Effect of Temperature on Magnetization

This is an eye catching demonstration which clearly shows how the property of a ferromagnetic material is affected by temperature.

You need a bar magnet, sewing needle, plastic stand, a small wooden or plastic platform, thread and candle.

Procedure

1. Tie the needle to a thread and suspend it from the top of a plastic stand.
2. Keep the bar magnet at the edge of a small platform and bring this arrangement near the stand.
3. See that the needle gets attracted towards the magnet and the string makes a certain angle with the vertical stand.
4. Now heat the needle at the end close to the magnet by using the candle. Do not the touch candle to the needle.
5. See that after a few seconds the needle is no longer attracted to the magnet and the string becomes vertical. Why?

Discussion

The needle made of iron is a ferromagnetic material. A ferromagnetic material has magnetic domains which are many magnetic dipoles aligned in the same direction. In the absence of an external magnetic field, these magnetic domains are randomly oriented and their net magnetic field is zero. But in the presence of external field (here field due to the bar magnet) there is a realignment of these domains and as a result the ferromagnetic material is strongly attracted to a magnet.

Increase in the temperature of the needle above a certain value results in the breakdown of these domains and the needle loses its ferromagnetic character. This happens at a characteristic temperature called the Curie temperature.

If the needle is cooled below the Curie temperature, it again starts showing a ferromagnetic character and gets attracted to the magnet. The string makes an angle with the stand. Horizontal and vertical components of the tension in the string balance the force of attraction and the weight of the needle respectively at equilibrium.

Variant: Pass current through heater coil. The temperature of heater coil increases and it looses its ferromagnetic properties. At this time, the magnet falls down. We developed a small set up that operates at 6-12 V. Made stand using a 4 in by 6 in piece of plywood. A 30 cm scale was fixed on one end to hold thread attached to the magnet. A 15 cm scale was cut into two pieces and these pieces are attached on the side of stand. This become holder for heating element. The heating element is a 3 cm piece of 1000 watt heater coil. Other things can be fine tuned.