
> rotational kinetic energy of the magnet by exactly the radiated energy.
Very good reply. Congratulations Timo Nieminen.
At last someone understands that a torque must be maintained during
time unit to have power radiated, no matter it is an antenna or a
spinning magnet.
I'm very glad that you mentioned the experimentally measured torque
reactions on an antenna. If torque was not measured on an antenna all
my understanding of induction will fail.
The spinning magnet doesn't radiate because the speed of the
magnetic flux lines is always fast them the feasible speed that
a real magnet can spin.
The theory predicts the speed of the magnetic flux lines to be the speed
of light and since it is not possible to spin a real magnet fast then the
speed of light the magnetic flux lines will never expand outwards creating
radiation.
> The roots of the theoretical predictions go back to 1880 or so, direct
> experimental verification using a radiating antenna was done in 1967,
> experimental verification using receiving antennas or rotating scatterers
> was done earlier.
Sorry to ask, but what is exactly a rotating scatter?
> Note particularly that the theory applies to non-conductive scatterers.
> I'm not aware of any experiments using magnetic (ie mu not equal to mu_0)
> non-conductive particles, but there have been quite a few using
> non-conductive particles with epsilon not equal to epsilon_0.
>
> Scattering by a rotating non-conductive particle is essentially radiation
> by a polarisation source (ie the source is a dipole moment density, not a
> charge or current density). In such a case, there is a rotational
> frequency shift, and a torque exerted on the scatterer, which does work on
> the scatterer exactly equal to the energy change in the radiation due to
> the rotational frequency shift.
It looks like that the receiver makes all the difference, as I've said
often.
> Now, I assume that all the theory in the world cannot convince you that a
> spinning magnet might radiate. Experiment, OTOH, would be decisive.
I don't know for sure if a spinning magnet radiates or not.
Theory predicts charges which doesn't exist and fields that could exist
or not. What I do know for sure is that a spinning magnet causes
induction and since the receiver must be always electric the interaction
between both is electromagnetic.
If the receiver is an other permanent magnet it is impossible to flow
energy from the source to the receiver, because in the second half
cycle the receiver will give back the energy to the prime source and
so on. We cannot convert mechanical energy into electricity by means
of two magnets, nor is possible to dissipate energy by means of two
permanent magnets, but with a magnet and an electric circuit is easy,
or vice-versa. The receiver does all the difference.
> Since you ask for a convincing experiment using a macroscopic
> magnet (electrons don't count!), one must ask if such an experiment
> is feasible.
I guess the experiment will be very difficult.
Nevertheless, instead of a spinning magnet one can use a stationary
solenoid fed by high frequency AC current.
Will the solenoid radiate? I don't think so!
The theory predicts that the solenoid consumes nothing from the AC
power source in the inductor (only RI^2 in the wires), unless an
inductance receiver is placed nearby.
The consumption (amount of radiation) will only depend on the
receiver for an ideal solenoid.
> Basically, it's very difficult to spin a magnet fast enough, free of other
> forces and torques.
I agree.
> However, a magnetic sphere, with permittivity equal to
> that of the surrounding medium, illuminated by a circularly polarised
> wave, is exactly a spinning magnet, spinning at the frequency of the
> incident field. Will the sphere scatter the incident radiation? If so, the
> spinning magnet radiates.
??????????
If you illuminate a spinning magnet the magnetic field will interact with
light, like it does with any other electromagnetic field.
> The theory is not too difficult, basically
> Lorenz-Mie scattering. I don't know if the experiment has been done, but
> it's something to look for. Actually, I'd consider a difference in
> scattering between magnetic and non-magnetic spheres of the same
> permittivity to be a conclusive demonstration; this experiment is more
> likely to have been done (but a quick search turns up only theory and
> computation, not experimental results).
Again you can use a solenoid fed by high frequency AC.
Will the solenoid radiate?
I guess not, otherwise antennas will be large solenoids.
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