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Problem:

Take some dust particles whose size is big. Bring a magnet near them. They do not get attracted to the magnet and thus we conclude that they are non-magnetic. However, when you grind the particles to the smallest particle size you can go to (any practical size such as .5mm or .2mm) and then bring a magnet near it - the particles get attracted to the magnet!

Conclusion: The particles are, in-fact, magnetic

Why does this happen?

Is it possible that the dust particles have become magnetic with the decrease in size due to their para-magnetic property taking over (takes over because as the particle size decreases, there are less chances of the dipoles being cancelled in the material, since there are less dipoles in the material. So the dipoles can align in one direction more easily than in a bigger material)? (My Explanation)

I did find a link on the net related to this, however it was not that helpful: Non-Magnetic Aluminium Particles get attracted to magnet when they are cut into smaller pieces!

Please help! Thanks a lot!

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Conclusion: The particles are, in-fact, magnetic

This is doubtful. There are two possibilities here:

  1. If the objects used to grind (let's call them grinders) the dust grains were made of a ferromagnetic material like iron/steel and/or nickel, then it is possible that the grinders deposited some material onto the dust. There is no reason to think that breaking dust down using physical force would induce a net magnetization.
  2. The act of grinding the dust induced a static electric charge on the dust grains, similar to what happens through the triboelectric effect. If the dust grain has a net charge, then it can respond to a magnetic field.

Is it possible that the dust particles have become magnetic with the decrease in size due to their para-magnetic property taking over

I highly doubt this (though I would need more specifics to confirm). Magnetic domains in a ferromagnetic materials tend to be on the order of ~0.001-0.010 mm, which is still 50 times smaller than your small dust grains. This is ignoring the fact that most dust is made of some sort of silicate compound, which are generally not magnetic materials unless they include a magnetic material like magnetite. So unless the dust itself had some intrinsic mangetic properties, the more likely answer is one of the two possibilities I listed above.

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  • $\begingroup$ Why/ How would a charge respond to magnetic field? $\endgroup$ – Tejit Pabari Jul 1 '16 at 3:51
  • $\begingroup$ Ultimately, it's the Lorentz force... $\endgroup$ – honeste_vivere Jul 1 '16 at 16:49
  • $\begingroup$ Isn't Lorentz force only in the case of a moving charge? $\endgroup$ – Tejit Pabari Jul 1 '16 at 16:54
  • $\begingroup$ For single particle motion yes, but it has slightly different forms for currents and/or inductive fields. I suppose if ferromagnetic material had been deposited on the dust during the grinding, then the force would be something like $$\mathbf{F} = - \nabla \left( \boldsymbol{\mu} \cdot \mathbf{B} \right)$$... $\endgroup$ – honeste_vivere Jul 1 '16 at 16:58
  • $\begingroup$ Thnx for the answer. Is there any way to see the formula you have written? $\endgroup$ – Tejit Pabari Jul 1 '16 at 17:01

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