Spinning magnetic coins
A simple, clear demonstration of the effect of magnetization!
- 0.33L cans;
- neodymium magnet;
- iron coins
Construct a bridge: suspend a ruler between two supports. Set a magnet on top of the ruler. Suspend some coins by the neodymium magnet under the “bridge,” forming a coin chain. Blow on the lowest coin through a straw. Notice how fast the coin rotates. Modify this experiment by constructing a tower of coins balanced on a can, attracted to the magnet on the ruler above. Now you can spin the whole tower!
Ferromagnetic materials, such as iron or nickel, can become magnetized if placed in an external magnetic field. We used coins made of steel, an alloy consisting of mostly iron, a small amount of carbon, and some additives. When these coins encounter an external magnetic field, they become magnets themselves and are therefore attracted to each other, able to form a chain or tower. In the chain, the strong neodymium magnet at the top counteracts the force of gravity; further, the magnet allows the tower to maintain a position of equilibrium so that the entire stack of coins doesn't tip over. But the magnet can only support a certain number of coins. Magnetic forces weaken with increasing distance. Each additional coin is magnetized weaker than the last, and consequently, the hanging chain can't be infinite. The attractive forces on the lowest coin are very weak – it’s practically hanging in the air. This allows it to rotate rapidly, relatively unhindered by friction. This principle also applies in the case of the coin tower – the magnet on the ruler attracts the topmost coin, which attracts the next coin, and so on. The bottom coin relies minimally on the can supporting it, and consequently, the frictional forces there are reduced. The small contact area and the low reaction force allow the coin to rotate practically freely. Such a seemingly simple and visual experiment allows us to explore many fundamental properties of magnets and ferromagnetic materials.