Sciencemadness Discussion Board

Question about magnetic fields going through non-magnetic materials

Blasty - 31-3-2011 at 21:29

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 - 1-4-2011 at 05:10

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 - 1-4-2011 at 07:01

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 - 2-4-2011 at 07:42

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 - 2-4-2011 at 11:17

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 - 5-4-2011 at 02:50

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 - 5-4-2011 at 05:16

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 - 5-4-2011 at 13:28

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 - 7-4-2011 at 09:47

A Moo-Metal audio transformer from Sowter, having now fallen out of popular use due to the advent of solid state

watson.fawkes - 7-4-2011 at 12:24

More on moo-metal: http://www.youtube.com/watch?v=rX_hdUmVyZ4

IrC - 7-4-2011 at 16:04

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]

arsphenamine - 7-4-2011 at 17:57

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 - 7-4-2011 at 18:09

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.

watson.fawkes - 9-4-2011 at 12:37

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 - 13-4-2011 at 18:27

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.

Blasty - 13-4-2011 at 21:19

Quote: Originally posted by MagicJigPipe  


I find this subject incredibely fascinating and would love to learn much more.


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.