George Holz wrote:
Stephen A. Lawrence wrote:
In this case, the energy was put in by spinning up the ring to start
with, and that's the energy we get back out. Whether it resided in the
magnetic field of the ring while the ring was spinning, or in the
inertia of the ring itself, is something which is beyond me just now
(and luckily, for this problem it doesn't really make a difference...).
Agreed from a COE point of view.
The point I was trying to lead up to is the way magnets compare
to the spinning charged ring. Consider the problem as though we
were using another spinning charged ring (B) in place of the dipole box.
To start, the dipole box charged ring (B) is already spinning.
To get attraction, the two rings need to spin in opposite directions.
As we spin up the original charged ring (A), ring B will slow down
as it outputs energy to the increasing magnetic field. It will slow down
more as the two rings are attracted together increasing the field energy
and providing the kinetic energy for the rings relative motion.
Assuming similar rings with equal rotation rates at the time when
both are spun up, both rings will subsequently provide equal energy.
Energy is conserved at all times during these operations.
A permanent magnet acts like a dipole box and not like a spinning
charged ring. A permanent magnet acts like a spinning charged ring
that does not slow down as it supplies energy.
But this isn't quite correct, is it? The internal currents in a
permanent magnet apparently _do_ change, quite a lot, when an external
field is applied to it.
In particular, the "induced" magnetic field exhibited by a piece of
unmagnetized iron when it's in a magnetic field, as well as the changes
in the magnetic field of a permanent magnet when it's in an external
field, are due to changes in the internal currents (or whatever black
magic one believes is taking place inside the magnet to give it the field).
Remember, E and B fields (apparently!) follow the law of superposition,
which means overlapping fields themselves don't interact; they just
sum. But we're all quite familiar with the way the B field is
"conducted" through iron filings (or ferrite transformer cores, for that
matter). There's no "conduction" going on, really -- what we're seeing
is the sum of the original B field, and the induced B field in the iron
filings or transformer core.
The "black box" I hypothesized, which produces a dipole field which is
_fixed_, is rather unrealistic, really -- any real magnet changes its
field as a result of external fields being applied to it. (Perhaps the
box contains a wire loop driven by a current source...) The part which
I, personally, don't understand is what determines _how_ the field of a
piece of iron changes when an external field is applied to it -- the
fact that it _does_ _change_ as the applied field changes, on the other
hand, makes sense, and is, in fact, inevitable if there are really tiny
currents of some sort inside the magnet causing the field.
This is where questions of ZPE as the source of energy in permanent
magnets become involved. With a PM in the box, energy is not
conserved during this operation unless we include the energy source
from which the PM keeps a constant dipole contribution.
But, as I pointed out, it _doesn't_ keep a _constant_ dipole contribution.
In finite
element magnetic field simulation programs the PMs are modeled
using soft magnetic materials with conductor loops driven by
current sources at the poles as the required energy source.
This brings us to the way that energy conservation in magnets
needs to be described. No net energy is provided by the PM over any
complete cycle that returns the magnet to the same
field conditions.
Here's another good one: Does a free-falling charge radiate?
As an experimentalist, I would suggest that the answer might
be found by studying black holes.
Crikey, what kind of lab setup do you have??
I do not have much confidence in many of the theories of
modern physics and prefer to direct my efforts to doing
experiments suggested by a more conservative approach
to how much we think we know.
When it comes to black holes, I certainly agree with you! :-)
George Holz
Varitronics Systems
