Thermomagnetic Motor
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The following article was originally published in the journal Młody Technik (eng. Young Technician) (1/2015):
Curie Temperature
The name in the title is familiar to us all. But this time, it’s not about Marie Curie — it’s about her husband, Pierre Curie (Fig. 1).
Pierre Curie (1859–1906) was a distinguished physicist. He studied crystalline substances and, together with Marie, won the Nobel Prize in Physics in 1903. Among his many contributions, he investigated the effect of temperature on the magnetic properties of materials. He discovered that ferromagnetic materials lose their magnetic properties at a specific temperature, which is unique to each substance. This temperature is now known as the Curie temperature (or Curie point).
The existence of the Curie temperature makes it possible to build a simple thermo-magnetic engine.
To understand how it works, we must recall that materials can be classified by their magnetic properties into:
- Ferromagnetic – strongly attracted to magnets
- Paramagnetic – weakly attracted to magnets
- Diamagnetic – weakly repelled by magnets
Materials that are ferromagnetic will become paramagnetic once their temperature rises above the Curie point.
Building the Engine
You will need:
- a permanent magnet
- a small piece of ferrite (e.g. a fragment of a ferrite magnet)
- a copper wire with a diameter of 0.4–1 mm (about 0.016–0.039 in)
- a candle or an alcohol burner as a heat source
Bend a piece of wire into a pendulum shape and attach the ferrite piece to its end. Suspend the pendulum so that it can swing freely. Then gently bring the permanent magnet close to the ferrite fragment — at some point, the magnet will attract the ferrite, thus pulling the pendulum out of its equilibrium position. Now place a lit candle under the pendulum so that the flame heats the ferrite near the magnet. The experimental setup is shown in Fig. 2.
Initially, the ferrite is ferromagnetic at room temperature, so it’s attracted to the magnet and pulls the pendulum to one side. After a short while, as the ferrite heats up, oscillations begin: the pendulum swings down to its vertical position, then is pulled toward the magnet again. By adjusting the heat intensity (distance from the flame), the length of the pendulum, and the distance to the magnet, you can vary the oscillation frequency over a wide range.
So what explains this behavior? Ferrite has a relatively low Curie temperature — around 200–300°C (392–572°F). Once heated to that temperature, it becomes paramagnetic, and the magnetic force decreases. The pendulum then returns to the center. In that position, the flame no longer heats the ferrite, so it cools. As the temperature drops below the Curie point, the ferrite regains its ferromagnetic properties, and the pendulum swings back — completing the cycle.
But why does ferromagnetism disappear at a certain temperature? Below the Curie temperature, atomic or molecular magnetic dipoles are aligned in the same direction due to chemical bonding, forming magnetic domains. Above the Curie temperature, thermal vibrations disrupt this alignment and the dipoles become disordered.
Through continuous thermal energy input and the resulting magnetic property changes in the ferrite, the pendulum produces periodic motion. It’s a genuine heat engine — converting thermal energy into mechanical work via magnetic interaction.
References:
- Joint Publication, Encyklopedia Techniki – Elektronika, Wydawnictwa Naukowo Techniczne, Warszawa 1983, ISBN 83-204-0198-4
- Hassa R., Mrzigod J., Nowakowski J, Podręczny słownik chemiczny, Videograf II, Katowicem 2004, p. 79, ISBN 8371832400
- Chikazumi S., Physics of ferromagnetism, Oxford University Press, 2009, p. 118, ISBN 9780199564811
All photographs and illustrations were created by the author.
Addendum
The effect of this experiment can be seen in the following video:
Marek Ples