# I Iron magnetism

1. May 21, 2016

### Dyon

Hi All,
It is said that iron has a high "permeability" for the magnetic field, it lets 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

2. May 21, 2016

### Staff: Mentor

Screens static or dynamic magnetic fields?

3. May 21, 2016

### Dyon

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

4. May 21, 2016

### Staff: Mentor

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

5. May 21, 2016

### Dyon

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.

6. May 21, 2016

### Staff: Mentor

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

7. May 21, 2016

### Dyon

Ok, I attached it.

#### Attached Files:

• ###### MagneticScreening.jpg
File size:
81.5 KB
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139
8. May 21, 2016

### Staff: Mentor

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.

9. May 21, 2016

### Dyon

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.

10. May 21, 2016

### 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.

11. May 21, 2016

### Dyon

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.

#### Attached Files:

• ###### MagneticScreening.jpg
File size:
81.5 KB
Views:
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12. May 21, 2016

### marcusl

13. May 21, 2016

### Dyon

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.

14. May 21, 2016

### 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

15. May 21, 2016

### Dyon

Not very happy with numerical simulations but thanks anyway.

16. May 21, 2016

### 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.

17. May 21, 2016

Staff Emeritus
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.

18. May 21, 2016

### Dyon

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.

19. May 21, 2016

### 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.

20. May 21, 2016

### Dyon

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.

21. May 21, 2016

### marcusl

22. May 21, 2016

### Dyon

Ok, thanks anyway.

23. May 21, 2016

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.)

24. May 21, 2016

### Dyon

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.

#### Attached Files:

• ###### MagneticScreening2.jpg
File size:
94.4 KB
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25. May 22, 2016

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.