MagLev Trains: Superconductivity Explained
Watch science videos online and learn how superconductors, magnetic fields and magnetic levitation work, from flux-pinning to maglev trains at The Emergent Universe, an online interactive science museum about emergence.
This model train contains a piece of superconducting material. The superconductor repels the magnetic track, causing the train to float.
How does superconducting levitation work?
At room temperature, when the superconducting disk is in its normal non-superconducting state, the magnet's field – indicated in the video by field lines – can pass through the disk; consequently, there is no repulsion and the magnet sits on the disk. When the disk is cooled into its superconducting state, it expels the magnet's field, pushing up the magnet, and causing it to float. This is a demonstration of the Meissner-Ochsenfeld effect.
Making a (Bad) Train
Because superconductors expel magnetic fields, a track created from rows of magnets, each 3 magnets across with alternating North-South-North poles, as shown, can levitate a superconductor. But while this effect can keep the superconductor suspended, it doesn't have the forces necessary to keep the superconductor "train" on the track; instead the superconductor easily falls off. What's needed to make a good superconductor train that will stay on track? A phenomenon called flux pinning.
Trick of Attachment: Flux Pinning
When certain types of superconductors are cooled in the presence of a magnet (of appropriate strength), tube-like areas of the material remain in the non-superconducting state. These tubes continue to allow the magnetic field to pass, even as the surrounding material becomes superconducting and expels the field. Because the details of these tubes are specific to the magnetic field they were formed around, any change in the superconductor’s position relative to the field would disrupt the field’s flow through the tubes and cost energy. As a result, the superconductor remains pinned to the magnet.
Staying on Track: A Good Train
Flux-pinning keeps a good train on track. Because the magnetic field emanating from the track is the same all along the track, the flux-pinned superconductor can move along the track without disrupting the trapped field lines. Movements of the superconductor off or away from the track, however, would require disrupting these field lines, which costs energy. This cost keeps the train from leaving the track, enabling it to follow around curves.*
*Note that while commercial superconducting magnetic levitation trains do exist, they operate on different principles than the model trains shown here.
What Are Magnetic Field Lines?
Recall that all magnets, like the one shown here, have a north (N) and south (S) pole. When two magnets are brought together, their “unlike” poles (N and S) will attract each other. Conversely, their “like” poles (N and N, or S and S) will repel each other. Scientists draw “field lines” around a magnet to indicate how a compass (which is just another tiny magnet) would respond to that magnet at each point. Its north pole (blue) traces out the magnet's field lines and illustrates the magnet's force field.
How Does a Superconductor Expel a Magnetic Field?
When a superconductor is placed nest to a magnet, electric currents form near the surface of the superconductor. These currents form in just the right way to generate a magnetic field inside the superconductor that exactly cancels the field from the nearby magnet, leaving the interior of the superconductor with zero magnetic field. These currents also change the field outside of the superconductor, and the net effect is the expulsion of field lines from inside the superconductor to outside the superconductor.
Tubes and Vortices
The boundaries between the tube-like, non-superconducting areas and the surrounding superconducting region are created by tiny, whirlpool-shaped electric currents, called vortices. These whirlpool currents generate a canceling field in the surrounding region (effectively expelling the magnet’s field), but not in their central cores (which become the tubes through which the magnet’s field can pass). While in a perfectly pure material, these vortices could move around freely, the defects and impurities found in most materials trap the vortices, making it energetically costly for them to move in response to changes in the magnet’s field.