Frying pans and Levitation
Plastic tray Thick-bottomed aluminium frying pan Very strong magnet Liquid nitrogen
1 - Hold the plastic tray a 45 degree angle and slide the magnet down the side.
2 - Take the aluminium frying pan and slide the magnet down the underside of the frying pan. Compare the speed of the magnet between the plastic tray and the frying pan.
3 - Cool the aluminium frying pan down to minus 196 degrees centigrade by pouring in some liquid nitrogen. Repeat step 2 and compare the speed of the magnet with the frying pan at room temperature.
If you compare the magnet sliding down the tray and the frying pan, you'll notice that it moves much more slowly down that pan.
The big difference between the aluminium and the plastic is that the aluminium is a metal and can carry a current, whereas the plastic tray carries no current at all because it's an insulator. When you move a magnet past any metal, you will start a current flowing round it. This is very important for generating electricity because all electrical generation is done by moving magnets past coils, making voltages and currents that we use in our home. In the aluminium pan we're not doing anything as useful as that, but nonetheless, the moving magnet is making currents flow in the aluminium.
The currents are trying to hold the magnet up and if they kept on flowing, the magnet would stay suspended over the aluminium. But we saw that the magnet eventually falls off the frying pan. This is because the currents are dying away and allowing the magnet to fall. The plastic tray doesn't carry any electrical current and so it can't stop the magnet from sliding down.
The next step was to make the frying pan very cold with liquid nitrogen, and you should have seen the magnet sliding down very very slowly. By cooling down the frying pan, the atoms in the aluminium can only move about slowly. Slow moving atoms are less likely to get in the way of the electrons moving current through the aluminium. This means that the pan has a lower electrical resistance, and the currents flowing round the pan die away more slowly. Consequently, the magnet slides down the pan more slowly.
But what would happen if we managed to get rid of all the electrical resistance so that the pan conducts electricity perfectly? The answer is that the magnet would stay where it is. Unfortunately, it is not possible to eliminate all the resistance from an aluminium frying pan, but it is possible with a material called yttrium barium copper oxide. This is a ceramic that goes superconducting at the temperature of liquid air. The word superconducting means that when you start a current flowing around a material, it never stops. If the current never stops then the magnet will never fall.
Ted showed us an example of this, and when he lowered a magnet down towards the superconducting material, the magnet started to levitate! Why doesn't it drop onto the superconductor? Just like the aluminium, as the magnet is moved towards the superconductor, it started currents flowing around it. But these currents chasing their tails in little whirlpools underneath the magnet don't stop because in a superconductor there's nothing to stop them. This means that the magnet levitates, and it will continue to do so for as long as we keep the superconductor cold.