Fun With Delicate Magnets
Most recent answer: 07/04/2014
- Scott Martindale (age 46)
Alas, everything you describe makes perfect sense in our plain old picture of electromagnetism. No special mysticism is needed.
Here's the key. The stable state is the one with lowest free energy. In a little piece of ferromagnetic material, the free energy of a little region goes down when it magnetizes. That's why those materials can form magnets. In a big piece, though, the magnetic field energy of lining up all the parts is large enough that the free energy can go down if the magnetism breaks up into little domains pointing different directions, which removes the large-scale field. That's why your u-bolt and wrench are not normally magnetized on a large scale. Good permanent magnets require special materials to slow down that spontaneous demagnetization a lot.
In special geometries, however, that magnetic field energy is unimportant. Thin wires and closed loops are examples of those geometries. Basically, in a loop the field lines don't have to spill out of the material into a region where they especially increase the energy. So when you made a closed loop out of the u-bolt and wrench, it could get to lower free energy by magnetizing. By itself, it would take forever for that to happen, but your pulse of electromagnetism got things lined up in one of the two stable magnetic patterns.
What happens when you pull away the wrench? Now the free energy goes up. You can feel that because it takes a little work to pull the things apart. In fact, the free energy goes up to an even higher level than the ordinary un-magnetized state with the domains pointing all directions. So with a bit of thermal jiggling, they do just that. As their magnetism changes, you get a changing flux through the coils. That induces the current spike that you measured, just following Maxwell's equations.
So again, the key is that the free energy lowering from bringing the two parts of the loop together is enough to make the magnetized state stable. When they're separated it isn't quite stable.
(published on 07/04/2014)
Follow-Up #1: mesoscopic persistent currents
- Scott Martindale (age 47)
There is a slight relation. The persistent currents typically have two stable states, based on the ring geometry of the little samples. The current goes around one way or the other. That's similar to the two stable states of the magnetism in a little wire or loop. The details are quite different for these two effects, however. For the persistent currents, there's a little magnetic field looping around the wire. For a loop-like magnetic material, the field lines go around inside the loop, not circling around it.
(published on 06/27/2015)