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Magnet question

Discussion in 'Electronic Design' started by HardySpicer, Mar 16, 2009.

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  1. HardySpicer

    HardySpicer Guest

    Ok so I have a permanent magnet which I fix say 1 inch above the
    table.
    I then slide a screw under it and it pulls it up. It has done mgh
    amount of potential energy. If I keep doing this will the magnet
    eventually de-magnetise? ie the energy in the field has all gone in
    doing this work,


    Hardy
     
  2. Sylvia Else

    Sylvia Else Guest

    No. The energy is not in the field alone - it's in the combined system
    of magnet and screw. You put the energy back into the system when you
    pull the screw away from the magnet.

    Sylvia.
     
  3. HardySpicer

    HardySpicer Guest

    Really? I imagined it may be but I cannot see why?
     
  4. Sylvia Else

    Sylvia Else Guest

    When you pull the screw away, you have to apply a force, and move the
    screw and/or magnet through a distance. This means you are doing work on
    the combined system. The energy you put into the system cannot be
    destroyed. Instead, you get it back when you allow the screw to be
    pulled back towards the magnet.

    Sylvia.
     
  5. J.A. Legris

    J.A. Legris Guest

    Or, suppose you dip the magnet into a bucket of iron filings. Almost
    all of the magnetic flux ("lines of force") will be confined to the
    resulting fuzz ball, and the available energy will approach zero.
     
  6. HardySpicer

    HardySpicer Guest

    When you move the screw away though you generate an emf in the screw
    don't you?
    Ok so it isn't connected to a load. Let's suppose I shorted the screw
    to a resistor.



    Hardy
     
  7. Benj

    Benj Guest

    Actually if it's a metal screw it is it's own resistor! However by
    bringing this up you've opened a can of worms because inductive E
    fields are not conservative! :)
     
  8. The EMF generated is not conserved, but that is nothing to do with the
    force connecting the magnet to the screw. That is purely magnetics, not
    electric ( at least in the macro sense ).

    --
    Regards,

    Adrian Jansen adrianjansen at internode dot on dot net
    Design Engineer J & K Micro Systems
    Microcomputer solutions for industrial control
    Note reply address is invalid, convert address above to machine form.
     
  9. Sylvia Else

    Sylvia Else Guest

    So? It will take energy to remove the iron filings, and then the
    situation will be as it was before.

    Sylvia.
     
  10. Sylvia Else

    Sylvia Else Guest

    Depending on the orientation, it would then take a bit more energy to
    pull the screw away from the magnet, with the extra energy being
    dissipated as heat in the resistor. Not all of the energy would be
    recoverable when the screw is returned to the magnet. Indeed, more
    energy would be dissipated by the same mechanism as the screw returns to
    the magnet.

    But this still wouldn't be depleting any energy notionally contained by
    the magnetic field. You could repeat the cycle endlessly, and the
    magnetic field would remain unchanged.

    Sylvia.
     
  11. J.A. Legris

    J.A. Legris Guest

    Of course. I just figured that iron filings would demonstrate
    confinement of the flux better than a handful of screws.
     
  12. Guest

    Sorry Adrian, but all electromagnetic fields are conservative,
    including their results. Were it not so, electrical engineering would
    not work. It does.

    I suspect that your confusion is that you do not fully grasp the
    nature of an electromagnetic field. Many of the quacks believe that
    they can sneak around the conservative properties of an
    electromagnetic field though various complex and seemingly
    sophisticated mechanism, but at the bottom line, none of these work.
    When you better understand that nature of electromagnetic fields (take
    a course) you'll clearly see why.

    Harry C.
     
  13. Sylvia Else

    Sylvia Else Guest

    Seems to imply that will take more energy to heat up a magnet that's
    holding a screw than one that isn't - even if the screw is thermally
    insulated and doesn't itself heat up.

    Sylvia.
     
  14. HardySpicer

    HardySpicer Guest

    What about the other pole of the magnet (if it is say a bar magnet and
    only one side is covered with screws). Is there energy in both poles
    lost or just one pole.


    Hardy
     
  15. J.A. Legris

    J.A. Legris Guest

    If that's the case then the specific heat of a ferromagnetic material
    should be dependent on the flux density passing through it, which
    means that if you reduce a magnetic field on a piece of iron (or
    remove a screw from a magnet) it should cool down a little. I don't
    think it's quite that simple, but here's a related subject:

    http://en.wikipedia.org/wiki/Magnetic_refrigeration
     
  16. I think you should be talking to the OP, not to me.

    I know a little about electromagmetism, I do have a degree in physics,
    after all. And I was referring to the fact that the EMF generated by
    moving a conductor in a magnetic field generates eddy currents, which
    end up as heat dissipated, not conserved within the magnetics system.
    Sure, this does not come from the energy in the field, but from the work
    done in the movement, but thats a subtle difference.

    --
    Regards,

    Adrian Jansen adrianjansen at internode dot on dot net
    Design Engineer J & K Micro Systems
    Microcomputer solutions for industrial control
    Note reply address is invalid, convert address above to machine form.
     
  17. Sylvia Else

    Sylvia Else Guest

    If there's a complete circuit around a loop that contains flux, and the
    flux changes, you'd expect a current to flow. If there's resistance in
    the circuit, then that will cause an energy loss.

    It would depend on the orientation of the loop whether there's any flux
    through it, and thus whether there's any change in flux.

    Sylvia.
     
  18. Sylvia Else

    Sylvia Else Guest

    Thanks for the link.

    It occurred to me after posting that maybe the presence of the screw
    increases the effective curie point. If I understand the mechanism of
    the curie point, it's the point where the domains have sufficient energy
    to move away from their aligned positions, thus destroying the magnetic
    field. However, if the field is more intense, because of the presence of
    the screw, then the domains would presumably require more energy. That
    is, the the effective currie point temperature has increased.

    At the point where the screw falls away because the field has
    disappeared, the magnet would then be above the temperature it would
    need to reach to destroy the field in the absence of the screw. It now
    need to cool down somewhat before it can pick up the screw again. The
    extra energy is irretrievably lost, thus defeating any attempt to make a
    free energy machine this way.

    This wouldn't manifest as a change in specific heat except about the
    curie point temperature.

    Sylvia.
     
  19. Benj

    Benj Guest

    Harry, this statement is either total ignorance of electromagnetics or
    a complete lie. You tell us which one it is!
    The truth is that were it not for non-conservative fields electrical
    engineering would be quite simple and boring (no, batteries, no
    transformers, etc. Just balloons rubbed on cats...)
    I just don't understand how it is that the person publicly displaying
    the most blatant ignorance of subjects are the very ones who are
    always suggesting that someone "take a course". Harry, it is YOU who
    is in need of a course in electromagnetics. I'll even give you a FREE
    lesson.

    The following from the classic FRESHMAN physics text Halliday and
    Resnick Page 757.

    "Electric fields associated with stationary charges are conservative,
    but those associated with changing magnetic fields are
    nonconservative. See Section 8-1. Since electric potential can only be
    defined for a conservative force, it is clear that it has no meaning
    for electric fields produced by induction..."

    Note that shoving iron (screws or other ferromagnetic materials) in
    the flux path of magnets produces changes in the field's distribution
    in space hence, "changing fields".

    I hope you are pleased with yourself that you just demonstrated to the
    world in a public forum that you do not understand this subject even
    to the Freshman level. I urge you to consider a college education
    starting with a freshman course in basic physics. And I'm talking
    about a REAL physics course not the mathless "physics appreciation"
    courses that "science" teachers take today.

    Benj
     
  20. J.A. Legris

    J.A. Legris Guest

    Just to be perfectly clear, we cannot shield one end of a magnet. The
    covered pole just gives a magnet with one end bigger than the other.
    In other words, more screws will be attracted to the growing blob of
    screws until it's large enough to enclose the other pole and all the
    flux in between.
     
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