Solved: How a spinning magnet cause other magnets to levitate

Back in 2021 the Turkish scientist Hamdi Ucar published a paper describing how he attached a magnet to a motor and made it rotate very fast. He then moved it close to a second magnet that then started to rotate and suddenly hovered in a fixed position a few centimeters away.

While magnetic levitation is nothing new - the best-known example is probably Maglev trains that rely on a strong magnetic force for lift and propulsion - the experiment puzzled physicists as this phenomenon was not described by classical physics, or, at least, by any of known mechanism of magnetic levitation.

It is now, however. Rasmus Bjørk, a professor at DTU Energy, was intrigued by Ucar’s experiment and set out to replicate it with MSc student Joachim M. Hermansen while figuring out exactly what was going on. The replicating was easy and could be done by using off the shelf components, but the physics of it was strange, says Rasmus Bjørk:

“Magnets should not hover when they are close together. Usually, they will either attract or repel each other. But if you spin one of the magnets, it turns out, you can achieve this hovering. And that is the strange part. The force affecting the magnets should not change just because you rotate one of them, so it seems there is a coupling between the movement and the magnetic force,” he says.

The results have recently been published in the journal Physics Review Applied.

Several experiments to confirm the physics

The experiments involved several magnets of differing sizes, but the principle remained the same: By rotating a magnet very fast the researchers observed how another magnet in close proximity, dubbed a “floater magnets”, started spinning at the same speed while it quickly locked into a position where it stayed hovering.

They found that as the floater magnet locked into position it was oriented close to the axis of rotation and towards the like pole of the rotor magnet. So that, for instance, the north pole of the floater magnet, while it was spinning, stayed pointing towards the north pole of the fixed magnet.

This is different from what was expected based on the laws of magnetostatics, which explain how a static magnetic system function. As it turns out, however, the magnetostatic interactions between the rotating magnets are exactly what is responsible for creating the equilibrium position of the floater, as co-author PhD-student Frederik L. Durhuus found using simulations of the phenomenon. They observed a significant impact of magnet size on levitation dynamics: smaller magnets required higher rotation speeds for levitation due to their larger inertia and the higher it would float.

“It turns out that the floater magnet wants to align itself with the spinning magnet, but it cannot spin fast enough to do so. And for as long as this coupling is maintained it will hover or levitate,” says Rasmus Bjørk, and continues:

“You might compare it to a spinning top. It will not stand unless it is spinning but is locked into position by its rotation. It is only when the rotation loses energy that the force of gravity – or in our case the push and pull of the magnets – becomes large enough to overcome the equilibrium.”

FACT BOX:

The authors behind the paper Magnetic levitation by rotation are all from DTU, namely DTU Energy, DTU Physics, and DTU Nanolab.

See the focus story from the journal: Physics - How Rotation Drives Magnetic Levitation (aps.org)

See an online presentation of the results by co-authors Frederik L. Durhuus and Joachim M. Hermansen: Magnetic levitation by rotation (youtube.com).