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Q & A: fun with delicate magnets

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Most recent answer: 06/27/2015
Q:
Good evening! I have a question that before I ask, I wish to clarify that I am NOT a member of the "tin-foil hat" crowd, nor that I embrace any of the pseudo-science pervading the internet. Having said that, a short while back, a co-worker showed me his personal rendition of Edward Leedskalnin's "Perpetual Holding Device"; which, while it's been 20+ years since I graced the halls of my college's physics department, defied what I remembered to be the behavior of electromechanics, or what I have since been able to determine from Maxwell's publishings, or any other credible source. For those not familiar with this device, it simply consists of a piece of non-magnetized ferrous material bent into a "U" #essentially a "horseshoe"#, with two opposingly wound coils of insulated wire at the its ends #wired in series#, and an additional piece of non-magnetized bar stock bridging the ends. To "operate", simply place the bar across the top of the "U", and quickly touch the leads one or more times across a 9v, or say, a car battery... If one even comes close to following this "recipe" #I just used a 1/2" u-bolt, 30'/2 of insulated 16ga wire, and a 5/8" wrench#, the device will now be holding the bar; even though the current has been removed, or if you are so inclined, after physically cutting away the coils of wire-- And will continue to do so for as indefinitely long a period of time as environmental conditions, and/or the tolerances and materials used will allow. Even in my slapped-together test-case, it hung on my wall in this state for over a year, until I took it down, and inadvertently dropped it while packing things up for an upcoming move. That all said, here's my question #finally#: Would someone mind explaining how and why this works in a manner that #at least in appearance# goes against what is commonly taught regarding electromagnetism? Specifically... 1# Unless its constituent material actually becomes "magnetized" while energized, electromagnets stop being magnets once current is no longer flowing through them. This is not the case here; while as one unit #which as I understood things, it shouldn't even be doing#, the bar and U components outwardly behave, and exhibit the same behavior as a typical horseshoe magnet and bar would. However, when separated, neither of them retain any magnetic properties, and won't as much as attract or hold something as light as a staple. 2) Whenever the bar being held is eventually removed, a spike of nearly the exact amount of current and #inverted# voltage is "returned" back to the leads which had been used to originally energize the device; regardless of how long it was since it had originally been energized. There are more and more questions I have as I continue to look into, experiment, and otherwise poke-around at it, but these are the two basic ones I have, that if answered, may shed some insight into some of the equally odd and unexpected behaviors exhibited by this device. I make no claim to have more than perhaps a slightly more-than-average general understanding of magnetism/electromagnetism, but having pondering this for a fair amount of time, I >>believe
- Scott Martindale (age 46)
Casselberry, Florida
A:

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.

Mike W.


(published on 07/04/2014)

Follow-Up #1: mesoscopic persistent currents

Q:
Hi there, Mike. I wrote a follow-up a short while ago, and for whatever reason, didn't get posted. The shortened-version is, first off, thanks for the well-explained answer to my original posting! | Secondly, I've been playing around with these 'things' from time to time over the last year or so, and one odd behaviors I've observed, is that it (when in a 'holding' state) will periodically flip its magnetic polarity-- Which got me to thinking that there might be a related, or direct correlation to the fairly-new quantum behavior often referred to as 'Persistent Current' [ref: 'Science Magazine Oct. 9, 2009-- Persistent Currents in Normal Metal Rings' J. Harris; Dept. of Physics, Yale] Granted, the scale between the two is drastically different, but there are enough interesting similarities that I really wanted to run this past you. Thanks again!
- Scott Martindale (age 47)
Maitland, Florida
A:

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.

Mike W.


(published on 06/27/2015)

Follow-up on this answer.