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Is there a way to shield a magnets field

  1. Jan 24, 2007 #1
    I am wondering if there is a way to put shielding on a simple magnetic bar mad out of neodymium so that the magnetic field is not emanating from all over but just at the ends. I/E a magnet shaped like a can of soda. Putting some type of Shielding on what would be the label section so that the magnetic field would onlu come out of the top & Bottom section. Is that possible ?
     
  2. jcsd
  3. Jan 24, 2007 #2
    super conductors have that ability I believe.
     
  4. Jan 24, 2007 #3

    ranger

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  5. Jan 24, 2007 #4

    marcusl

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    I'm not following your question. The field from a bar magnet emanates almost entirely from the two ends.
     
  6. Jan 24, 2007 #5
    I don't think you can't really sheild magnetic fields.
     
  7. Jan 24, 2007 #6

    ranger

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    Exactly. When they say shield, they should really be saying redirect. As per my explanation in the post cited above, the magnetic field will simply flow through the mumetal because it has a higher magnetic permeability than the air around it.
     
  8. Aug 20, 2008 #7
    Is there any known material that will shield the magnets field? I am wanting to put a magnet on a machine and don't want it to adhere to. or magnatize the machine. I need somthing thin that I can put between the magnet and the steel that it will be fastened to that will shield the mannetic field.
     
  9. Aug 20, 2008 #8

    mgb_phys

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    Anything with a high permeability will work to some extent - as long as it is larger than the magnet, otherwise the field will just go around the egdes.
    Mumetal, as ranger said, is best but it is expensive and difficult to work with. Iron or silicon steels are pretty good and are generally used in AV speakers.

    Of course if you want the magnet to stick to the machine you rather have to let the field reach the machine !
     
  10. Aug 20, 2008 #9

    berkeman

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    Another strategy would be to put a "keeper" on the magnet. That will contain the magnetic field, and the magnet will not stick to the steel machine. I'm assuming that you are going to put this magnet on some kind of shelf or hook, right?

    A keeper looks like this for a horseshoe-shaped magnet:

    http://www.dorlingkindersley-uk.co.uk/static/clipart/uk/dk/sci_electricity/image_sci_elec017.jpg

    Basically a keeper is a piece of steel that clicks onto the two poles of the magnet (whatever shape the magnet is), and contains the field.
     
  11. Aug 20, 2008 #10
    What is mumetal? this is a new term for me.
     
  12. Aug 20, 2008 #11

    mgb_phys

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    Follow the link in rangers post, it's a special high magnetic permeability alloy.
     
  13. Aug 20, 2008 #12

    Dale

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    Any ferromagnetic material (e.g. iron) will also provide some shielding.
     
  14. Aug 20, 2008 #13

    berkeman

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    Everybody please keep in mind that netic and co-netic mu-metal shielding is meant for low- to moderate-field shielding applications. It is not meant for high magnetic field situations like permanent magnets. mu-metal saturates pretty easily, due to its very high mu values, so it is great for deflecting B fields, but not if the B field is high enough to cause saturation. mu-metal shielding might be used in some sensitive CRT shielding applications, for example, but would never be used in any permanent magnet environment that I can think of.
     
  15. Aug 21, 2008 #14
    I heard somewhere that molten(hot and liquified) ferrous metals disrupts a magnetic field such that the magnetic field can not pass through. Not sure if that's true though.
     
  16. Apr 11, 2011 #15
    I had a similar question and found this thread. So far nicely informative, however, I have a slightly different angle towards the same end. I am carrying around an antenna for my mobile broadband with a magnetic bottom. I was annoyed by its ostensible propensity for attracting certain objects, with interesting results... Always the amateur engineer I want to find a way to fix a problem, and being a lover of science I like certainty and precision.

    Be a curious sort, I found a steel box to put it on and noticed that most, but not all, of the magnet's strength was reduced. I am not yet in UP 2 {I think this topic might be touched upon then but I like to think about things whilst fresh on my mind.}.

    So my question: "Can the relationship between this re-routeing of magnetic force be related to the permeability And mass of the magnet and the ferromagnetic material respectively?"

    This question seems intuitive, and quite nearly rhetorical, but the real question is: "If this is true, then how does one go about actually discussing the forces and other aspects involved, i.e. to be able to plan and make something that performs this function?"

    This way I can find out, without trial and error, approximately how large a shield I must use.

    I have at this point assumed that shape of the shield is a separate issue. I am trying to simplify my question. This is only a curious musing at this point, no sensitive instruments involved.

    Thanks in advance for any help.

    Kyle
     
  17. Apr 11, 2011 #16

    marcusl

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    It is likely that you don't need a "shield" surrounding your base, but a "magnet keeper" at its bottom. Your magnet is designed to stick onto ferromagnetic plates (it's an antenna holder after all), so it likely has both poles at the bottom surface. A common geometry is that one pole occupies a central disk, the other pole an annulus surrounding it. Flux lines exit one pole and enter the other. A thick iron plate will effectively shunt the flux without much leaking out. How thick depends on the strength of the magnet, but 1/8" should be quite effective.
     
  18. Apr 12, 2011 #17
    I am not quite there yet, but certain materials have magnetic domains that can align properly to increase or decrease magnetic moments. Sure, changes in temperatures can alter the frequency of proper alignment. If there is just a way to align the domains inside certain conductors to shield the emanating magnetic fields, you will surely be contributing to the world of engineering.
     
  19. Apr 12, 2011 #18
    The location of the poles, I definetly think you are right. What I am after is sort of an experiment... last night I did find some promising information in my Physics book about magnetism, though general. It discussed the relationship between the permeability of the ferromagnetic material and the strength of the magnetic force. It seemed more toward electromagnetism, being measured in Teslas.

    So a re-newed question: How then is a permanent magnet rated for the strength of its force? {Once again toward the central purpose of selecting, without trial, an appropriately dimensioned keeper.}

    Thanks again for your assistance so far,

    Kyle
     
  20. Apr 12, 2011 #19

    marcusl

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    Calculating magnetic fields in the presence of ferromagnetic materials is surprisingly difficult, and requires the use of fast computers and big codes. These are usually finite element calculations, but there are other approaches such as Galerkin method. Here's one such code for a PC:

    http://www.fieldp.com/magnum.html

    The reason it's complicated is that the field at a point depends on the shielding by iron everywhere else. The shielding everywhere depends on the permeability at every point in the material. The permeability at every point, however, depends non-linearly on the field at the point. But that's where we started, so it's a circular problem. It's typically solved by dividing the materials into 10's of thousands to millions of meshes, estimating the fields and permeabilities at each (with large errors to start with), and iterating until a converged solution is reached.

    Before computers, the design of motors or magnet systems typically proceeded by estimation followed by extensive trial and error. Unless you have a computer code, I suggest you estimate and see how you do. Here's my estimate: mild steel 0.090" thick will be pretty good, 1/8" thick should give nearly complete "shielding".
     
  21. Apr 12, 2011 #20
    Thanks Marcus, I followed the link. I am pleased to discover that I have much more to learn.
    And seeing the value of computer simulation software such as comsol... the ilk.
    This topic seems to have a suprising amount of depth. Maybe I will run into it in grad school?
    That would be interesting.
    Kyle
     
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