Question: How does diamagnetism work?

Simone Sturniolo answered on 19 Jun 2013:

Nice specific question! Diamagnetism is the property that all materials have to contrast a magnetic field – in other words, when a strong magnetic field comes from the outside, the material develops its own magnetic field, weaker and opposite to the external ones. I say this is a property that “all” materials have because the mechanism that drives diamagnetism is true for all materials: however, some of them also are able to generate a much stronger magnetic field in the same direction of the external one (a dramatic example would be iron, that magnetizes) and in the end that’s the winning effect, which makes diamagnetism almost disappear by comparison.
Diamagnetism can be calculated exactly with the quantum mechanical equations of motion of electrons in a material. Since this is rather complicated, almost no one would understand, and seriously, how am I supposed to write QM formulas in this box anyway?, I’ll try to explain it in a ‘classical’ way (it’s not wrong, but not perfectly right either. Take it as is for now). You may know that a charged particle moving in a magnetic field tends to “bend” its course. In fact, if you have a charged particle (like an electron) moving straight and then it enters a magnetic field, it will curve and end up running around in circles: the stronger the field, the tighter the circle. On the other hand, electric current in a circuit generates a magnetic field. And what is a charged particle running around in circles if not a sort of electric circuit (without wires, of course)? So it generates a magnetic field because it runs in circles because of a magnetic field. But the new magnetic field (the generated one) will have a direction opposite to the external one, when you calculate everything properly. There comes your diamagnetism! Every material has electrons moving inside, every magnetic field will make these electrons move in circles, every electron moving in circles will generate a magnetic field countering the external one.

The nice thing about diamagnetism is that, being universal, you can use it to magnetically levitate pretty much anything! Like a frog:

Superconductors are especially funny, because they have PERFECT diamagnetism (the reason in this case is entirely different from the mechanism I described earlier though. Superconductors are strange beasts and you can’t explain their properties without quantum mechanics), which allows them to fly much more easily than other things (say, frogs). This is how magnetic levitation trains are built: