I have a question about magnetism

[F Gump]

I was wondering if anyone could help me with a question. I am a long time knife collector, and after a block of steel is formed it gets demagnetized...after the steel block is then ground into shape and finally sharpened on a grinder, it some how becomes slighty magnetic. Knife makers say its from grinding and sharpening. Can someone explane how that would induce magnetism? Thanks Forest.

 

[clive]

Hi Forest

You probably know that steel contains Iron i.e. a ferromagnetic material. In a ferromagnetic material, some spins (especially 3d) are well aligned in certain directions. But in a big sample (like a steel knife for example) the total energy would be too big if all spins are aligned in the same direction. So, inside the sample will nucleate some regions with spins aligned on different/oposite direction (ferromagnetic domains) in order to minimize the total energy of the sample. So the "magnetism" is already inside the sample but it is self-compensated (as you have many small magnets with random orientations). Under an external influence (magnetic field, mechanical tensions, etc.) the size of some ferrmagnetic domains can change and accordingly the sample get a non-zero magnetic moment.

By grinding and sharpening it is possible to induce some modification in the ferromagnetic structure of the knife and it becomes "magnetic".

 

[Quarkycharm]

Yes, you can produce a magnet from steel by stroking its surface with a magnet or ferrous material. This will allign the domains of surface - they were first randomly ordered - in one direction producing an ordered magnetic field around the steel knife.

 

[Chi Meson]

Does the magnetic field of the Earth have anything to do with the magnetic moment the knives get? Do the knives get random polarization, or are all tips north-seeking etc?

 

[Antiphon]

Non-magentic alloys like stainless steel can become magnetic.

If they are subjected to heating for example, the "stainless steel
molecule" can locally break down into the magnetic iron components
plus other non-magnetic metals. This goes along with a gross change
in the other properties of the alloy, such as it may rust after that
(no longer "stainless" steel.)

Edit: I was assuming that the knives were stainless. If not, I apologize
for hitting the thread.

 

[F Gump]

Yes they are stainless, but they get sharpened on a stone which is not steel. Yet somehow they become slightly magnetic. Now I am thinking maybe its where i leave them , ie...near my cell phone, on top of the tv..etc...thanks for any help guys.