Hi,
I would like to know what is a magnetic field. I mean what is it composed
of.
I searched google , asked people around me , no one seems to know.
Obviously everyone knows where how, but not what.
I thought it was electrons, but that cant be.
thank you
ken
A magnetic field is part of a theory we use to describe the interaction
between charged particles, which are themselves just part of the theory.
At the end of the day, science only ever describes. It never explains
things except in terms of other descriptions. By asking what a magnetic
field is out in the real world, you're asking for something that is
beyond the scope of measurement, and therefore unknowable even in principle.
Sylvia.
Try quantum physics.
Ken wrote:
A magnetic field is part of a theory we use to describe the interaction
between charged particles, which are themselves just part of the theory.
At the end of the day, science only ever describes. It never explains
things except in terms of other descriptions. By asking what a magnetic
field is out in the real world, you're asking for something that is
beyond the scope of measurement, and therefore unknowable even in principle.
Sylvia.
A field is a region of influence hence a magnetic field is a region of
magnetic influence i.e. there are effects due to the magnetism. Beyond that
one gets into "what is magnetism"
Basically, both electric and magnetic fields store electromagnetic
enegy. Electric fields can be created by something as simple as a
battery, and magnetic fields come from magnets or from electrical
coils while a current flows. Why magnets make magnetic fields is a
question for quantum physics.
With respect to Sylvia, James Clerk Maxwell expressed a relationship
between electric and magnetic fields in the mid 1800's. Maxwell's
contribution was to show that a magnetic field is created as an
electric field changes, and an electric field is created as a magnetic
field changes. The result is that if I switch an electric field off
and on, I get a magnetic field that swells and collapses with each
onoff transistion.
If I flip the swith fast enough at some frequency, I will find that
the magnetic field comes and goes with the same frequency. But, since
a changing magnetic field will create its own electric field, the
effect moves out to this new field, which makes another magnetic
field, and so on, and that is radio.
Kevin
I'm not disagreeing with Kevin, but there is a simple thought experiment
that shows how careful one must be about taking a theory, such as that
of James Clerk Maxwell, and attempting to use it as anything more than a
description.
Take two electrons, separated in space, stationary relative to some
observer. There's an electric field, obviously, but no magnetic field.
Now take another observer moving perpendicularly to the line joining the
electrons. This observe sees the electrons in motion. Electrons in
motion are an electric current, and an electric current produces a
magnetic field, so for that observer there is a magnetic field present.
So one observer finds a magnetic field present where another observer
finds none. The notion that a magnetic field has a concrete existence is
clearly problematic. This paradox doesn't appear in the theory itself,
because it simply tells you what will happen (or more exactly, what your
measurements will show). It doesn't say anything about what is "really"
there.
Sylvia.
i was pretty sure that the magnetic field was the horizontal plane where
alnico, samarium cobalt, and neodymium magnets were stored prior to
instillation in speakers and microphones.
but after some consideration i decided it must be a nickname for 3M stadium.
perhaps its just a wild gauss chase but i wouldn't get too fluxed up over
it.
This is all a bit over my head, but presumably the first observer (the one
who doesn't see the magnetic field because it doesn't exist for him) sees
something else; whatever the second observer sees as a magnetic field
manifests itself somehow for the first observer? Don't conservation laws
say that elements might vary, but the total sum must be the same?
Probably not...
in article snippedforprivacy@4ax.com, Kevin Kilzer at
snippedforprivacy@mindspring.com wrote on 9/26/04 7:59 PM:
Whatever a magnetic field may be, quantum physics is not going to explain
it. Quantum physics will merely use the magnetic field as part of the
hamiltonian in a quantum equation such as the Schrödinger equation. From
that, quantum consequences of magnetic field will be derived but not
explained.
Bill
in article 4157853f$0$20129$ snippedforprivacy@news.optusnet.com.au, Sylvia Else at
snippedforprivacy@not.at.this.address wrote on 9/26/04 8:13 PM:
Think special theory of relativity and Lorentz transformation. Realize that
both electric field and magnetic field are part of a tensor, that is
relatavisticly invariant.
What is a tensor you my ask. Rather than giving a circular argument I will
present *stress* as an example. Inside a stressed medium there will be a
combination of tensile and shear stresses. This combination is an entity by
itself, in which tensile and shear stresses cannot be separated out. If you
carefully twist a piece of blackboard chalk (if it still is available)
without bending it, it will break with a 45 degree break. Even though you
are applying rotational shear alone, there are directions, at 45 degrees to
the shear where tension is produced. The chalk is weaker in tension than in
shear and the tension ends up causing the break. Stress is a tensor that is
an entity where where shear and tension are not independently present.
The same is true for electric and magnetic fields. The entity is a tensor
that mixes electric and magnetic fields.
Bill
I
in article RAM5d.5110$ snippedforprivacy@news01.roc.ny, John at snippedforprivacy@john.com wrote on
9/26/04 9:23 PM:
Whatever one observer sees is what another observer sees when transformed ty
the appropriate Lorentz transformation.
Bill
will
entity by
Shear stress can always be seperated out: you can represent any
combination of shear and tensile stress as pure tensile stress (google
for Mohr's Circle).
The Mohr's Circle operation is just a graphical way of diagonalizing the
stress matrix (well, it only works in 2D where the tensor is of rank 2).
Ed
It isn't really composed of anything. It's a region in which a magnetic
force can be detected. Ever seen a police car on the motorway? Everyone
within 100 yards drives exactly at the speed limit. the police call this a
bubble or zone of legality. The zone isn't composed of anythig but you can
"feel the force".
Hi Sylvia,
Just a little note here, in your post the observer is moving relative
to the electrons not the electrons relative to each other so there is
no change in the electric potentials, unless the observer is at a
different electrical potential themselves, of course then they are not
a 'neutral observer" so the stationary observer would then see the
field created as the other non stationary observer moved past the
stationary electrons. The question I come up with is exactly what does
the "at differential" moving observer see, since they are part of the
emf/cemf event.
Matt
in article cj8eqs$9kd$ snippedforprivacy@pita.alt.net, Checkmate at snippedforprivacy@The.Edge
wrote on 9/27/04 12:15 AM:
Planck introduce the concept of a quantum of energy in order to explain the
spectral distribution of black body radiation. Einstein and others were able
to extend the concept to explain specific heat and photoemission. Bohr first
applied it to atomic physics. Heisenberg developed the first modern theory
of quantum mechanics. Shrödinger formulated a wave equation that was much
more familiar to physicists of the day. The big surprise was that the
Schrödinger formulation gave identical results to that of Heisenberg's in
spite of appearing to be very different. The Schrödinger formulation was
much easier for making calculations while the Heisenberg formulation was
better suited to understanding essence of what quantum physics was all
about.
Bill
in article snippedforprivacy@my.sig, E. Rosten at snippedforprivacy@my.sig wrote on
9/27/04 2:31 AM:
Mohr's circle is a tensor on the cheap. In a sense, it was developed for
engineers who were not formally trained in tensors or their matrix
representations. The days when that was necessary, I hope, are over.
Another crutch for tensors was developed under the name of *dyadics*.
Another example of tensor quantities is that of the dielectric constant
tensor. Displacement (D field) is not in the direction of the electric field
(E field) for an anisoptripic material like a crystal.
Even more complicated are the piezoelastooptic tensors used for
electrooptic devices where there are simultaneous stresses and optical
index changes.
Bill

>
> Take two electrons, separated in space, stationary relative to some
> observer. There's an electric field, obviously, but no magnetic field.
> Now take another observer moving perpendicularly to the line joining the
> electrons. This observe sees the electrons in motion. Electrons in
> motion are an electric current, and an electric current produces a
> magnetic field, so for that observer there is a magnetic field present.
>
> So one observer finds a magnetic field present where another observer
> finds none. The notion that a magnetic field has a concrete existence is
> clearly problematic. This paradox doesn't appear in the theory itself,
> because it simply tells you what will happen (or more exactly, what your
> measurements will show). It doesn't say anything about what is "really"
> there.
>
 This is all a bit over my head, but presumably the first observer (the one
 who doesn't see the magnetic field because it doesn't exist for him) sees
 something else; whatever the second observer sees as a magnetic field
 manifests itself somehow for the first observer? Don't conservation laws
 say that elements might vary, but the total sum must be the same?
 Probably not...
If there is motion between the observer and the electrons, the observer
might THINK he sees a magnetic field. But is it really there? That lies
in the ability to observe. How do you tell if a magnetic field is there
or not? You measure it by seeing how it acts on something. One way is
with a coil and ampmeter. But now that's electrons again. The net effect
is that something which is electrically changed has ultimately caused an
electrical current where movement is involved.
On the other hand, how do we know there is an electron there? Maybe it
only looks like it because that would explain a magnetic field, which is
what we are measuring. Maybe what we sense as an electric/static charge
is really a sensation of a magnetic current because we have to move in
some way to know there is a charge and where it is.
One cannot be without the other, but the real question is whether there
are really two things or not. I suggest that the answer is that what is
really there is one thing that simply is characterizsed both ways.
 It isn't really composed of anything. It's a region in which a magnetic
 force can be detected. Ever seen a police car on the motorway? Everyone
 within 100 yards drives exactly at the speed limit. the police call this a
 bubble or zone of legality. The zone isn't composed of anythig but you can
 "feel the force".
It is composed of something called "fear".
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.