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If you melt a magnet, what happens to the magnetism? Does the liquid metal retain the magnetism or does it go away?

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Questa volta abbiamo cercato una curiosità scientifica: If you melt a magnet, what happens to the magnetism? Does the liquid metal retain the magnetism or does it go away?

Ed ecco le risposte degli esperti:

Ferromagnetism (what most people think of as “magnetic” is a property of solids. A liquid cant be magnetized in the same way.

Metals are made up of a bunch of tiny little crystals. Each crystal is a miniature magnet. A permanent magnet has been treated in such a way that most of the mini magnets are all aligned in one direction, causing the larger chunk of metal to act as a magnet.

When metal melts, it stops having crystals. Even if the crystals survived in the liquid form, they would be able to move around and rotate, so that it would stop being magnetic very quickly even if it started all lined up.

In fact, metals stop being magnetic before they melt, due to a change in the type of crystals within the metal as it heats up. The shift in magnetism is called the “Curie Temperature”, and will be different for different types of metals.

Blacksmiths use the Curie Temperature to estimate the temperature of steel as they heat it. They will touch the steel with a magnet to see if it is still attracted. When the magnet stops attracting the steel, they know it has reached a certain temperature.

Sometime before it melts, the Curie temperature will be exceeded and it’ll lose its ability to retain a magnetization in the absence of an external field.

All molecules in matter have charges, it’s just they’re all facing different ways. In a magnetic material, the crystal lattice of the material means that all of the molecules line up in exactly the same way. This is what creates the perceptible magnetic field. When you heat the magnet (or even if you drop it), these molecules will start to become unaligned with each other and you will lose the property.

You can actually track the history of Earth’s magnetic poles (north and south) by studying the magnetism in rocks formed at various times and places, whether the rock crystallized slowly deep in the earth, or formed rapidly at the surface. Cool stuff.

Tempering steel, you can heat it till it loses its magnetism and then quench it. That keeps it from forming crystals large enough to weaken the metal. I think that’s important in any iron components of airplanes, and of course ships, because you don’t want substantial magnetism interfering with navigation devices (compasses in particular) or inducing current in moving parts that aren’t made for that.

They stop being magnetic

There are some cool videos floating around of a magnet hanging from a wire that is stuck, at an angle, to a metal pole that is off to the side.

The magnetic is blowtorched and turns red hot. at some point it just swings free from the pole as it loses its ability to be magnetic.