When I was
10 or 11 years old 9 years old, my big sister took me to the single theater we had in our small home town to see “Back to the Future, Part II”. I hadn’t seen the first part yet. But that didn’t matter. Frankly, I wasn’t so much interested in the past. It was the future that made me dream. My eyes opened wide as I beheld Marty McFly jump onto a curiously 80’s styled hover board. He weightlessly zipped around trying to avoid Griff and what I can only describe as rejects from the cast of Mad Max. We’ve come a long way in the last few decades. We have maglev trains now. To my knowledge, we don’t have a skate-board equivalent to the mag lev train. These trains are, however, active in nature. They require power in order to operate. The hover board depicted in the clip was passive and required no power to run, it just hovered. The operator had to provide the momentum to make it go. Also, Griff points out that “you need power” when Marty gets stuck out on the water. So is my dreams of retro hot pink flying planks just too far fetched? I though so. That is, until I ran across this demo on YouTube courtesy of the Association of Science-Technology Centers (ASTC). Not only was this little disc working just like the hover board in the movie, but it was also completely passive! There was no energy reserves required to make it levitate. I dare say it seemed to work better than the hover board. But how did it work? I set out to investigate. Not surprisingly it all comes down to magnets. But not just magnets. Magnets and electricity. But not electricity that comes from a battery or off the grid. Induced electricity. Ok, this might all seem just a bit complicated, so let me see if I can break it down. Let’s take a closer look at electricity and magnetism. Most people know what electricity is. It’s simply the flow of electrons. Anywhere electrons are flowing from one point to another, you will find electricity. (It’s actually a bit more complicated than this.. But I’ll write an article on that some other time.)
Magnetism, on the other hand, is not as dynamic as electricity. A magnetic field is typically static in nature and can be described as invisible lines of force pushing in a closed path. There is nothing flowing or moving along the magnetic field line. It’s just a force that exists.
So what do these things have to do with each other? Well, as it turns out, when electrons flow from one point to another, they create a magnetic field around the path through which they are flowing (again, this is a simplistic view, but it works for this discussion). Also, if you move a conducting material through a magnetic field, those lines of force push the electrons within the material creating an electrical current. So what would happen if you were to take a chunk of conducting metal and place it into a magnetic field? Well, as the metal moved through the field, the electrons inside the material would be given a push. As long as the material kept moving through the magnetic field, the electrons inside would continue to be pushed. And, since moving electrons create a magnetic field of their own, a second magnetic field would be created around the metal object. The second magnetic field created by the moving electrons would perfectly oppose the magnetic field through which we were moving our metal object. But nothing is perfect. A typical metal conductor contains vibrating molecules. The vibration is from thermal energy trapped in the metal. Anything above absolute zero (approx -273 degree Celsius) has thermal vibration in its atomic structure. As electrons flow through the material, they interact with the vibrating network of atoms. Through this interaction, their forward momentum is absorbed into the metals’ atomic structure as thermal energy. The result is the metal gets hot, and the electrons eventually stop flowing. In the case of your run-of-the-mill magnet and chunk of metal conductor, you wouldn’t really notice anything. So How does this help us? Well, we can play a trick. We can freeze the conductor all the way down to absolute zero. Once we do so, something interesting happens. Once all atomic vibration stops, there is nothing left to slow down the electrons. The material is now referred to as a “Superconductor” because it can now allow electrons to flow (or conduct) without any resistance. Once the conductor is moved into the magnetic field and those electrons start moving around, they don’t stop. The consequence of this is that the equal and opposite magnetic field also does not go away. And of course, if it doesn’t go away, the two magnetic fields now perfectly repel each other. This phenomenon is called the Meissner effect. But now things are a bit of a balancing act.. If the two metals are repulsing each other, why do they not just fly apart? Well, there’s a few other phenomenons at play here. First off, you may noticed they do not use a metallic conductor in these videos. Instead, they use a ceramic material. The ceramic material being used enters a superconducting state at around -200 degrees Celsius making it relatively easy to achieve. Once in the superconducting state, the repulsion of the magnetic field around it causes it to become pinned in space. The magnetic lines of force (called flux lines) are pushed out and squeezed on the outer boundary of the superconducting material. Flux lines are also pushed in through any defects in the material. This phenomenon is known as flux pinning. This nervous graduate student at Cornell University does a good job explaining flux pinning: Together the phenomenon of flux pinning and the Meissner effect create a pretty near hover board like effect. Of course you do need to either have magnets in your hover-board and a super-cooled floor, or a magnetic floor and a supply of liquid nitrogen on your hover board.. But those are just minor details. The important thing is it that it takes us one step closer to this: In his defense, I would have taken it from the little girl, too. – Marc Cabot Bsc. Engineering, University of New Brunswick.