Curie Point - When a piece of iron gets too hot, it is no longer attracted to a magnet

 

Curie Point

 

When a piece of iron gets too hot, it is no longer attracted to a magnet. 

 

A piece of iron will ordinarily 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 that they can't line up and create a magnetic field. Here is a simple demonstration of this effect. 

 

• A small magnet. (Radio Shack's disk magnets work fine.)
• A stand to hold the magnet pendulum and wire. (The stand can be easily made from Tinkertoys™ or pieces of wood.)
• One 6-volt lantern battery (or other 6-volt power supply).
• 2 electrical lead wires with alligator clips at both ends (available at Radio Shack).
• One 3-inch (8 cm) length of thin iron wire, obtainable by separating one strand from braided picture-hanging wire.
• String, about 1 foot (30 cm) long.
• Adult help.

(15 minutes or less)

Make a stand from Tinkertoys™ or other wood as shown in the diagrams. Suspend the magnet from the top of the stand with a string. Make a pendulum at least 4 inches (10 cm) long. Stretch the iron wire between two posts so that, at its closest, the wire is 1 inch (2.5 cm) from the magnet.

(15 minutes or more) Touch the magnet to the iron 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 iron wire, and touch the other clip lead to the iron wire on the opposite side of the magnet. Current will flow through the iron wire, causing the wire to heat up. (CAUTION: The wire will get hot!) As the iron heats up and begins to glow, the magnet will fall away from the wire. Take a clip lead away from the iron wire. Let the iron wire cool. When the iron 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. 
 

The iron wire is made of atoms that act like tiny magnets, each one having a north and south pole of its own. These iron atoms usually point in all different directions, so the iron has no net magnetic field. But when you hold a magnet up to the iron, the magnet makes the iron atoms line up. These lined-up atomic magnets turn the iron into a magnet. The iron is then attracted to the original magnet.

High temperatures can disturb this process of magnetization. Thermal energy makes the iron 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 iron loses its magnetism. The temperature at which this occurs is called the Curie point

Inside the earth, there is a core of molten iron. This iron is at a temperature above the Curie point and therefore can't 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.