Levitation
What is levitation?
Levitation is when a something rises into the air or something floats in the air without visible means of support.
Levitation piece of YBCO superconductor above a
magnetic track
Can we levitate everyday objects, or even a person?
This is an interesting question to consider
scientifically, as it's often thought to be realm of magicians
and movies!
We might have had experience as a child of playing with magnets.
But no matter how carefully we try to balance them, it seems to
be impossible to get one magnet to float above another - they
just flip over and snap together.
Diamagnetic Levitation
Despite this initial failure, using magnetism is actually the way forward! A diamagnetic material responds to oppose any external magnetic field the material is subject to.
Diamagnetism is a quantum mechanical effect arising from the change in the orbital velocity of the electrons around the nucleus of the atom in response to an applied magnetic field.
All materials show this diamagnetic response to an applied field although, because this is a weak effect, it is overpowered in those materials which also exhibit ferromagnetism or paramagnetism.
Common materials such as water, organic compounds
(such as wood, plastics), metals (copper, gold, mercury) and even
your body are all diamagnetic, but only very slightly. This ability
to repel magnetic fields gives us the possibility of being able
to levitate objects!
Graphite levitating above magnets
At room temperature, one of the best diamagnets is the metal bismuth. Slightly more diamagnetic (in one direction) is pyrolytic graphite. This grows very slowly by a process called chemical vapour deposition creating a highly ordered material where the carbon atoms form a layered hexagonal structure.
Because of the low density of pyrolytic graphite (compared with bismuth) a thin sheet will be repelled by a sufficiently strong neodymium magnet. A thick piece will be too heavy as the material above about a half of a millimetre does not contribute much to the lift.
If the piece is thin enough, it will simply slide right off the side of a single magnet, and refuse to sit still on it. To get a piece of pyrolytic graphite to sit still above a magnet, we need to find a way to force it to the centre of a magnet.
Layout of poles of magnets
This can be achieved by using a set of four magnets (creating a magnetic potential well). The pole of each of the magnets (where the field is strongest) forces the graphite to the centre. This effectively pins the graphite above the magnets. The diagram below shows the magnets coloured green and blue (with their North and South poles arranged as shown) and the pyrolytic graphite in grey in the centre.
Magnetic field strength (colours) pinning graphite
(grey) above magnets
This creates a stable room temperature levitation demonstration where the graphite is levitated about 1mm above the magnets. When pushed gently down, the graphite moves, but if the pressure is removed it levitates again.
Graphite levitating above magnets
Magnetic fields are measured in units of Tesla. The earth's magnetic field is around 0.0001 Tesla and the field from the neodymium magnets used for the graphite levitation is around 1T.
In Nijmegen in the Netherlands, there is a special scientific laboratory dedicated to performing experiments at very high magnetic fields. These are created using large coils of wire and very high currents and create fields more than 300,000 times larger (upto 33T) than the earth's magnetic field. These magnets are strong enough to levitate even weakly diamagnetic objects such as frogs, strawberries or grasshoppers!! They have a very good website, with many videos - click here.
Superconducting Levitation
In the same way that these objects we've already seen can levitate above magnets because they are weak diamagnets, superconductors which are perfect diamagnets can also levitate above a magnet. Here are a selection of YouTube videos showing this.
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A piece of superconducting material (YBCO), cooled to below
its superconducting transition temperature (92K) with liquid
nitrogen (77K) will levitate above a magnet.
This is an example of the Meissner effect - the weak magnetic field from the nearby permanent magnets can be expelled by the superconductor. This is because current flows in the surface of the material, generating a perfect mirror repulsive magnetic field i.e. the superconductor is a perfect diamagnet.
A further demonstration can be performed using a magnetic track: the superconductor is seen to levitate and oscillate back and forth. This can also be adapted into a oval/circular track which superconductor circles around.
The magnets in the track need to be set up to create a magnetic valley - so the track needs to be three magnets wide! Each magnet along the length of the track needs to have a top face of one pole and the opposing downward face of the other pole. Looking across the track three magnets are needed, the two edge magnets have, say north upwards and the middle magnet has south upwards.
Side view of arrangement of magnets in magnetic
track. Yellow is south and grey is north.
Top view of arrangement of magnets in magnetic
track. Yellow is south and grey is north.
On a linear track, the superconductor can be stopped and sent in the reverse direction at the ends by reversing the arrangement of the magnets so that the outer magnets are south upwards and the inner magnet in north upwards.
Technical note - there is also another effect at work here which explains what is seen in the last video. Here the piece of black superconductor comes along when the magnet is raised. This sample was cooled in the presence of the magnetic field and so the magnetic field is 'pinned' into the sample by the presence of impurities in the superconductor. The effect of pinning is also demonstrated by running the magnetic track demonstration upside down!
Detailed step-by-step instuctions for setting up these demonstrations as well as risk assesments can be downloaded by clicking the links. Within a room, liquid nitrogen is best poured from a transport dewar into a wide-neck insulated liquid nitrogen container. From here, it can be viewed/poured by dipping in by hand a polypropylene plastic drinks cup (a clear one is best).
Using the superconducting levitation apparatus, a snail or other small creature can be levitated in a plastic weighing container.
Resources
Liquid Nitrogen Risk Assessmnets
Detailed Instructions for Levitation Experiments
Yellow Tape - Yellow 1350 polyester tape is made by 3M and can be ordered from Farnell order code 753002