Re: rotating magnetic field

On Apr 12, 11:51 pm, Salmon Egg <Salmon...@xxxxxxxxxxxxx> wrote:
`
The currents are those from the spins in the magnet.

That is incorrect. The current loops are circulating atomic currents,
i.e. the currents that result from the motion of the electrons inside
the atom. It was Ampere who first suggested that the magnetism of
matter came from circulating atomic currents, hence the term “Amperian
currents.” See details at http://farside.ph.utexas.edu/teaching/em/lectures/node73.html

Magnetization is a combination of Amperian currents from the orbital
motion of the electrons and the spin of the electrons. See details at
http://en.wikipedia.org/wiki/Magnetization


On Apr 15, 12:27 pm, Benj <bjac...@xxxxxxxxxxx> wrote:

I'm not saying math is useless. I'm not even saying that erroneous
theories are useless. What I'm saying is that one should be AWARE..

But when you start pushing the limits (as the Faraday disk generator
does) it suddenly becomes necessary to start looking at your
philosophy of how things "work". A practical answer that what you've
already calculated isn't good enough. A Theory that only works some of
the time, doesn't quite do it either.

I know we agree on all this stuff so theres no debate here. The
debate is with people who think that mathematical theories are reality
and that erroneous theories are correct. If you are looking for a
practical answer in regions where given theories are not correct, what
is a person to do? I suggest you first start by realizing that what
you've pinned your hopes on is probably not going to work.

I’d like to get one point straight here – The entire purpose of using
mathematics in science (especially physics) is to precisely *describe*
phenomena. Your comment “people who think that mathematical theories
are reality” has no meaning to a physicist. E.g. Faraday’s law is
*not* a mathematical theory. It’s a physical theory. You are thinking
of it’s mathematical description and calling it a mathematical theory.
That is completely wrong.

No scientist would ever hold that “erroneous theories are correct”.
There is no area in electrodynamics in which the laws of physics don’t
hold true and in all such cases it can be an has been described
mathematically. What I see happening in this thread is that people
have wrong ideas about things like field lines and their motion. A
decent physicist knows what the limits are on his tools. And the idea
of a field line is just that, a tool. They don’t even exist so its
even less meaningful to speak of their motion. They are very useful
ideas though and that’s why the idea was created and held onto. If you
studied physics in college then your physics textbook would tell you
that as soon as the notion was introduced. I’m looking at a physics
text that is being used in undergraduate courses right now. The author
writes

---------------------------------------------------
Another way to picture the field is with the use of the *magnetic
field lines.* These are imaginary lines drawn through a region of
space so that

* A tangent to a field line is in the direction of the magnetic field,
and
* The field lines are closer together where the magnetic field
strength is larger
---------------------------------------------------


Richard Feynman has it right. From “The Feynman Lectures on Physics,”
Feynman, Leighton and Sands, V-II, page 1-10 – Referring to electric
field lines
---------------------------------------------------
Not only is it not possible to say whether the field lines move or do
not move with the charges – they may even disappear completely in
certain frames of reference.
---------------------------------------------------

The main problem with the notion of moving field lines is this exact
problem, i.e. people get the wrong idea and start thinking that the
field of a magent rotates with it. This is discussed in the article
“On the Concept of Moving Magnetic Field Lines,” Carl-Gunne Fälthammar
located at
http://plasma.colorado.edu/phys7810/articles/Falthammar_MovingFieldLines_2007.pdf

The author writes
---------------------------------------------------
Another example of how the concept of
moving magnetic field lines can be deceptive
is that of a homogeneously magnetized conducting
sphere surrounded by vacuum and
rotating around its axis. For someone thinking
of magnetic field lines as entities that
can ‘move,’ it is a not an uncommon fallacy
to believe that the magnetic field lines outside
the spherical magnet ‘rotate with the magnet’
and that this rotating field is capable of exerting
a force on a test charge at rest, due to the
perceived ‘relative’ motion between the test
charge and the magnetic field.
---------------------------------------------------

Let’s say for the sake of argument that the field actually did rotate
with the magnet. The field would rotate like a rigid object. The force
on a stationary charge would be non-zero and therefore the moving
field generates a static electric field. You could then easily find an
expression for the electric field of a long magnet in the region near
the axis. If you were to then take the divergence of that field you’d
get a non-zero value. Since the divergence of the electric field
equals the value of the charge density at the point of interest you
end up with a contradiction. But never mind that – If the field
actually did rotate with the magnet and since the magnetic field
extents to infinity then there must be portions of the field that move
at the speed of light. If that were true then the energy density of
the field would be infinite. Further out the energy density of the
field would be imaginary. Since these are nonsensical results the
notion of the field rotating with the magnet must not be true.


On Apr 16, 7:06 pm, Don Kelly <d...@xxxxxxxxxxxx> wrote:

And how is this a problem- other than making connections to the coil.
you still have a flux in the axial direction and external observation
wont see the difference between this and a rotating bar magnet.

A rotating current loop will not have the same electric field when its
rotating. In fact a rotating current loop won’t become charged at
all.

Are you implying that a bar magnet must be rotating to produce a field?

Magnetic field? No. Electric field? Yes.

I suggest that you look at the whole situation with these little current
loops.

Your comment suggests that you ignored the reference I posted since I
explained all this in terms of current loops. The link I posted above
is
http://www.geocities.com/physics_world/em/rotating_magnet.htm

Possibly the following site might help. The little loop currents
cancel internally but result in a net external loop current equivalent.

The currents cancel when the object is not rotating. If the object is
rotating the charge densities don’t cancel. This is a result of the
fact that a moving current loop is charged. This is a result of the
special theory of relativity. It’s all worked out on that page.

On Apr 16, 7:42 pm, Salmon Egg <Salmon...@xxxxxxxxxxxxx> wrote:

To see that lines of force do exist and rotate with the magnet, just dip
the magnet in iron filings.

Ever watch an aerospace engineer use smoke in wind tunnels in order to
watch the streamlines of air move around the airframe? There are no
“real” lines in the air. They just “look” like lines. They are a
useful representation of the air flow around he airframe. The reason
you “see” magnetic field lines is because the iron filings orient
themselves head to tail and attract each other and orient themselves
tangent to the direction of the applied force. That’s a great reason
to invent the fiction of field lines. But if you thought that they
were real things then you have the wrong idea.

You can see the lines and they will rotate
with the magnet. ;=) Seeing is believing, isn't it?

Nope. In fact you just gave a perfect example of an exception to that
rule.

Pete
.

Relevant Pages

  • Re: Is charge conserved between frames?
    ... And obviously none of the electrons in the ... reciprocal electrodynamic action of a magnet and a conductor. ... cases in which either the one or the other of these bodies is in motion. ... energy, producing a current at the places where parts of the conductor are ...
    (sci.physics.relativity)
  • Re: Magnetic fields doing work?? No way ... but...
    ... He has a loop of wire carrying a current suspended ... between a magnet and a weight attached to the wire. ... the problem appears to be purely "classical" (charge carriers are ... That force would be the force of electrons colliding with the ...
    (sci.physics.relativity)
  • Re: Magnetic fields doing work?? No way ... but...
    ... He has a loop of wire carrying a current suspended ... between a magnet and a weight attached to the wire. ... problem appears to be purely "classical" (charge carriers are simple ... That force would be the force of electrons colliding with the surface ...
    (sci.physics.relativity)
  • Re: Is electromagnetic field theory unified?
    ... >>I provide below the answers which are currently accepted in physics. ... >>Electromagnetic radiation. ... Classical mechanics can not fully explain the behaviour of electrons. ... > at the origin (the magnet at rest, not radiating, with closed magnetic ...
    (sci.physics)
  • Re: Is electromagnetic field theory unified?
    ... > problem has to be revamped so that the perfect bar magnet is a bunch ... > electrons whose poles are aligned. ... > would see the magnetic field be invariant with rotation of the ... > Now, what if we replace the bar magnet with a single loop, and vary ...
    (sci.physics)