A single magnet between two sheets of graphite makes an intriguing demonstration of levitation.
If asked which materials are magnetic, the standard answer would be Iron, Cobalt and Nickel. As a matter of fact, those are the only three elements on the periodic table that exhibit magnetic properties. We don’t normally consider materials such as wood to be magnetic. However, they do exhibit another property – diamagnetism, but very weakly.
Under a very strong magnetic field, a diamagnet will repel and be repelled by the field. They are able to do this by slightly tweaking the orbits of their electrons so that the electrons always oppose the magnetic field. These forces are extremely weak. By “weak”, I’m talking millions of times weaker than the forces a magnet can exert on objects. However, under a very carefully set up set of conditions, you can get it to exhibit levitation, which is what this assembly does.
The magnet I used in this demonstration is an N52 NIB (Neodymium Iron Boron) magnet. These magnets are different to the regular black ceramic magnets than you see almost everywhere and lie under their own class: Rare-earth magnets. They are extremely powerful and handling them unsafely can lead to serious injury. The manufacturing process of these magnets is very extensive, but briefly:
- They gather the raw materials and place them in a vacuum furnace. The furnace melts the raw materials to produce the main ingredient
- They then mill the main ingredient into a very fine powder in preparation for the next step
- Then, they press this powder under high temperature and pressure until the particles stick to each other and form a single solid
- They then trim the solid down to the desired shape and size
- They plate the to-be magnet with three layers: nickel, copper and then nickel again. This protects the material from corrosion, which can make it lose its magnetic properties in the presence of water
- Then, they place the to-be magnet inside a huge coil, which they switch on in a small pulse. This exposes the to-be magnet to a very powerful magnetic field, which magnetizes it. A magnet is born!
The magnets are generally graded on an Nxx scale, where xx’s are a number between 35 and 52, with 52 being the strongest.
So, let’s talk about the levitation assembly now. The assembly contains two sheets of pyrolytic graphite. This material exhibits strong diamagnetic properties. The goal is to cause the floater magnet to levitate above the bottom sheet. Since we are using a strong magnet and a diamagnetic material, the floater magnet experiences a small upwards force. But there is a problem: this force is not strong enough to overcome the weight of the magnet itself.
To solve this problem, we use a lifter magnet. This is connected to the screw that you see at the top of the set up. The use of this magnet is to solely counter the force of gravity so that the effect of the upwards force from the bottom sheet of graphite is visible, causing the magnet to float. So, why the upper sheet? It simply adds stability. As the floater magnet rises and moves towards the upper sheet, it experiences a downwards force from the graphite on the top sheet, stabilizing the magnet in between the two sheets. A diagram is below:
It does take a little effort to get the balance just right. Usually, the magnet would either end up falling to the bottom or sticking to the top, so careful fine-tuning of the lifter magnet (and a lot of patience!) is critical to getting the demonstration to work as expected. But, once it is set up, it looks great! The magnet just floats there and can even be set spinning by blowing air through a straw.