Superconductors – Floating Magnet
Cool the superconductor with liquid nitrogen and the magnet floats.
Imagine a classroom of students fixated on a tiny magnet the size of the head of a pencil. The magnet is resting on a mysterious black disc, but the magnet does not stick to the disc. Both the black disc and the magnet sit on top of an upside-down Styrofoam cup. Here comes the fun part… when the teacher pours liquid nitrogen on the black disc, the tiny magnet lifts off the disc and spins in the air. The students cheer… and the teacher is ready to explore the science of superconductors with her students.
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- A small amount of liquid nitrogen
- A superconductor, like the one used in this demonstration - yttrium-based (YBa2Cu3O7)
- Small magnet
- Heavy protective gloves
- Safety glasses
- Carefully pour a small amount of liquid nitrogen into the dish or styrofoam cup until the liquid is about a quarter of an inch deep. The liquid boils furiously for a short while. Wait until it stops boiling.
- Using the provided tweezers, carefully place the black superconductor disk flat in the liquid until its top is just flush with the surface of the liquid nitrogen. Again, the nitrogen boils around the disk. Wait until this boiling stops too.
- Using the tweezers, pick up the provided magnet, and attempt to balance it on top of the superconductor disk. Instead of settling down onto the surface of the superconductor, the magnet will simply ‘float’ a few millimeters above the superconductor. This is a demonstration of the Meissner Effect (see below).
How Does It Work?
The following information was supplied by the scientists at Colorado Superconductor, Inc. and reprinted with their permission.
One of the properties of superconductors most easy to demonstrate, and also the most dazzling, is the Meissner Effect. Superconductors are strongly diamagnetic – that is to say that they will repel a magnet. The superconductor used in this demonstration is yttrium-based (YBa2Cu3O7). Imagine a ‘perfect’ conductor of electricity that simply has no resistance to the flow of an electric current. If a conductor of electricity is moved into a magnetic field, Faraday’s Law of Induction would lead us to expect an induced electrical current in the conductor and its associated magnetic field which would oppose the applied field. The induced electrical current would not dissipate in a ‘perfect’ conductor, and thus the associated magnetic field would also continue to oppose the applied field. Conversely, if the ‘perfect’ conductor was already in a magnetic field, and then that applied field was removed, the same physical law would indicate that an electrical current and its associated magnetic field would appear in the conductor which would attempt to oppose the removal of the applied field. If we were to do an experiment in which we placed a magnet on top of a material that by some process then became a ‘perfect’ conductor, we would see no physical effect on the magnet. However, were we to attempt to remove the magnet, only then would we feel an opposing force.
This experiment can also be conducted by placing the magnet on top of the superconductor before it is cooled in liquid nitrogen. As predicted by the Meissner Effect, the magnet will levitate when the temperature of the superconductor falls below its Critical Temperature. As explained earlier, there is no material other than a superconductor which could have shown this effect. If you carefully set the magnet rotating, you will observe that the magnet continues to rotate for a long time. This is a crude demonstration of a frictionless magnetic bearing using the Meissner Effect. The rotational speed of a cube-shaped magnet can be increased by using a plastic drinking straw to blow a stream of air at one of the edges or corners of the cube.
Another way to increase the magnet’s rotational speed is to cut out a small rectangular hole in a piece of paper. The hole is positioned over the levitated magnet such that half of the magnet projects above the plane of the paper. A stream of air directed along the upper surface of the paper will cause the magnet to rotate rapidly. The cubical magnet naturally is slowed by the resistance of air. Consequently, it can be expected to stop after a while. A cylindrical magnet will rotate for much longer since it is rotationally streamlined. However, the cubical magnet makes this demonstration much more graphic. A research group at Cornell University has demonstrated a frictionless superconducting bearing that can turn at a rate of one million rotations per minute. A bearing constructed in this manner, using the Meissner Effect, is much more convenient and safe than a conventional magnet bearing because of the ‘self-centering’ nature of the Meissner Effect on account of flux pinning.