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Why Molten Iron Just Isn't Attracted to Rare Earth Magnets

Release time:2017-04-18 09:26     Author:Nature

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Keywords: Rare Earth Magnets

 The Backyard Scientist is usually good for a nice bit of spectacle, whether it's shooting potassium into a tank
full of water to turning gasoline-filled tennis balls into bouncing fireballs.
 
For his latest experiment, he got his hands on a large rare earth magnet, and then used thermite to melt down
iron to pour down on and near the magnet. The results? Less than exciting. The molten iron doesn't react to the
magnet—it's only when the iron has cooled that anything seems to happen.
 
So what's going on here? The answer is what's known as the Curie point. To greatly simplify, iron is attracted to
magnets because the atoms in a solid chunk of iron have a stable magnetic spin. This means when a magnetic
field is applied, the magnetic poles of all the atoms are aligned in the same direction, creating magnetic attraction.
 
But when iron hits its Curie point (1,043° Kelvin, or 1,417.73° Fahrenheit), the atoms are now moving around
quickly enough that their magnetic spin is no longer stable—the magnetic poles are swinging around every which
way. It should be noted this happens well before the iron actually enters its liquid state—iron's melting point is
2,800° F. But ironworkers will actually use this the Curie point to test when the iron is ready to be molded—once it
stops reacting to a magnet, you have a very good sense of how hot it is.
 
Regardless, the Curie point is why when a stream of molten iron is poured directly next to a magnet with tremendous
amounts of pull, the iron falls in a straight line instead of being pulled towards the magnet. But once the molten magnet
cools back down below the Curie point, the magnet has no problem picking up the random blobs of metal.
 
So grab some thermite and iron and lay down a rare earth magnet in your backyard if you want to, but all you're gonna
get is mainly some scorched grass and one kinda junked-up magnet.


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