Blasty
Hazard to Others
Posts: 107
Registered: 25-7-2008
Member Is Offline
Mood: No Mood
|
|
Question about magnetic fields going through non-magnetic materials
Many years ago while reading Lester R. Moskowitz's Permanent magnet design and application handbook (1976), I stumbled upon a passage where,
if memory serves me right, the author said something about people assuming that a magnetic field from a permanent magnet will go through any
non-magnetic material unaffected, as if it wasn't there, but that this is not exactly true, that the magnetic field is reduced when it goes through
something. He then gave an example of a magnetic field being reduced by a certain amount when it goes through brass plates of some thickness (can't
remember the exact specifications he gave.) Unfortunately, I no longer have the book with me so I can't check this. If anyone around here has the book
handy and can check for this passage, please let me know what exactly is it that he said.
Also, does anyone know if different non-magnetic materials will affect a magnetic field from a permanent magnet differently? Would such things as the
density of a non-magnetic material matter much in this? For example: Would a lead plate have a more diminishing effect on the magnetic field than a
brass or copper plate of the same thickness?
|
|
|
m1tanker78
National Hazard
Posts: 685
Registered: 5-1-2011
Member Is Offline
Mood: No Mood
|
|
Magnetic fields can't be shielded in the traditional sense. They can be redirected through a material that has very high magnetic permeability. Off
the top of my head, steel has a moderately high permeability. Lead is effective for shielding EM waves (ex: x-rays) but is probably totally
ineffective as a magnetic 'shield'.
Are you building a project or did you just need the reference in that book?
Tom
|
|
|
smuv
National Hazard
Posts: 842
Registered: 2-5-2007
Member Is Offline
Mood: Jingoistic
|
|
I am no magnetism expert,so i can't get super detailed...but, nearly everything is magnetic. It may not be ferromagnetic or ferrimagnetic (ie. the
two types of magnetism, that in lay-terms we refer to as 'magnetism'), but just about everything is diamagnetic and many things are also paramagnetic
(which is a stronger interaction which usually overshadows diamagnetism). These are both much weaker interactions, but they cannot be ignored.
So essentially, lots of things perturb magnetic fields, the question is, to what extent.
[Edited on 4-1-2011 by smuv]
|
|
|
Blasty
Hazard to Others
Posts: 107
Registered: 25-7-2008
Member Is Offline
Mood: No Mood
|
|
Quote: Originally posted by m1tanker78  | | Magnetic fields can't be shielded in the traditional sense. They can be redirected through a material that has very high magnetic permeability. Off
the top of my head, steel has a moderately high permeability. Lead is effective for shielding EM waves (ex: x-rays) but is probably totally
ineffective as a magnetic 'shield'. |
They can't be stopped, but as I seem to remember from that passage in Moskowitz's book, they are somewhat diminished by going through something that
is not magnetic. What I am trying to find out is: do all non-magnetic materials affect the magnetic field of a permanent magnet in the same degree as
it goes through them or do such things as the density, chemical composition, or other physical/chemical properties make any noticeable difference on
this diminishment? As far as I can remember, Moskowitz only mentioned brass plates of a certain thickness and the diminishing effects they have on the
magnetic field.
| Quote: | Are you building a project or did you just need the reference in that book?
Tom |
A bit of both. It usually annoys me a bit when I can't remember details of something I read that struck my fancy back in the day. I might be
misremembering it. I should have copied the passage when I read it.
|
|
|
Morgan
International Hazard
Posts: 1694
Registered: 28-12-2010
Member Is Offline
Mood: No Mood
|
|
Just a tidbit on the different qualities of the same material ...
I have some stainless steel objects that aren't very magnetic or hardly discernible if at all, but I discovered where there are bends in the metal
they will attract and hold a magnet. Funny how the stressed areas do that. Some stainless steel spice jars you can't feel any attraction on the
bottom, but the sides which have been rolled or spun/worked stick to a neodymium magnet enough to support it's weight.
"If the alloy is mechanically deformed, i.e. bent, at room temperature, it will partially transform to the ferritic phase and will be partly magnetic,
or ferromagnetic, as it is more precisely termed."
http://www.scientificamerican.com/article.cfm?id=why-dont-ma...
[Edited on 2-4-2011 by Morgan]
|
|
|
lavenatti
Harmless
Posts: 17
Registered: 19-12-2006
Member Is Offline
Mood: No Mood
|
|
Anyone remember the "Magic Wand" used to tune adjustable inductors on old radios?
They were a wooden (or plastic) rod with brass on one end and iron on the other. One end was inserted or held near the inductor in question
effectively raising (the iron end) or lowering (brass end) it's inductance. This way one could tell if the inductor needed adjustment and in which
direction prior to messing with it.
|
|
|
peach
Bon Vivant
Posts: 1428
Registered: 14-11-2008
Member Is Offline
Mood: No Mood
|
|
Magnetic shields can be made from superconductors. The field decays very rapidly (exponentially over nanometers). Interestingly, this is not due to
them having zero resistance.
They are used around super conducting quantum interference detectors (SQUIDs), which measure magnet fields very accurately. <--- Another link.
Both SQUIDs and superconducting shields are used in magnetoencepholography (MEG) scanning, which is a passive method of detecting the minute magnetic
fields neurons produce as they depolarise; unlike MRI, this doesn't rely on injecting a signal or watching a concurrent event (like oxygenation of the
blood) to deduce activity.
Gradiometers are added to exclude magnetic fields that are being emitted from more remote sources by subtracting them from the more local results.
The big cylinder over his head is a dewar with the all this in.
|
|
|
MrHomeScientist
International Hazard
Posts: 1806
Registered: 24-10-2010
Location: Flerovium
Member Is Offline
Mood: No Mood
|
|
Wow! That guy looks like he's about to become someone's arch-nemesis when his prototype mad scientist brain-enhancement machine malfunctions.
To contribute to the thread, Mu-metal is also an interesting material that is very effective at shielding magnetic fields, because it has a very high
magnetic permeability. From what I understand, it essentially provides a "low resistance" path that field lines can follow and redirects them around a
shielded, enclosed area.
http://en.wikipedia.org/wiki/Mu-metal
|
|
|
peach
Bon Vivant
Posts: 1428
Registered: 14-11-2008
Member Is Offline
Mood: No Mood
|
|
A Moo-Metal audio transformer from Sowter, having now fallen out of popular use due to the advent of solid state
|
|
|
watson.fawkes
International Hazard
Posts: 2793
Registered: 16-8-2008
Member Is Offline
Mood: No Mood
|
|
More on moo-metal: http://www.youtube.com/watch?v=rX_hdUmVyZ4
|
|
|
IrC
International Hazard
Posts: 2710
Registered: 7-3-2005
Location: Eureka
Member Is Offline
Mood: Discovering
|
|
Ultraperm 80 here: http://www.goldmine-elec-products.com/prodinfo.asp?number=G1... or: http://www.goldmine-elec-products.com/prodinfo.asp?number=G1...
Brass is an alloy and the magnetic properties of each component comes into play. Diamagnetic atoms are going to oppose the impinging field thereby
altering what is seen on the other side. Plus the mention of brass tuning wands needs to include the fact of alternating or moving fields. Mere copper
has a reducing effect to a moving field as seen on the other side due to eddy currents.
Admit it Peach. The picture is really you, sitting in your house with your thinking cap on.
[Edited on 4-8-2011 by IrC]
"Science is the belief in the ignorance of the experts" Richard Feynman
|
|
|
arsphenamine
Hazard to Others
Posts: 236
Registered: 12-8-2010
Location: I smell horses, Maryland, USA
Member Is Offline
Mood: No Mood
|
|
Mu-metal has unusual permeability but saturates at only .08 Oersteds. When you do BH plots for soft ferromagnetics, its obvious that each material
has an optimum range.
Depending on your application (shielding, signal transformers, variable reluctance transducers, etc), there are dozens of possible candidates such as
HyMu-90, nickel, type-400 stainless steel, and silicon steels.
|
|
|
IrC
International Hazard
Posts: 2710
Registered: 7-3-2005
Location: Eureka
Member Is Offline
Mood: Discovering
|
|
Build a barrier of several sheets, say 5. Spaced at the proper distances. You will see a major improvement in both maximum field reduction and an
increase in the effective saturation level.
"Science is the belief in the ignorance of the experts" Richard Feynman
|
|
|
watson.fawkes
International Hazard
Posts: 2793
Registered: 16-8-2008
Member Is Offline
Mood: No Mood
|
|
| Quote: Originally posted by IrC | | Admit it Peach. The picture is really you, sitting in your house with your thinking cap on. |
Nah. That's him
getting his hair done. Really done, with science.
|
|
|
MagicJigPipe
International Hazard
Posts: 1554
Registered: 19-9-2007
Location: USA
Member Is Offline
Mood: Suspicious
|
|
The way I understand it is since all magnetic fields are caused by moving charges (in the case of a ferromagnet it would be the magnetic
moments/dipoles of the electrons in an iron atom lining up) and are always effected by other magnetic fields. Since all matter contains moving charge
(I include spins here) then all matter will effect a magnetic field. The question is how? Will it be repulsed, attracted or have no net reaction?
Even if the material is non-magnetic it will still be affected slightly. Sort of like even though gamma radiation will easily pass through a piece of
paper, there will still be some gamma photons that will impact an electron or nucleus in the paper reducing it's intensity (of course only negligibly)
There are 3 types of magnetic materials and I'm sure you know about those.
It is probably extremely complex to examine an effect that's so weak. So many different combinations of effects can come together in such a simple
material as say, PTFE. I don't even know enough to describe the interaction of a magnetic field with the electrons in such a molecule (I'm just not
there yet). And in most of these non-magnetic materials I'm sure the effects cancel each other out until the effect is essentially non-existent.
Attempting to find a material that causes "degradation" of a magnetic field more than others just starting with these principles would be extremely
difficult. I haven't been able to find some sort of list online.
I find this subject incredibely fascinating and would love to learn much more. If I have an inaccurate understanding PLEASE let me know.
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
|
|
|
Blasty
Hazard to Others
Posts: 107
Registered: 25-7-2008
Member Is Offline
Mood: No Mood
|
|
Have you got access to the book I referred to in the first post? I had access to it through my local public library a couple of decades or so ago, but
no longer (I think someone stole their copy; they also stole their copy of McLain's Pyrotechnics From The View Point of Solid State
Chemistry) I seem to remember that the author briefly addressed this topic by pointing out the diminishing effects that brass plates of a certain
thickness have on the magnetic field of a permanent magnet. I wish I had copied the passage when I was reading the book. It's a topic that does not
seem to be usually addressed in the literature about permanent magnets.
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
