Copper ingot in MRI machine

  1. Hey there,

    inspired by this exhibit in the SF exploratorium, I was discussing with my colleague what might happen if you put a thick piece of pure copper (say, a 5kg ingot) in the bore of a 3.4 T MRI machine. (What about 7T?) We have the MRI machine, all we need is a big piece of copper...
    Will it float? Will it resist when force is applied?


    [​IMG]
     
  2. jcsd
  3. f95toli

    f95toli 2,349
    Science Advisor
    Gold Member

    Nothing would happen. Copper is not a magnetic material.
    In fact, some parts of the actual MRI machine might be made from copper for that very reason (abnther reason being that it s good conductor of heat, which is usueful for cryogenic applications). The wire used to make the solenoid is definitly partly copper (it is superconducting, but in a matrix of copper for cooling, structural integritiy and protection in case of a quench)

    We have a 14T magnet where I work, and much of the "strucural" material (sample holders etc) are made from copper (the rest is made from non-magnetic stainless steel, brass and aluminium).

    The only thing you need to watch out for when using non-magnetic metals is eddy currents.
     
  4. Or would it?

    I'm aware of that.


    And that's where it's getting interesting. Check out the link above. The Magnet between the slabs of copper can be suspended in mid air for a couple of second due to the eddy currents. I'm basically wondering if it also works the other way around: If you have a big magnet with a slab of copper instead of two big pieces of of copper and a smaller magnet, would you achieve a similar effect? You need a large chunk of copper (>4cm thickness) for it to work the classical way. But what's the math behind it?
     
  5. OK so we all have huge magnetic fields, who has a big piece of copper to wield around?
     
  6. First thing which comes in mind is that depending on which speed the copper ingot is inserted in the magnetic field, it'd get hot due to inducted currents.
     
  7. marcusl

    marcusl 2,100
    Science Advisor
    Gold Member

    Eddy currents are induced by changing flux, so you will induce eddy currents that will will produce resistance to motion when you try to rotate the conductor. It is not desirable to use a huge ingot of the type you picture--apart from being heavy and unwieldy, a cylinder will experience no eddy currents when rotated about its axis, since the flux enclosed by the conductor in a uniform field will be constant. Find an aluminum (it's light and easy to handle) disk, maybe 1 cm thick, instead. Put it into the field so that the field is normal to the plane of the disk, and rotate it around an axis perpendicular the field (like turning the vane in an air duct damper). You will feel resistance due to eddy currents and Lenz's law.

    BTW, observe good safety practice when doing this experiment. Empty your pockets of metallic, and especially ferromagnetic, objects, remove your belt, etc., and don't approach a strong magnet if you have a pacemaker, metallic medical implants, aneurism clips, etc. Talk to the safety officer responsible for this magnet first.
     
  8. Borek

    Staff: Mentor

  9. marcusl

    marcusl 2,100
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    Gold Member

    Thanks Borek, that's a perfect demonstration!
     
  10. Borek

    Staff: Mentor

    To be precise, that's Al. Not that it matters.
     
  11. DaleSpam

    Staff: Mentor

    When I give tours of our MRI systems I usually demonstrate this using an aluminum sign. You want something flat so that you can get big eddy current loops, so an ingot is not ideal. You can push it along the field lines quite easily, but making it twist is very slow.
     
  12. http://www.youtube.com/watch?v=30oPZO_z7-4
    If you hung onto the magnet in the above video and let the copper tube fall, it should be pretty much the same effect as the aluminium falling in Boreks vid.
    So if you placed a big peice of copper into an MRI ,say in the centre of the machine it would probably float slowly to the base in a similar fashion.
     
  13. marcusl

    marcusl 2,100
    Science Advisor
    Gold Member

    Buckleymanor, that's not wrong. A piece of copper in the center of a uniform field will not float, it will fall as though there were no magnet present. The key is that translating a metallic object through a uniform field (such as that in the center of the MRI magnet) results in no flux change and no eddy currents. Rotating a flat plate, however, results in a large flux change. That's what is demonstrated in Borek's video.
     
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