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Topic: Metal not be attractive towards magnet and not allow magnetic fields to pass?  (Read 5898 times)

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Offline tallatghazi

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Hi,

Can anyone help me in finding out a Diamagnetic or Paramagnetism or Ferromagnetism or any other metal which has following capabilities:

1- Should not be attractive towards magnet.

2- Should not allow magnetic fields to pass through inside.

Thanks!

THG

Offline Enthalpy

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Welcome, tallatghazi!

Not attract magnets means, in a first approximation, not ferromagnetic.
Then, some peple distinguish ferrimagnetism from ferromagnetism, but the result is the same.
Dia- and para- magnetic materials still react to magnets with a tiny force, but usually it's negligible. Antiferro- needs cold.

https://en.wikipedia.org/wiki/Ferromagnetism
https://en.wikipedia.org/wiki/Ferrimagnetism
https://en.wikipedia.org/wiki/Paramagnetism
https://en.wikipedia.org/wiki/Diamagnetism
https://en.wikipedia.org/wiki/Antiferromagnetism

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"Not pass through" depends on what you mean.

If the field shall pass as if the metal were not there, this means para- or dia- magnetic metals, and exclude the ferromagnetic (and ferri-) ones. So this is the same constraint as "not attract magnets".

But if you want an object to repel the magnetic field, then only the type I superconductors do that. They need cold: liquid helium or, for some materials, liquid nitrogen, and the repelled induction can't exceed some 10mT depending on the material. Some superconductors (type II) work at a higher induction but they don't repel the field.
https://en.wikipedia.org/wiki/Superconductivity
https://en.wikipedia.org/wiki/Meissner_effect

By the way, repelling the field would inevitably let push the magnet away.

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Most metals are para- or dia- magnetic.  All common aluminium alloys, copper alloys, titanium alloys... Some iron alloys are not ferromagnetic because that's a molecular property, not an atomic one: for instance austenitic stainless steel, among which the 17-12 variant stays non-magnetic even after deformation.

Other solutions exist besides the material. While a few Soviet submarines were built of titanium alloy, most submarines are made of steel, and to avoid deforming Earth's magnetic field hence being detected, they have big coils where the current is adjusted to let pass the same magnetic flux as if the submarine were not there.

Offline tallatghazi

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Please find the attached image.
I think this image will more clear you, what I am actually looking for.

Offline Enthalpy

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Then, you really need a shield against the permanent magnetic field.

A first possibility is a ferromagnetic shield, thick and permeable enough to short-circuit the flux created by each magnet, but:
  • The shield will attract each magnet, like a piece of iron does. Is that better than a force between the magnets?

A second possibility type I superconductors that repels the field.
  • You need cold. Liquid helium or nitrogen.
  • You need a superconductor. Some are expensive and brittle, especially those for liquid nitrogen, but they can often be deposited as thin film.
  • The maximum repelled induction is small: a few 10mT. Type II superconductors work as type I under small induction.
  • The superconductor will repel each magnet. Is that better than a force between the magnets?

One could also imagine an active solution, where currents pass through metal coils to compensate each magnet's field.
  • It needs power. Often more than is realistic.
  • I may well need cooling. Often more than is realistic. But around room temperature.
  • It needs many coils, sensors and control electronics.
  • This will repel each magnet too.

Offline Irlanur

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I think the problem lies a bit deeper here.

Your picture says "Magnetic fields of both sides should not interact with each other."

This sentence is at least ill-defined. You should be able to state this in a physically meaningful way.

Offline tallatghazi

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Hi,
Please allow me to ask in an extremely simple words and situation.
With a help of any single or combination of materials (shield):
1- Both magnets' SOUTH poles should not repel each other, when they will be brought near.
2- Neither any of the magnet should be attracted towards the shield.

I hope this will be simple :)

Offline Arkcon

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It is a simple explanation.


It is also impossible, in that it utterly ignores basic physics.

Enthalpy: Was kind enough to give you some specifics and Irlanur: Asked you to build on basic physics, but there is no thin, "magnetic field" barrier substance.  Because the underlying physics simply doesn't work that way.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline Enthalpy

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A passive shield... If it doesn't attract the magnets, it will repel them - that would be the superconducting shield, which needs cold and works for weak fields only.

Without attracting nor repelling, I firmly believe there is no solution with passive shields.

---------- End of the simple answer

With seriously complicated active means, I believe to see a possibility. Put a ferromagnetic material to short-circuit the flux of both magnets, add many coils over each face of the ferromagnetic wall, plus sensors and electronics, and try to adjust the currents to impose everywhere at one side of the ferromagnetic short-circuit an induction as if there were only the magnet alone.

Might perhaps work, with a heap of electronics and software. Not what was asked.

Offline tallatghazi

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Dear All,
Thanks for your kind solutions and advices. Based on these advices, I am going with a solution, kindly give your thoughts on following:

Attaching one IRON shield with south pole of first magnet (this shield will BLOCK Magnetic fields of this magnet).
Now on other side of this iron shield, I can attach another shield (layer) of Pyroltic Carbon Graphite or Bismuth with the iron shield (this will NEUTRALIZE the magnetic fields of second magnet). And like this both magnets will behave independently without interacting with each other.  WILL THIS BE A CORRECT APPROACH?

Offline Arkcon

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Attaching one IRON shield with south pole of first magnet (this shield will BLOCK Magnetic fields of this magnet).
Now on other side of this iron shield, I can attach another shield (layer) of Pyroltic Carbon Graphite or Bismuth with the iron shield (this will NEUTRALIZE the magnetic fields of second magnet).

OK, I have a question.  If an iron shield will work, why do you need carbon or bismuth.  And if you need carbon or bismuth, why do you think iron shield will work?

Quote
And like this both magnets will behave independently without interacting with each other. 

WILL THIS BE A CORRECT APPROACH?

No.  I don't think it will work at all.  Why do you believe so?
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline tallatghazi

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@Arkcon
Using 2 layers will do the job better (I think)
First Iron shield will control/block the magnetic fields of 1st magnet.
But now 2nd Magnet will be attracted towards this Iron shield.
So, second Bismuth shield should be added (between 2nd magnet & Iron shield) to balance/neutralized the magnetic filed of attraction of 2nd magnet towards Iron shield.

hope this gives required results.

Offline Arkcon

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« Last Edit: May 07, 2018, 12:24:22 PM by Arkcon »
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline Enthalpy

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[...] second Bismuth shield should be added (between 2nd magnet & Iron shield) to balance/neutralized the magnetic filed of attraction of 2nd magnet towards Iron shield. [...]
You can forget that. It won't work. Graphite and bismuth have nearly zero effect on the field.
And, no, there is no solution that works as you hope.

But if you accept to attach iron to the permanent magnets, yes. You can have soft iron attached to each magnet, and with proper shape, the magnet+iron compounds will create very little induction on that side, so they attract an other very little. That's how electric motors are designed: despite comprising huge electromagnets, they create very little induction outside their casing.

Offline Enthalpy

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Pictures there of an (electro-) magnet that creates a field (and a force) at its front side only
http://www.first4magnets.com/other-c89/40mm-dia-x-20mm-thick-electromagnet-with-m5-mounting-hole-25kg-pull-8w-0-33a-p9715#ps_1-9985
it produces very little induction at its back side, especially if the iron is thick and magnetically soft.

Two such magnets wouldn't attract an other from their back sides, especially if optimized for that and if not very close to an other. But whether this design is possible depends on the primary use of the magnets.

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