Curie Point of Iron

Location:

Concepts Shown:

Magnetic materials. To demonstrate that when metals are normally attracted to a magnet, they can be heated to a temperature (Curie point) where they

Equipment:

small house magnet (hole in middle, Radio Shack), stand to hold magnet pendulum and wire, alligator clip leads, and extension cord, 6" (+) length of thin iron wire (braided picture hanging wire), string (1-2'), stand to stretch wire across, power source (Variac Variable Transformer, or large lantern battery).

Procedure:

Construction:

Make a stand to stretch the wire across. This can be made out of tinkertoys or wood. Build a stand to hang the magnet from (about 1' to 2' high).

[eq].

Use a small block to give stability to your post. Stretch the wire between the small stand, use staples to hold it down, or nail posts to wrap it around.

Make a small loop of wire and run it through the hole in the magnet. Connect the string to the loop and to the post. Cut the female end off an extension cord, and attach the alligator clips to the end of the extension cord.

For above picture use the following pieces of wood: 2'6" post, 12"x12" Base, 2"x2"x4" support, and 6"x2"x4" stand for wire. Presentation: Connect the alligator clips to the wire and plus the other end into your power source (if using a battery, connect magnet first). Now touch the magnet to the braided wire. Plug the power source in and let the magnet fall away. (With Variac, turn it up until the wire just glows red.) Turn off power, and let the wire cool. Touch the magnet to the wire again, and it will stick.

Science:

Metals have a transition temperature, called the Curie point (Tc), at which the magnetic properties are drastically changed. For iron, this temperature is 770 C, above which Fe begins to transform from BCC to FCC. The low-temperature BCC iron is ferromagnetic. When an external (in this case from the house magnet) magnetic field is applied to the iron, it produces a spontaneous alignment of the spins of electrons in the valence bands, which in turn produce a net magnetic moment, M, inside the iron.  Below the Curie point, this magnetic moment in the material is self-stabilizing and it persists even when the external magnetic field is removed.

As the iron wire heats above 770 C, it begins transforming to FCC Fe, with the iron becoming completely FCC at 910 C. FCC iron is paramagnetic, meaning it has no net magnetic moment until an external field is applied to align the electron spins in the material.  A paramagnetic material can be weakly attracted to an external magnet due to this alignment.  However, in this experiment the high-temperature thermal vibrations of the atoms in the FCC Fe act to randomize the spins of the electrons in the iron and so it will not be attracted to the external magnet.

Other:

If your wire is not hot enough, move the alligator leads closer together. Or if your magnet is not sticking at all, use more wire, but this may require a larger power source.

References:

Author:
Credits:


Related Equipment
Related Supplies