Posted by “I see miracles all around me
Stop and look, it’s all astounding
Water, fire, air and dirt
Fucking magnets, how do they work?
And I don’t wanna talk to no scientist,
Y’all motherfuckers lyin’, and gettin’ me pissed.” -Insane Clown Posse
While music certainly has the power to be uplifting, the Insane Clown Posse simply won’t make the cut for this site. You’d do better listening to LZ Love‘s song,
Even though scientists will claim otherwise, magnetism isn’t great (sic) understood.
This declaration of ignorance is followed by a — let’s be generous — partially correct explanation of how this “quantum levitation” works. But let’s see if we can’t get it right! Let’s start with the basic type of magnetism you all know: ferromagnetism.
Image credit: Robert Krampf.
Ferromagnetism is how permanent magnets work, from iron blocks capable of picking up paper clips to the magnets sticking to your refrigerator. The basic principle is that you apply an external magnetic field, and not only does your ferromagnetic material wind up internally magnetized in the same direction as the external field, it remains magnetized even after that field is turned off!
Image credit: Wiley.
Although this is the type of magnet we’re most familiar with, nearly all materials are not ferromagnetic. Why not?
Because most materials don’t remain magnetized once that external field is removed. So what happens inside these other materials when you apply an external magnetic field? They are either diamagnetic, where they magnetize anti-parallel to the external field, or paramagnetic, where they magnetize parallel to the external field. (Incidentally, all materials exhibit diamagnetism, but some materials are either also paramagnetic or ferromagnetic, which can easily overwhelm the effect of diamagnetism.)
Image credit: Skulls in the Stars. It's a science blog, really!
At normal temperatures, you’ve probably heard of Faraday’s law of induction, which says that if you change the magnetic field inside of a material, it generates an internal current to oppose that change! Well, if you bring a material with any sort of conduction at all into or out of a magnetic field, you’re going to create tiny currents inside of the material — known as eddy currents — that oppose the internal change in the magnetic field.
Image credit: Looks like it's from CEDRAT, as far as I can tell.
Now, at normal temperatures, these currents are extremely temporary, as they encounter resistance and decay away.
But what if you eliminated the resistance? What if you drove it down all the way to zero?
Believe it or not, you can drive the resistance down to zero in pretty much any material; all you have to do is bring it down to low enough temperatures, until it becomes a superconductor!
Image credit: Piotr Jaworski.
But just what is it that happens when you drop the temperature of a material below its critical temperature, to make it superconducting? It expels all the magnetic fields from inside! This is known as the Meissner Effect, and it turns a superconducting material into a perfect diamagnet.
“Hang on,” you may say, “how does that explain this quantum levitation?”
Well, it doesn’t, of course. Because what I just told you is for a Type I superconductor, like aluminum, lead, or mercury.
But there’s another type of superconductor, one with impurities in it, like the one at the video atop, and also in the amazing video, below.
Image credit: Department of Engineering Physics, GITAM University.
In other words, in a Type II superconductor there are impurities where the magnetic field lines can penetrate. And if the magnetic field can get through, guess what else it can do? Make those eddy currents! And with the resistance driven so far down by these ultra-low temperatures, these currents don’t simply decay away; they’re sustained.
Image credit: Philip Hofmann, file provided by an old friend.
So in the superconducting regions, the fields are expelled, and you get a perfect diamagnet. In the impure regions, the magnetic field lines are concentrated, pass through and cause sustained eddy currents, and this is what pins the superconductor in place! (When you hear the term flux pinning, these confined field lines in the impure regions are what they’re talking about!)
Two years ago, I happened to cross paths with Matthew Sullivan of Ithaca College, who has applied this to create some amazing resources, including — for eye candy — the video below!
October 23, 2011
October 23, 2011
October 23, 2011
October 24, 2011
October 24, 2011
October 24, 2011
October 24, 2011
October 25, 2011
November 29, 2011
