Re: Question about magnetic forces beyond saturation



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"Jessie" <jessie.taylor91@xxxxxxxxxxxxxx> wrote in message news:a810cf18-6d7e-4db6-ba4b-d8317c0e40ac@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
This discussion is getting very interesting! :)

Don Kelly wrote:

The difficulty is due to geometric conditions- not the consideration of
non-existant monopoles.

I'm just trying to think how one would approach computing magnetic
fields and their effects on nearby bodies (conductors, ferro/para/
diamagnetics) in a more robust manner using computers.

Would one break up a bar magnet into tiny domains of characteristic
length (how do you determine the lengths?) and then have theoretical
monopoles on each domain? Or after reading what Dr. Campbell says,
consider each domain to be a little solenoid with the resulting
magnetic field to be summed up over the entire body?
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your bracketted question answers the main question. We don't have any size, or other information with regard to these domains or even know if they are all aligned- so that approach is a dead end.
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With the situation that you indicate- the bar magnet
is equivalent to a solenoidal electromagnet (as seen externally). The
analysis with respect to permanent magnets is covered by Dr. Campbell
www.magnetweb.com/design2.htm

Very interesting website. I'm currently reading Section 1A but it is
on my reading list. It will take me a while to get anywhere up to
speed so please be patient with me if my questions are covered by
later chapters.

The best approximation in most practical
cases is to ignore the iron part of a path and consider only the air gaps
involved- on the basis that, typically 1 mm of air is roughly equivalent to
5m of iron. Hence the drive for small air gaps. That changes in the
situation where saturation occurs and, in cases of heavy saturation, the
permeability of the iron approaches that of air.

So if you have an electromagnet whose field is on the cusp of inducing
saturation in an iron bar, and then increase the field some more, can
the new field be found by superposing the original field up to
saturation with one where the iron bar has been replaced by "air"?
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No; superimposition implies linearity. Saturation is a non-linear effect. Note that the ignoring of the iron path is a valid approximation for larger air gaps. Consider a lifting magnet: If there is a gap between the magnet and the object being lifted- a first approximation is to ignore the iron. The problem with this is that for a given ampere turns, the force tends to infinity at 0 gap. In that case the iron must be considered. Depending on the dimensions invloved, the approximation may be excellent down to 1 or 2 mm.
If there is saturation- then both iron and air must be considered and as the saturation may depend on the air gap length, it gets messy as the total flux vs mmf curve changes.
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How does the magnetic permeability of plasmas compare with normal air?
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I really don't know.
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Generally, can the attractive force of all permanent magnets be
improved by coating them in a nanoscale ferromagnetic fluid to improve
contact with different test pieces to act analogous to thermal paste?
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Holding force would likely be better with such a coating on the pole face but I don't see an improvement once there is a gap.
Holding force is less of a problem than attractive force at some distance because the force will be greatest at 0 gap in any case.
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>
How do the metallurgical properties (composition, alloying, heat
treatment) of ferromagnetic materials affect their coercivity and
permeability?
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Considerably, Taking cast iron as a basis, transformer steel is designed, through metallurgical factors to have a relatively high and narrow (small area) hysteresis curve to reduce losses and to get a reasonably high flux density. Permanent magnet materials are designed to have a fat hysteresis curve to be less easily de-magnetised. Transformer steel makes a lousy permanent magnet. Ferromagnetic materials have different requirements- also met by the metallurgical design.
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Suppose you swung a magnetic pendulum bob between two metallic plates.
Just by virtue of the plates being conductive, eddy currents would be
generated in them (and I suppose second order eddy currents in the
bob). To what extent as a result would the pendulum lose kinetic
energy? Is is purely in relation to its velocity (according to Lenz's
Law)? How will this answer change if the plates are superconducting,
diamagnetic, paramagnetic, ferromagnetic and a permanent magnet
themselves?
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The eddy currents cause losses which are dependent upon the dimensions of the plates and their material- this implies that the material has some resistance. The losses are alsovelocity dependent. If the losses are large, the pendulum will stop more quickly. Consider regenerative braking. I am assuming the bob is magnetized, not unmagnetized material. Superconducting -what is the resistance? Paramagnetic, etc- Off the cuff, I would expect additional braking (maybe not for diamagnetic materials) in comparison to non-magnetic conductors. Permanent magnets would cause considerable braking. I have thrown a nickel between the poles of a very strong permanet magnet- most times the coin simply stops dead and attaches to one or both poles of the magnet. Sometimes, the coin will pass the magnet and then reverse direction befor contacting the magnet.

I do believe that you are trying to run before learning to walk.
There are electromechanical energy conversions that deal with electromagnetic forces. I would suggest looking at the principles involved there before dealing with permanent magnets where these principles are applicable.

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Don Kelly
dhky@xxxxxxxxxxxx
remove the x to reply

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