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magnetic field

Discussion in 'Misc Electronics' started by Ken, Sep 26, 2004.

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  1. Ken

    Ken Guest

    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
     
  2. Sylvia Else

    Sylvia Else Guest


    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.
     
  3. Brian

    Brian Guest

    Try quantum physics.




    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.
     
  4. Ryan Wheeler

    Ryan Wheeler Guest

    look at this one http://www.who.int/peh-emf/en/

    search word in google is: emf
     
  5. Don Kelly

    Don Kelly Guest

    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"
     
  6. Sylvia Else

    Sylvia Else Guest

    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.
     
  7. TimPerry

    TimPerry Guest

    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.
     
  8. John

    John Guest

    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...
     
  9. 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
     
  10. Whatever one observer sees is what another observer sees when transformed ty
    the appropriate Lorentz transformation.

    Bill
     
  11. E. Rosten

    E. Rosten Guest

    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

    --
    (You can't go wrong with psycho-rats.) (er258)(@)(eng.cam)(.ac.uk)

    /d{def}def/f{/Times findfont s scalefont setfont}d/s{10}d/r{roll}d f 5/m
    {moveto}d -1 r 230 350 m 0 1 179{1 index show 88 rotate 4 mul 0 rmoveto}
    for /s 15 d f pop 240 420 m 0 1 3 { 4 2 1 r sub -1 r show } for showpage
     
  12. CWatters

    CWatters Guest

    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".
     
  13. Guest

    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





    --
    "It's not what folks don't know that gets 'em in the most trouble,
    it's the things they do know that ain't so" Will Rodgers

    "Any sufficiently advanced technology appears as magic" Arthur C.
    Clarke
     
  14. Guest

    |
    |>
    |> 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.
     
  15. Guest

    | 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".
     
  16. Clint Sharp

    Clint Sharp Guest

    Sod that, I overtake the buggers. Never been pulled yet.
     
  17. E. Rosten

    E. Rosten Guest

    More like symmetric eigen decomposition done graphically. It just
    happens that the matrix it operates on is a 2D stress tensor. But you
    can still diagonalize it (ie represent shear as tension only).
    Your hopes may be elevated, sadly. Though I'm not sure.
    They also are for rank 2 tensors aren't they?

    -Ed

    --
    (You can't go wrong with psycho-rats.) (er258)(@)(eng.cam)(.ac.uk)

    /d{def}def/f{/Times findfont s scalefont setfont}d/s{10}d/r{roll}d f 5/m
    {moveto}d -1 r 230 350 m 0 1 179{1 index show 88 rotate 4 mul 0 rmoveto}
    for /s 15 d f pop 240 420 m 0 1 3 { 4 2 1 r sub -1 r show } for showpage
     
  18. Isn't it assymetrical, with a constant current producing a constant
    magnetic field?
     
  19. It is only assymmetrical because there are no magnetic charges. Magnetic
    charge does not appear Maxwell's equations. The symmetries or lack of them
    show up more strongly in the four-vector relativistic formulations.

    Bill
     
  20. E. Rosten

    E. Rosten Guest

    IIRC, magnetic charges (known also as magnetic monopoles) are
    theoretically possible (according to some physicists). This leads to the
    very minor addition to Maxwell's Equations that

    div B = m

    which is analogous to

    div D = rho


    However, magnetic monopoles are thought to exist at such high energies
    that can never be observed.

    -Ed




    --
    (You can't go wrong with psycho-rats.) (er258)(@)(eng.cam)(.ac.uk)

    /d{def}def/f{/Times findfont s scalefont setfont}d/s{10}d/r{roll}d f 5/m
    {moveto}d -1 r 230 350 m 0 1 179{1 index show 88 rotate 4 mul 0 rmoveto}
    for /s 15 d f pop 240 420 m 0 1 3 { 4 2 1 r sub -1 r show } for showpage
     
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