Superconductors and Magnetic Levitation

A superconductor is a substance that has zero electrical resistance. Usually this occurs at extremely low temperatures, but there is class of objects called high-temperature superconductors which achieve zero electrical resistance at easily achievable temperatures. The substance used in the video below, Yttrium barium copper oxide, acts as a superconductor (drops below its critical temperature) at temperatures below 95 K (-178°C), and was the first material that was found to be a superconductor at temperatures above the boiling point of nitrogen (77 K, -196°C). This is significant because instead of wasting large amounts of electricity to cool it down, it can be submerged in widely available liquid nitrogen to achieve superconductivity. Superconductivity is a very interesting property, and a current flowing in a superconductive object will continue to flow with no outside power source. The following is an experiment done by Professor Tom H. Johansen in the Superconductor Laboratory at the University of Oslo, Norway, in which a superconductor is immersed in liquid nitrogen which causes a magnet to float on top of it.

What actually causes the magnet to levitate is a phenomenon known as the Meissner-Ochsenfeld effect. When a magnet is brought near a superconductor a current is induced which repels the magnetic force due to its diamagnetic properties. A diagram of the force of the magnetic field (B) and current (I) is shown in the diagram to the right for temperatures above and below the critical temperature (Tc). Because the repulsive force is created because of the magnet on top, pushing it slightly to the side will cause it to be pulled back into place, but pushing it hard will cause it to move slightly and then reestablish equilibrium at a different point. As the superconductor heats up above its critical temperature, the magnet is gradually lowered and finally comes to rest on top of the object. This experiment is interesting, not to difficult to recreate, and very cool.

Magnetic levitation can be produced in many ways, and every object creates some diamagnetic resistance in a magnetic field, but for most objects the force is very small. With a strong enough electric field nearly any object can be levitated. Below is a video of a frog being levitated at the High Field Magnet Laboratory in Nijmegen, Netherlands, with a magnetic field of 16 Teslas, 320,000 times the magnetic field of the earth.

Another interesting use of magnetic levitation is the Maglev Train, a transport system where a train is levitated on a track using electromagnets in order to reduce friction. Currently the technology is being improved, and it is estimated that a Maglev train has the potential to travel at 4000 mph (6437 km/h) in an evacuated tunnel. Currently the fastest recorded speed of one of these trains is 361 mph (581 km/h), which was achieved in Japan in 2003. To the right is a picture of the JR-Maglev at the Yamanashi test track in Japan.

In addition to magnetic levitation, there are two techniques that achieve similar effects known as optical levitation and acoustic levitation. Optical levitation was developed by a scientist named Arthur Ashkin, and causes objects to float by shooting photons at them. Auditory levitation uses sound waves to keep objects in the air, and at the Otsuka lab in Japan, scientists were able to levitate objects with sound frequencies above the human auditory range. Below is a video of an acoustic levitation chamber that uses three speakers to control the x,y, and z coordinates of the objects inside by adjusting the wavelength of a constant, resonating, 600 Hz sound.

Levitation is one of the world’s phenomenon that can be described as having a surreal or magical effect because it goes against everything we think we know about gravity. Since sound and magnetic fields are invisible to us, it causes us to ignore one of our most important senses and accept that things are not always what they seem. In any case, magnetic, acoustic, and optical levitation are some of the most interesting scientific effects, and they may have a large part in technology developed in the near future.

Sources: Meisner Effect, Superconductor, Hoffman lab, Optical levitation, Acoustic levitation.