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Curie Point
When a piece of iron gets too hot, it loses its attraction to a magnet.
A piece of iron ordinarily will be attracted to a magnet, but when you heat the iron to a high enough temperature (called the Curie point), it loses its ability to be magnetized. Heat energy scrambles the iron atoms so they can’t line up and create a magnetic field—this Snack is a simple demonstration of this effect.
CAUTION: Electricity can really heat things up. The wires can get really hot when you're doing this Snack. Be careful.
Tools and Materials
- A small magnet such as a ceramic disk magnet
- String, about 1 foot (30 cm) long
- A 3-inch (8-cm) length of thin steel wire, obtained by separating one strand from ordinary braided galvanized picture-hanging wire
- A stand to hold the magnet pendulum and wire (examples shown are made from PVC, a plastic cup, and wooden sticks, but feel free to improvise using available materials of your choice)
- Two alligator-clip leads
- One 6-volt lantern battery (or other 6-volt power supply)
- Tape
A note on materials: Braided copper wire and aluminum wire are available, but will not work here; iron wire can work but is not commonly available. Whatever you use, be sure to stay away from plastic-coated wire, which can burn if it gets hot.
Assembly
- Make suitable stands either as shown in the photos above or of your own design.
- Suspend the magnet from the top of the stand with a string. Make the pendulum at least 4 in (10 cm) long.
- Stretch the wire between two posts so that, at its closest, the wire is 1 in (2.5 cm) from the magnet.
To Do and Notice
Touch the magnet to the wire. It should magnetically attract and stick to the wire.
Connect the clip leads to the terminals of the lantern battery. Connect one clip lead to one side of the wire, and touch the other clip lead to the wire on the opposite side of the magnet. Current will flow through the wire, causing it to heat up. Be careful—the wire will get hot! As the wire heats up and begins to glow, the magnet will fall away from the wire.
Take a clip lead away from the wire and let the wire cool. When the wire is cool, notice that the magnet will stick to it once again.
If the wire does not heat up enough to glow red, move the clip leads closer together.
What’s Going On?
Steel wire is made of atoms that act like tiny magnets, each of which has a north and south pole of its own. These atoms usually point in all different directions, so the steel has no net magnetic field. But when you hold a magnet up to the wire, the magnet makes the steel atoms line up. These lined-up atomic magnets turn the steel wire into a magnet. The steel is then attracted to the original magnet.
High temperatures can disturb this process of magnetization. Thermal energy makes the steel atoms jiggle back and forth, disturbing their magnetic alignment. When the vibration of the atoms becomes too great, the atomic magnets do not line up as well, and the steel loses its magnetism. The temperature at which this occurs is called the Curie point.
Going Further
Inside the earth, there is a core of molten iron. This iron is at a temperature above the Curie point and therefore cannot be magnetized. Yet the earth is magnetized, with a north and a south magnetic pole. The magnetic field of the earth comes from an electromagnet—that is, from electrical currents flowing inside the liquid metal core.
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This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Attribution: Exploratorium Teacher Institute
