Iron & Magnetic Fields: Permeability or Screening?

[Dyon]

Hi All,
It is said that iron has a high "permeability" for the magnetic field, it let's the magnetic field pass through it more easily.
But it is also said that iron screens the magnetic field? Don't these two contradict themselves?

Thanks,
Ionel

 

[Drakkith]

But it is also said that iron screens the magnetic field?

Screens static or dynamic magnetic fields?

 

[Dyon]

Screens static or dynamic magnetic fields?

Static magnetic fields, the magnetic field of a permanent magnet, for example.

 

[Drakkith]

Static magnetic fields, the magnetic field of a permanent magnet, for example.

I wasn't aware of that. I though it only blocked dynamic magnetic fields. Very interesting...

 

[Dyon]

I wasn't aware of that. I though it only blocked dynamic magnetic fields. Very interesting...

I can send you a scan from a book of physics where this is stated. But I don't know how to upload pictures (or PDFs) here.

 

[Drakkith]

I can send you a scan from a book of physics where this is stated. But I don't know how to upload pictures (or PDFs) here.

You should be able to click the "Upload" button at the bottom right and attach a file or picture.

 

[Dyon]

You should be able to click the "Upload" button at the bottom right and attach a file or picture.

Ok, I attached it.

 

[Dale]

high "permeability" for the magnetic field, it let's the magnetic field pass through it more easily

I don't think that this is an accurate description of what high permeability means. If a material has high permeability then it becomes highly magnetized in response to an external field. The result of that would probably be better characterized by saying that it pulls magnetic field lines into itself.

 

[Dyon]

I don't think that this is an accurate description of what high permeability means. If a material has high permeability then it becomes highly magnetized in response to an external field. The result of that would probably be better characterized by saying that it pulls magnetic field lines into itself.

A big plate of iron placed in front of a bar magnet (plate area say 100 times that of the pole of a magnet) reduces the magnetic field behind it.
If the iron plate became highly magnetized in response to the field of the bar magnet, it wouldn't have this effect, the plate would develop an equal pole on the other side of it and transmit the field to its rear. So rather than saying it pulls the field lined to itself, it looks like iron really prevents the lines from passing through.

 

[marcusl]

That's not how it works. To shield a point, a sensor, say, place your iron sheet near the sensor but parallel (not perpendicular) to the field lines. The field is pulled into the iron as Dale said and conducted along it. This reduces the field at the sensor.

 

[Dyon]

That's not how it works. To shield a point, a sensor, say, place your iron sheet near the sensor but parallel (not perpendicular) to the field lines. The field is pulled into the iron as Dale said and conducted along it. This reduces the field at the sensor.

Ok, but from the figure attached, it seems that the iron rather blocks the magnetic field than transmitting it. Hence probably iron shouldn't be described as having high "magnetic permeability", on the contrary, it has low "permeability" since it blocks the magnetic field. Compare it to a plate of wood of identical dimensions, where the magnetic fields passes through easily.

 

[marcusl]

There's no "blockage;" field lines are being conducted through the slab. Compare it to field lines incident on a superconducting slab, which does "block" them. See the difference?

https://www.google.com/url?sa=i&rct...YbE4t5FIpq9mLjtBZV5B2kYA&ust=1463939164204453

 

[Dyon]

There's no "blockage;" field lines are being conducted through the slab. Compare it to field lines incident on a superconducting slab, which does "block" them. See the difference?

https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwiRi8TM3OvMAhVXE1IKHbfbAEUQjRwIBw&url=http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Rodney_Guritz%20Folder/magnetism.htm&bvm=bv.122448493,d.aXo&psig=AFQjCNGtm6YbE4t5FIpq9mLjtBZV5B2kYA&ust=1463939164204453

Ok, I see the difference. But, I find it quite strange to call iron "high permeability" material and not to have a strong magnetic field on the back of an iron plate right opposite the pole of a magnet. Rather I suspect something similar with a superconductor might be taking place: lines enter the iron superficially and then travel along the surface - while for a superconductor the lines do not enter at all.

 

[marcusl]

Completely wrong. Field lines are conducted ("permeate") into the the bulk of the iron, and flow through it. Looking at conceptual drawings like the one you showed and the one I linked to can be misleading. If you look at a proper numerical simulation of the fields, you'll see more clearly. Here are field lines incident on a hollow ferromagnetic cylinder. You can see field lines "attracted" to the iron and conducted along it at top and bottom. The fields completely penetrate the material. Notice also that the left and right sides of the cylinder, which are normal to the field, do not block field lines at all. All the action occurs parallel to, not normal to, the fields.

http://www.magneticshields.co.uk/en/technical/magnetic-shielding-how-does-it-work

 

[Dyon]

Completely wrong. Field lines are conducted ("permeate") into the the bulk of the iron, and flow through it. Looking at conceptual drawings like the one you showed and the one I linked to can be misleading. If you look at a proper numerical simulation of the fields, you'll see more clearly. Here are field lines incident on a hollow ferromagnetic cylinder. You can see field lines "attracted" to the iron and conducted along it at top and bottom. The fields completely penetrate the material. Notice also that the left and right sides of the cylinder, which are normal to the field, do not block field lines at all. All the action occurs parallel to, not normal to, the fields.

http://www.magneticshields.co.uk/en/technical/magnetic-shielding-how-does-it-work

Not very happy with numerical simulations but thanks anyway.

 

[marcusl]

I don't know why you would say that. Finding magnetic fields in the presence of ferromagnetic material is a highly non-linear problem, due to the extremely non-linear shape of hysteresis loops. There is virtually no way to solve such a problem without numerical solvers, unless fields are kept so weak, distances so large and iron so thick that a linear approximation holds.

 

[Vanadium 50]

Not very happy with numerical simulations but thanks anyway.

Are you planning on complaining about every answer you receive? You got a lot of good answers here, coming at this from various directions. But you don't seem to like any of them. Not liking an actual calculation is, well, hard to argue with.

 

[Dyon]

I don't know why you would say that. Finding magnetic fields in the presence of ferromagnetic material is a highly non-linear problem, due to the extremely non-linear shape of hysteresis loops. There is virtually no way to solve such a problem without numerical solvers, unless fields are kept so weak, distances so large and iron so thick that a linear approximation holds.

Back to the plate in front of a magnet, when a magnetic line coming from the pole of the magnet enters the iron plate perpendicularly, I don't see why it just doesn't go straight on, if iron has such a "high permeability" material. The line doesn't go straight but curves and goes through the plate following its shape close to the surface. This looks like the material blocks the field rather than letting it pass (like a plate of wood does, for example).

Thanks again, waiting for other opinions.

 

[marcusl]

You are being fooled by the fact that you've chosen to put the plate in front of a localized source (pole). The fields are already curving around to return to the distant pole, so the slab just enhances that tendency. (In fact, for a dipole there is mathematically only a single field line that impinges on the slab at normal incidence. The majority of field lines impinge on the slab at an angle.)

If you examine a slab inserted at right angles into a wide region of completely uniform field, you will field plenty of field intensity on the opposite side.

 

[Dyon]

You are being fooled by the fact that you've chosen to put the plate in front of a localized source (pole). The fields are already curving around to return to the distant pole, so the slab just enhances that tendency.

It is the fact that the slab enhances that tendency (of lines to return) that bothers me. It just doesn't square well with the fact that iron has "high permeability". It looks more like iron "reflects" the lines than letting them pass on.

 

[marcusl]

Stick to your incorrect view if you prefer. I can't seem to help you.

 

[Dyon]

Stick to your incorrect view if you prefer. I can't seem to help you.

Ok, thanks anyway.

 

[Charles Link]

I do think in your first attachment, (post #7), parts of the iron shield do become magnetized with a strong magnetization. The magnetization is not uniform, but runs upward on the upper half of the shield and downward on the bottom half of the shield. You might expect the magnetization $M$ and the magnetic field $B$ inside the shield to point to the right, but I think your attachment is accurate in the result that can occur with the proper shield/slab geometry. The permanent magnet also has strong magnetization $M$and strong lines of flux /magnetic field $B$ , that will be somewhat stronger than any magnetization or field strength that occurs inside the shield. (In the shield the flux lines get re-routed in two directions.) The description I'm giving is qualitative, but can be quantified with the equation $B=\mu_oH+M$. In order for the iron shield to have these lines of flux $B$ conducting through it, it necessarily develops a strong magnetization vector $M$ along the direction of the flux lines. (Again the direction of $M$ in the shield is upward in the upper half and downward in the lower half.)

 

[Dyon]

I do think in your first attachment, (post #7), parts of the iron shield do become magnetized with a strong magnetization. The magnetization is not uniform, but runs upward on the upper half of the shield and downward on the bottom half of the shield. You might expect the magnetization $M$ and the magnetic field $B$ inside the shield to point to the right, but I think your attachment is accurate in the result that can occur with the proper shield/slab geometry. The permanent magnet also has strong magnetization $M$and strong lines of flux /magnetic field $B$ , that will be somewhat stronger than any magnetization or field strength that occurs inside the shield. (In the shield the flux lines get re-routed in two directions.) The description I'm giving is qualitative, but can be quantified with the equation $B=\mu_oH+M$. In order for the iron shield to have these lines of flux $B$ conducting through it, it necessarily develops a strong magnetization vector $M$ along the direction of the flux lines. (Again the direction of $M$ in the shield is upward in the upper half and downward in the lower half.)

Hi, thanks for your comment.
I understand that the iron plate develops poles at the edges, but they are more to the magnet side and not to the rear side.
If iron was such a "high permeability" material for the magnetic field, it would develop poles at points A also (at the rear of the plate). This is what strikes me as odd. Instead, no magnetism is detected at A, all of it looks like reflected back to the magnet side, and this to me looks like a screen and "low permeability" material - especially when comparing the iron plate with a glass plate of similar dimensions and seeing that magnetism passes through the glass plate almost undiminished.

I also begin to suspect that the magnetic lines do not really pass through the iron bulk but only very close to its surface, because really nobody obtained magnetic field lines with iron filings inside bulk iron. The magnetic field lines in such pictures as this and in the one I attached to this comment are really obtained outside not inside of the iron piece.

 

[Charles Link]

The lines of flux of the magnetic field do pass through the bulk iron, but if you look closely at your first attachment, they seem to have them running more to the left side of the slab that is doing the shielding. (The lines of magnetic flux do not go to point A. Instead they circulate through the iron shield,( to the top and bottom) and return to the "-" end of the permanent magnet.) The magnetic field around iron does not behave like a superconductor where the magnetic field vanishes in the interior. It also does not behave like the (static) electric field when it encounters a good conductor where the electric field vanishes in the interior (with the electric charges distributing themselves on the outer surface of the conductor material to make this occur.) One concept that is used in the magnetic flux is that the flux lines need to travel in continuous loops, e.g. the flux lines that circulate through a permanent magnet and emerge out of the "+" pole end will loop around and feed into the "-" end. (in more mathematical terms, this is the statement (one of Maxwell's equations) that $\nabla \cdot B =0$. In the case of iron slabs as shields, you could probably find detailed calculations somewhere, (e.g. computer simulations), that accurately predict both the path of the flux lines as well as the actual field strength along with the distribution of the resulting induced magnetization vector throughout the material (magnitude and direction), The text that has the two diagrams you have displayed seems to be quite good.

 

[Dyon]

The lines of flux of the magnetic field do pass through the bulk iron, but if you look closely at your first attachment, they seem to have them running more to the left side of the slab that is doing the shielding. (The lines of magnetic flux do not go to point A. Instead they circulate through the iron shield,( to the top and bottom) and return to the "-" end of the permanent magnet.) The magnetic field around iron does not behave like a superconductor where the magnetic field vanishes in the interior. It also does not behave like the (static) electric field when it encounters a good conductor where the electric field vanishes in the interior (with the electric charges distributing themselves on the outer surface of the conductor material to make this occur.) One concept that is used in the magnetic flux is that the flux lines need to travel in continuous loops, e.g. the flux lines that circulate through a permanent magnet and emerge out of the "+" pole end will loop around and feed into the "-" end. (in more mathematical terms, this is the statement (one of Maxwell's equations) that $\nabla \cdot B =0$. In the case of iron slabs as shields, you could probably find detailed calculations somewhere, (e.g. computer simulations), that accurately predict both the path of the flux lines as well as the actual field strength along with the distribution of the resulting induced magnetization vector throughout the material (magnitude and direction), The text that has the two diagrams you have displayed seems to be quite good.

Ok, so it is clear to me that most of you (if not all) have a deep belief that the magnetic field lines pass through the bulk of iron, and that the magnetic field lines are not confined to a very thin layer at the iron surface.
Thanks very much for your answers, I will search more on this issue and I will probably write an article arguing against this common belief.

 

[Charles Link]

Ok, so it is clear to me that most of you (if not all) have a deep belief that the magnetic field lines pass through the bulk of iron, and that the magnetic field lines are not confined to a very thin layer at the iron surface.
Thanks very much for your answers, I will search more on this issue and I will probably write an article arguing against this common belief.

The geometry of an slab shielding a magnetic field (as in your first figure) is a semi-complex one, but from your discussion, I believe for the case of a soft iron sphere placed in a uniform field, you would conclude that the magnetic field remains outside of the iron. This clearly is not the case. In fact, the spherical geometry in a uniform field yields an interesting solution where the magnetization vector $M$ and the magnetic field $B$ are both uniform inside the sphere, and they both point in the direction of the applied magnetic field.

 

[Dyon]

The geometry of an slab shielding a magnetic field (as in your first figure) is a semi-complex one, but from your discussion, I believe for the case of a soft iron sphere placed in a uniform field, you would conclude that the magnetic field remains outside of the iron. This clearly is not the case. In fact, the spherical geometry in a uniform field yields an interesting solution where the magnetization vector $M$ and the magnetic field $B$ are both uniform inside the sphere, and they both point in the direction of the applied magnetic field.

My whole point is that you have no experimental proof to support the belief that the magnetic field is uniform inside the sphere, indeed that it goes through the bulk at all.

Speaking about your example with the sphere, I was thinking about making myself a hemisphere of soft iron and stick the pole of a magnet at the center (so that the pole is at the center of the sphere form which the hemisphere is cut). Thus, all the points on the hemisphere surface would be equally far from the pole, through the iron. I guess if the magnetic field does pass through the bulk iron hemisphere you will find equal magnetism on the surface of the hemisphere. But I think that, since this is not the case and magnetism does not pass through the bulk iron, you will not find equal magnetism on the surface of the hemisphere but magnetism will be weakest at the point right opposite the pole, because it is farthest from the pole when going along the surface. Would that make it an "experimentum crucis" to settle this whole argument?

 

[Charles Link]

My whole point is that you have no experimental proof to support the belief that the magnetic field is uniform inside the sphere, indeed that it goes through the bulk at all.

Speaking about your example with the sphere, I was thinking about making myself a hemisphere of soft iron and stick the pole of a magnet at the center (so that the pole is at the center of the sphere form which the hemisphere is cut). Thus, all the points on the hemisphere surface would be equally far from the pole, through the iron. I guess if the magnetic field does pass through the bulk iron hemisphere you will find equal magnetism on the surface of the hemisphere. But I think that, since this is not the case and magnetism does not pass through the bulk iron, you will not find equal magnetism on the surface of the hemisphere but magnetism will be weakest at the point right opposite the pole, because it is farthest from the pole when going along the surface. Would that make it an "experimentum crucis" to settle this whole argument?

Any demonstration of this sort would not be very definitive. I think if you have any kind of resources available to work with magnetic materials of various kinds, you could do a lot of interesting things including experiments to create permanent magnets (even spherical ones), and even see what kind of field is necessary to reverse the magnetization. I do suggest if you have further interest in the subject, you google about experiments that have already been performed.

 

[jim hardy]

Back to the plate in front of a magnet, when a magnetic line coming from the pole of the magnet enters the iron plate perpendicularly, I don't see why it just doesn't go straight on, if iron has such a "high permeability" material. The line doesn't go straight but curves and goes through the plate following its shape close to the surface. This looks like the material blocks the field rather than letting it pass (like a plate of wood does, for example).

Maybe a picture will help.

First thing to straighten out is magnetic flux , like current, makes its way back to the source. Think "Closed Loops".So in OP's first picture
here's what i would add to hopefully resolve the misunderstanding.

Flux (in red) will prefer the permeable iron slab , not the impermeable air behind it. (well, permeability of just 0.00000126)

That image in MarcusL's post 14 is just great , shows how an iron pipe routes flux around its hollow middle. That's why we use iron conduit for some sensitive signal wires in the power plant.

 

[Dyon]

Maybe a picture will help.

First thing to straighten out is magnetic flux , like current, makes its way back to the source. Think "Closed Loops".So in OP's first picture
here's what i would add to hopefully resolve the misunderstanding.

View attachment 101112

Flux (in red) will prefer the permeable iron slab , not the impermeable air behind it. (well, permeability of just 0.00000126)

That image in MarcusL's post 14 is just great , shows how an iron pipe routes flux around its hollow middle. That's why we use iron conduit for some sensitive signal wires in the power plant.

Thanks for taking the time to draw the magnetic lines in red. But I go back to my initial point and say you don't have any experimental proof that the lines actually go so deep in the iron plate, I think they enter the plate (if at all) very superficially, most of the lines just glide past the surface (iron screens them), that's why the poles seen at the edges are towards the magnet side and not also to the rear side. Also I have a great deal of trouble understanding why a N pole is not induced at A on the plate since iron is such a "high permeability" material, which also tells me iron's screening is real and not due to "high permeability".

The iron pipe routing the flux around its hollow middle can be seen also as a proof that iron is not permeable to magnetism, thus prevents the magnetic field from passing through. If the pipe were made of plastic, that would be a "highly permeable" material to the magnetic field since it let's the magnetic field pass through.

 

[Dyon]

Any demonstration of this sort would not be very definitive. I think if you have any kind of resources available to work with magnetic materials of various kinds, you could do a lot of interesting things including experiments to create permanent magnets (even spherical ones), and even see what kind of field is necessary to reverse the magnetization. I do suggest if you have further interest in the subject, you google about experiments that have already been performed.

Thanks.
In order to avoid the magnetic field lines' tendency to return to the other pole, I think I will work with a very long magnet to have the magnetic lines leaving the pole radially.

 

[Dale]

I will probably write an article arguing against this common belief.

I think it is time to close this thread.