Re: magnetism question

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<muknot@xxxxxxxxx> wrote in message
news:1134271465.046108.6960@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> 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.
>
Here is an analogy- apply a voltage across a gap which is partially filled
with wet toilet paper and partially with copper. What is the current
distribution?
Most of the current will flow in the low resistance copper but some will
flow in the wet TP. The magnetic "resistance" (reluctance -inversely
proportional to permeability) of iron is much lower than air so the magnetic
flux(i.e "current") flows mainly in the iron. Iron is a preferred path.
Hence any electromagnetic device such as a motor is designed to minimise any
air gaps as much as possible given the intention of the device (eg. a motor
doesn't turn worth a damn when there is no gap between rotor and stator and
a transformer performs rather poorly when there is a gap).

As to the situation where you apply an mmf to a piece of iron, such that it
saturates, the difference between the iron and the air is moot as the
permeability of the iron when in saturation is near that of air while, at a
lower mmf, it is higher by a factor which is typically near 5000. Saturated,
the pit will have little effect but when the iron is unsaturated it can have
a great effect on the local flux distribution. A small pit in the iron will
not cause "flux leakage" bypassing the iron but it will affect the local
flux distribution because the local geometry has changed.
However, the preferential treatment that iron gets is eliminated or reduced
when saturation exists so the flux says "why should I be concentrated as the
air path is just as good?"
As for the flux spreading in air- tthere are several parallel paths of
different lengths between two points a and b and if a potential (mmf) is
applied across the a-b gap, then the the actual flux in each of these paths
is dependent on the path length and the materials in each path. Hence you
get fringing at gaps.
The actual distribution, except in trivially simple cases, is not all that
easy to work out but iterative computer methods do exist (as well as some
analog methods) but the principles behind modelling this are the same for
magnetic flux as for electric flux or water and air flow. There are flux
plotting programs-free- at least for 2 dimensions- on the net. You can set
up a magnetic system and plot the flux distribution.
--

Don Kelly @shawcross.ca
remove the X to answer


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