Re: magnetism question

operator jay wrote:
> "Sue..." <suzysewnshow@xxxxxxxxxxxx> wrote in message
> news:1134283147.866512.68990@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > Sue... wrote:
> > > muknot@xxxxxxxxx wrote:
> > > > Hi I have a question:
> > > >
> > > > Say you have a flux distribution due to a magnet, then you place a
> > > > piece of iron beside the magnet and you get a new flux distribution.
> > > > The flux density in the iron will be greater than the flux density in
> > > > the surrounding air. Why is this? Is it because the magnetization of
> > > > the iron creates additional flux in it, thus leading to more flux
> > > > density, or is it because the flux from the magnet prefers to travel
> > > > through the iron (rather than the surrounding air), thus leading to a
> > > > greater flux density or do both these effects contribute to a greater
> > > > flux density in the iron? Also if the flux from the magnet prefers to
> > > > travel through the iron, what's the reason for this (just saying that
> > > > it has higher magnetic permeability isn't much of an explanation)?
> > > >
> > > > Another question I have is if you place a piece of iron in a magnetic
> > > > field that saturates it, and then remove a bit of iron (create a pit),
> > > > from the surface of your sample, the flux will "leake" out of your
> > > > sample. I don't understand why this happens, why doesn't the flux just
> > > > continue to travel through the air (where the pit is) with the same
> > > > distrubiton it had in the iron -- why does it have to take up a
> > > > greater volume in the air?
> > > >
> > > > Thanks.
> > >
> > > Would you mind asking easier questions?
> > > << Note that the simple result (345) can only be
> > > obtained from the Biot-Savart law after some non-trivial
> > > algebra. Examination of more complicated current
> > > distributions using this law invariably leads to lengthy,
> > > involved, and extremely unpleasant calculations. >>
> > > http://farside.ph.utexas.edu/teaching/em/lectures/node39.html
> > > Unpleassant calculations:
> > > http://mathworld.wolfram.com/MultipleIntegral.html
> > This is better because it has pics:
> > http://en.wikipedia.org/wiki/Multiple_integral
> > > http://farside.ph.utexas.edu/~rfitzp/teaching/302l/lectures/node62.html
> > >
> http://web.mit.edu/8.02t/www/802TEAL3D/visualizations/magnetostatics/index.htm
> > >
> > > :o)
> > > Sue...
> >
>
> Biot-Savart appears to be a superposition type calculation based on
> linearity, and it appears to implicitly assume free space or similar by use
> of mu-not. I think in the situation with multiple iron bodies, Biot-Savart
> might not work. The linearity is not there due to regions of different mu,
> and the mu-not assumption goes out the window. One might have to hack out
> an answer using magnetic circuits (mmf, reluctance, flux, etc.), or use
> numerical methods based on Maxwell. Considering it as a magnetic circuit
> may answer some of the OPs questions qualitatively. I expect that flux
> through the neighboring body is high due to both the 'affinity' of the flux
> for the iron path, and the polarization of that iron body itself. These
> might both be rolled up in the low reluctance value of the path through the
> iron body. As to the question about the pitted surface, I'm not certain
> what does and does not happen. At saturation I would think you'd start to
> see some flux stray from the material all over. If the OP's description of
> events is accurate, I'd attribute it to the flux taking as many parallel
> paths as it can to find the lowest reluctance. Similar (dual?) to the
> fringing effects of a capacitor plate.
>
> j
<< Also if the flux from the magnet prefers to
travel through the iron, what's the reason for this (just saying that
it has higher magnetic permeability isn't much of an explanation)? >>

The OP's question asks about the mechanism underlying
permeablilty. That is why we have to go all the way back to the
fundamental unification with Coulomb force using the triple
integral. The material doesn't matter because the result is
fundamental field equations that are predictive of the
material's atomic structure.

Sue...

.

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