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I Iron magnetism

  1. May 21, 2016 #1
    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. jcsd
  3. May 21, 2016 #2

    Drakkith

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    Screens static or dynamic magnetic fields?
     
  4. May 21, 2016 #3
    Static magnetic fields, the magnetic field of a permanent magnet, for example.
     
  5. May 21, 2016 #4

    Drakkith

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    I wasn't aware of that. I though it only blocked dynamic magnetic fields. Very interesting...
     
  6. May 21, 2016 #5
    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.
     
  7. May 21, 2016 #6

    Drakkith

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    You should be able to click the "Upload" button at the bottom right and attach a file or picture.
     
  8. May 21, 2016 #7
    Ok, I attached it.
     

    Attached Files:

  9. May 21, 2016 #8

    Dale

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    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.
     
  10. May 21, 2016 #9
    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.
     
  11. May 21, 2016 #10

    marcusl

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    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.
     
  12. May 21, 2016 #11
    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.
     

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  13. May 21, 2016 #12

    marcusl

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  14. May 21, 2016 #13
    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.
     
  15. May 21, 2016 #14

    marcusl

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    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
     
  16. May 21, 2016 #15
    Not very happy with numerical simulations but thanks anyway.
     
  17. May 21, 2016 #16

    marcusl

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    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.
     
  18. May 21, 2016 #17

    Vanadium 50

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    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.
     
  19. May 21, 2016 #18
    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.
     
  20. May 21, 2016 #19

    marcusl

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    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.
     
  21. May 21, 2016 #20
    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.
     
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