Friday, April 12, 2013

Lenz's Law, Induction, and Levitation

+Bruce Elliott asked an excellent superconductor levitation question, so, no lab stuff, journal articles, or homework problems today, just superconductors.  A few days ago, I posted the following video of eddy current levitation to Google + with this explanation...

Staying with the eddy current theme, what if you played the game a different way?  The video below shows a coil of wire driven by wall current, (60 Hz here in the USA).  The alternating current creates a rapidly changing magnetic field.  The eddy currents in the aluminum plate, (aluminum like copper is not magnetic), oppose the original magnetic field created by the coil and cause it to levitate.  The coil gets very, very hot in the process because of all the energy from the wall current being turned into heat by the electrical resistance of the coil, (video 1).

What the post and video are demonstrating is a combination of Faraday's Law of Inductance, (a changing magnetic field will induce a changing electrical current in a conductor), and Lenz's Law, (the current produced in Faraday's Law will always run in a direction to create a new magnetic field that will oppose the change in the first magnetic field.)  Here's the second post I made with a video showing the levitating coil from the side...

Here's the same eddy current levitation demo seen from the side.  Notice that the coil won't stand still it drifts off to the edge.  This is an example of Earnshaw's theorem that states you can't get stable levitation if all you're using is a set of charges or magnetic fields that repel each other. 
So, to review, Faraday's law says that a changing magnetic field will induce an electric current in a conductor, (like the aluminum plate shown here).  Lenz's law says the current will move in a direction that will create a second magnetic field that will oppose the change in the first.  That means the magnetic field created by the current in the aluminum repels the magnetic field created by the coil.  Earnshaw's theorem says the levitation won't be stable and off the coil goes.

Bruce pointed out that in the following video of the levitation of a superconductor, there are none of the stability issues I just mentioned above and asked why.  By the way, this is soooo not UFO levitation technology!  Everyone knows they use phase conjugated microwave beams :)

So, why's the levitating superconductor stable while the levitating coil wasn't?  The levitation mechanism, is exactly the same.  As the superconductor nears the magnetic field a current is setup inside it that exactly repels the magnetic field outside and voi la, levitation!  This is called the Meissner effect in superconductors and  it's also why you'll hear the phrase 'superconductors expel all magnetic fields from their interior."

So, again, why is the levitation stable?  As it turns out, the stability is also caused by eddy currents, they're just acting in a different direction, (sideways) than the eddy currents that cause levitation   Things become a little more clear if you repeat the expulsion phrase above more completely: "Type I superconductors expel all magnetic fields from their interior."  Type II superconductors actually allow some magnetic field to penetrate.  Picture 1 shows what's going on.

The type I superconductor on the left expels all magnetic field.  If you try to levitate a magnet above a type I superconductor  it will just slide off the edge like the coil over the aluminum plate[1].  Type II superconductors allow some magnetic field to penetrate at what are called pinning sites.  It's this pinned field that provides stable levitation.  It all goes back to Lenz's law again. Looking at picture 1, imagine one of the red magnetic field lines tries to move left or right across the superconductor.  The moving magnetic field line is a changing magnetic field.  That field induces a current inside the superconductor   According to Lenz's law the new current will setup a magnetic field that will oppose the change in the original magnetic field.  In other words, the newly induced field will push back on the magnet to keep it from sliding across the surface of the superconductor   You can see this stabilizing force at action in the video of a magnet suspended above a superconductor that NASA loaned us below.

Paradoxically, (physicists love to say paradoxically), the same property that allows superconductors to expel magnetic fields can also  make them behave as magnets.  Watch the video below of a Type II superconductor that was cooled into its superconducting state with a magnet sitting directly on top of it.  Since it wasn't superconducting before it cooled, the field from the magnet could easily penetrate   When the superconductor entered its superconducting state  the field was stuck where it penetrated and the superconductor and magnet were stuck together.

NASA is looking into using this propensity of magnets and superconductors to maintain the magnetic field lines between themselves once established as a way to assemble clusters of satellites in outer space[2], (picture 2).

So, to wrap it all up, eddy currents can be used both to levitate things using magnetic fields, and, in the case of superconductors, make that levitation stable.  Their are quantum mechanical reasons that superconductors don't resist electrical currents, but as for the levitation, it's just plain old eddy currents.

A Few Research Notes
Interestingly, a theory paper a few years ago purports to have shown that while flux pinning certainly is a way to get stability, it's not necessary[3].

1.  On the instability of Type I superconductor levitation
Šimon I. (1953). Forces Acting on Superconductors in Magnetic Fields, Journal of Applied Physics, 24 (1) 19. DOI:

2.  NASA on flux pinning and satellite control

3.  Stability and pinning
Perez-Diaz J.L. & Garcia-Prada J.C. (2007). Finite-size-induced stability of a permanent magnet levitating over a superconductor in the Meissner state, Applied Physics Letters, 91 (14) 142503. DOI:

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