Magnestism Question

[HippocratesX]

Ok, so I was watching Myth Busters last nite and they were testing some myth about 2 hammer heads being slammed into eachother with the strongest possible human force and seeing if the heads explode on impact...or something like that.

And they had a part in there where the metal was heated to very high temperatures, at which point a magnet would not work on the metal. Why is this? What does the heat have to do with de-magnetizing the metal? Can anybody explain this? Thx

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[dr.kicia]

Ok, so I was watching Myth Busters last nite and they were testing some myth about 2 hammer heads being slammed into eachother with the strongest possible human force and seeing if the heads explode on impact...or something like that.

And they had a part in there where the metal was heated to very high temperatures, at which point a magnet would not work on the metal. Why is this? What does the heat have to do with de-magnetizing the metal? Can anybody explain this? Thx

metal has little domains inside of it. If you align them(by puting the metal in magnetic field) it will become a little magnet itself. You might have noticed that a metal after contact with magnet becomes a little magnet itself (ever stick a magnet into a bag of nails and then noticed how they will stick together even after magnet is removed?) Metal (iron, steal etc.) it is a very weak magnet because that domain alignemnt is not permanent. Heat destroys the abbility for metal to align its domains (heat is energy, electrons go crazy and if everyone goes wherever you cannot align) that is why the magnet stops working when you heat the metal.
Hope that helps.

 

[DiverDoc]

I could be wrong, but maybe the T exceeded the Curie temp, thereby making the metal paramagnetic. ?

 

[HippocratesX]

I could be wrong, but maybe the T exceeded the Curie temp, thereby making the metal paramagnetic. ?

I'm not sure if this is the case, because after heating the metal, they held up a regular u shaped ferromagnet up to the hot metal and it just dropped to the ground. so there was an externally applied magnet present. so would it still be paramagnetic? hmm...i dont know.

 

[CATallergy]

im not sure either, but I believe that when making magnets, the molten metal is held in a strong magnetic field while it reforms, thus allowing the domains mentioned by dr kicia to be fixed with the same orientation. when they orientations are all random, their effects cancel out and the metal will not have strong magnetic properties.

if anyone agrees with this, then pls explain how temporary magnetism work?

 

[dr.kicia]

it looks like i was right. This is from wiki:
A magnetic domain is a region in which the magnetic fields of atoms are grouped together and aligned. Magnetic domains are miniature magnets within a material. In an unmagnetized object, like an piece of metal which has just solidified from a liquid state, the different magnetic domains are pointing in random directions. When the metal becomes magnetized (which is what happens when it is rubbed with a strong magnet) all like magnetic poles are lined up and point in the same direction. The metal becomes a magnet. It becomes demagnetized when its magnetic domains are returned to a random order, for example by heating.

i am just trying to figure out why heating makes disalignment to happen. Heat=energy, excited electrons, everyone goes "bannanas" in all directions, big mess?
I acctually emailed my prof. to answear this question. I got really curious about this. I will post once i hear back from her

 

[dr.kicia]

ok i got a reply from my prof.
When metal is heated the atoms move to fast for domains to stay aligned. That is why the metal looses its magnetic properties when heated.
LOL gotta love mythbusters!