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Classical magnetism

  1. Nov 15, 2014 #1
    a stationary magnet attracts another magnet or a piece of ferromagnetic material.
    1) I am trying to quantify (generic formulae) the distance (from the magnet) at which the attracted material starts moving towards the magnet -
    2) If I were to try interrupting the attraction at some point after initiation, could a magnetic shield (diversion shield, actually, since the lines of magnetic field must close) be introduced on the path in useful time?
    What would be the position of its insertion relative to the stationary magnet?
    Is it possible at all?

    Thank you in advance.
     
  2. jcsd
  3. Nov 15, 2014 #2

    mfb

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    As soon as the attractive force exceeds friction. In general, determining this force can be quite complex.
    If your system reacts fast enough, sure. A good position will depend on the setup.
     
  4. Nov 16, 2014 #3
    Thank you very much, mfb!!
    It looks like I need to obtain some data from actual experiments. Especially, what would be the longest distance ever achieved so far...
     
  5. Nov 16, 2014 #4

    mfb

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    That mainly depends on the size and field strength of the magnet. There are many magnets around that can attract iron from meters away, and the big particle physics experiments have magnets that could probably influence light particles enough over 100m if you remove their return yokes (they are designed to contain the field to the detector, where it is used). Depending on the orientation of the magnetic field in the planned INO, they could get an even larger "range".

    All those values are just theory - the experiments are too expensive to let magnetic things fly around.

    If you have enough time, you can also put a magnetic particle in an orbit around earth and watch how the magnetic field slowly changes its orbit from thousands of kilometers away.
     
  6. Nov 16, 2014 #5
    Thanks again, mfb!

    Embarrassingly enough, my question is way more prosaic than that. It is about objects of dimensions measurable in feet (meters) and pounds (kg).
    I scramble around for data but no luck yet.
    I am working on something which could go either way:
    1) If a mass of several pounds cannot be attracted from several feet, it's all a wash for the project. The larger the numbers, the better.
    2) If the above is doable, then I may be onto something. And if I am, then the apparent triviality of my question will turn into its opposite.
     
  7. Nov 17, 2014 #6

    mfb

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    It can. With a massive magnet that is so expensive that you don't want heavy objects flying around.

    What do you want to do?
     
  8. Nov 17, 2014 #7
    The idea has been in the workings of my mind for many years. Intuition and all theory I ever came across indicate it works.
    I have been circumspect about revealing anything so far - sensible reasons, one may gather...
    I can say, at this point, only that the magnet is used merely as a corrective on objects that already have momentum. The latter don't start from rest and don't encounter friction (don't drag on a surface) - this may relax the demand on the magnet's size and cost.

    If this happens to be your field of expertise, and also have empiric (not only theoretic, like me) knowledge, and if I progress on other, totally different facets of the overall project, then I would like to correspond.

    Thank you again, and I hope you approve of my obfuscation at this time.
     
  9. Nov 18, 2014 #8

    mfb

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    I have worked with large (larger than a meter) magnets with ~1 Tesla multiple times. I would not call it "my field of expertise", however.

    Or make it more problematic because the time the object is in the field could be short.

    Vague questions just allow vague answers.
     
  10. Nov 23, 2014 #9
    I have arrived at a simplified description, with some putative values:
    I need to act on the load from 1m above it, and lift it through that distance.
    The load is 50 kg.
    I need to cut the flux at some point close to the end of the distance.
    What strength and size must the magnet be?
    What can be said about the shield - size, shape?
    Costs?

    These elements are not the whole project, so costs are aggregating from all quarters. This would be understandable because the materials are quite novel.
    This is why I consider keeping the magnet and shield costs lower to be crucial.

    Thank you again.
     
  11. Nov 23, 2014 #10

    mfb

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    Oh, good luck. Switching those magnets on and off can be tricky. Sure they all have an emergency off mechanism, but then it can happen that they need hours (!) to cool down again.

    The large electromagnets used to move cars around could be interesting. They are faster with switching.
     
  12. Nov 24, 2014 #11
    Oh, so my intuition was right that this is where my fantasies might crash!

    I would have needed around 7 seconds between reiterations of the process, but your warning about cooling down is quite ominous.

    That would mean shelving the whole thing.

    I greatly appreciate your time put in this and your implicit help in shortening my lala-land time!

    Best to you.
     
  13. Nov 24, 2014 #12

    mfb

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    Large magnetic fields store a lot of energy - about 500kJ for 1 cubic meter and 1 Tesla. If you want to switch the magnet off, this energy has to go somewhere. The regular way is to dump it into some resistors somewhere, that can take a while. The fast version (emergency off) is to dump the energy into the coils of the magnet, then cooling it down to operation temperature can take a while (depends on the coil).

    To power the magnet again, you'll need this energy again.
     
  14. Nov 25, 2014 #13
    In light of all this, I considered reconfiguring the geometry of the pertinent sections.
    But it doesn't give me more leeway than several seconds.

    How about permanent magnets?
    The natural ones, I assume, are not strong enough.
    What about the permanently magnetized materials? They wouldn't need cooling-off time, correct?
     
  15. Nov 25, 2014 #14

    mfb

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    Where is the point of permanent magnets? You cannot switch them in a reasonable way at all (that's why they are called permanent).

    You can move magnets (independent of the type) around, that could be easier than switching them on and off.
     
  16. Nov 25, 2014 #15
    I have been considering either of the following:
    - moving the magnets;
    - inserting the shield - you confirmed that they do work, but from your exposition it results that the electromagnets would be far less amenable to this action.

    Would you please enlighten me as to which one of the options above would be more feasible with permanent magnets?
    If this is doable, then I am back on track and hope to collaborate.
     
  17. Nov 25, 2014 #16

    mfb

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    Without more details about what you want to do, it is impossible to suggest anything specific.
     
  18. Nov 26, 2014 #17
    I agree.
    You have already elucidated important points so far.

    I will take some time, and when I reach a comfortable stage I reconnect.

    Thanks again.
     
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