Will a Copper Ingot Float in a 3.4 T MRI Machine?

  • Context: Undergrad 
  • Thread starter Thread starter Milarepa108
  • Start date Start date
  • Tags Tags
    Copper Machine Mri
Click For Summary

Discussion Overview

The discussion revolves around the behavior of a copper ingot placed in the bore of a 3.4 T MRI machine, exploring whether it would float or resist motion. Participants consider the implications of magnetic fields, eddy currents, and the properties of copper in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that copper is not a magnetic material and would not float in the MRI machine, suggesting that it would behave as if there were no magnet present.
  • Others propose that eddy currents could be induced in copper, potentially leading to resistance when the ingot is moved through the magnetic field.
  • A participant mentions that the speed at which the copper ingot is inserted into the magnetic field could affect heating due to induced currents.
  • One participant suggests that a flat aluminum disk would be more effective for demonstrating eddy currents than a heavy copper ingot.
  • Another participant emphasizes that translating a metallic object through a uniform magnetic field does not induce eddy currents, while rotating it does.
  • Some participants reference personal experiences with high-field magnets and the materials used in their construction, noting that copper is often employed for its thermal conductivity.

Areas of Agreement / Disagreement

Participants express differing views on whether a copper ingot would float or fall in the MRI machine, with no consensus reached. The discussion includes multiple competing perspectives on the effects of eddy currents and the behavior of copper in magnetic fields.

Contextual Notes

Participants highlight the importance of the geometry of the object (e.g., flat versus cylindrical) and the nature of the magnetic field in determining the behavior of the copper ingot, indicating that assumptions about these factors are critical to the discussion.

Milarepa108
Messages
3
Reaction score
0
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?


http://exs.exploratorium.edu/wp-content/uploads/2009/09/600-flo-300x225.jpg
 
Physics news on Phys.org
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 definitely 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.
 
f95toli said:
Nothing would happen.
Or would it?

f95toli said:
Copper is not a magnetic material.
I'm aware of that.


f95toli said:
The only thing you need to watch out for when using non-magnetic metals is eddy currents.
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?
 
f95toli said:
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).

OK so we all have huge magnetic fields, who has a big piece of copper to wield around?
 
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.
 
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.
 
Thanks Borek, that's a perfect demonstration!
 
To be precise, that's Al. Not that it matters.
 
  • #10
Milarepa108 said:
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?
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.
 
  • #11
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 piece of copper into an MRI ,say in the centre of the machine it would probably float slowly to the base in a similar fashion.
 
  • #12
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.