The Meissner Effect: Exploring the Physics of Superconductivity

[Diane_]

I had to flip a coin between thermo and quantum for this one, so forgive me if the coin made the wrong decision. :)

I've been thinking about the Meissner Effect. (Well, a girl's got to think about something, anyway.) Suppose you take a piece of lead and shape it into a paraboloid. Chill it until it transitions to superconducting. If you drop a magnet into it, it will "support" itself on its field lines. This is true - I've seen it, and it's easy to find pictures of it on the Internet. Two questions occur to me:

1) It seems to me - and this is just "feel" - that can't be a permanent state. If you were somehow to keep the lead chilled forever, it still seems like something should give way eventually and the magnet should sink down into the bowl. Would it be right to say that the repulsion of the field lines eventually disarranges the domains in the magnet, turning it into just a lump of iron? If so, would that derangement occur faster if the magnet were pushed down deeper into the bowl? Or am I just out in left field somewhere?

2) Suppose you did this with an electromagnet, with the power off. Once it's in the bottom of the bowl, you turn on the power. There'll be a little bit of a power surge - partly until the back EMF of the coil builds up, but partly too (I should think) as the magnet "lifts" itself out of the bowl. The energy to raise it must come from somewhere, after all, and the battery is the logical candidate. Suppose you then turn off the power by switching the coil from a battery to a load of some sort. Would you get back the additional power that went into the magnet from raising it up as it sank back down? If so, what is the specific mechanism of that part of it - it feels like it should be an induction effect, but induction by what, through what?

I'm sure I could figure this out if I sat down and played with it, but it's been some time since my EM classes, so I figured I'd try here first. Thanks for your patience.

 

[pervect]

Well, offhand, I'd expect that a magnet would probably lose it's magnetization over a long enough time period, but it wouldn't matter particularly whether or not it was suspended above a superconductor. If you wait long enough, and the magnet has a non-zero temperature, the magnet will eventually evaporate due to it's vapor pressure, for instance.

 

[CharlesP]

Some superconducting experience

I have witnessed the Meissner effect in a test tube with liquid nitrogen and a small piece of superconductor. The tube constrained the 1/4" pieces to only vertical movement and something was backwards-maybe the superconductor was on top. It would fail after a minute out of the nitrogen.
I saw pictures of 4 degree K apparatus with a lead "frying pan" and a bar magnet.

I had my own Ti-Mo superconductors for thesis work at the University of New Orleans.
Ferromagnetism may decay but even at room temperature it takes a long time, maybe years.
The Meissner effect has the property that forcing "magnetic lines of force" into it requires energy, so going down for the magnet can mean going uphill in a potential energy diagram.
Similarly if an electromagnet is energized (your description) and brought nearer to the superconducting lead barrier, then loaded, I expect more energy could be removed than supplied by the battery. The difference having been supplied by the force moving the electromagnet.
I am trying to visualize this in terms of H, the magnetomotive force.
There may be a room temperature solenoid anology and an electrostatic anology. I did the electrostatic experiment at home and it is easy. Do you have a lab?

 

[Diane_]

I don't have a lab at the moment, other than my kitchen - and that's a long story in itself.

I do understand that a magnet will "demagnetize" over time. As I say, I have nothing to base this on other than my feelings about how this works, but it does seem to me that suspending the magnet on its own field would speed up the process. The alternative, that it would just hang there until random thermal fluctuations deranged the domains seems too much like getting something for nothing. I was hoping someone had some direct experience, either empirical or theoretical, that would answer the question for me.

I do appreciate the responses, though. Looks like I'm going to have to try my boyfriend's patience and play with this on my own.

 

[pervect]

I don't have a lab at the moment, other than my kitchen - and that's a long story in itself.

I do understand that a magnet will "demagnetize" over time. As I say, I have nothing to base this on other than my feelings about how this works, but it does seem to me that suspending the magnet on its own field would speed up the process.

I can't see any mechanism that would cause a significant change. You're not gaining "something for nothing", the levitating magnet is doing no work by hanging where it is. Work = force*distance, and while there is a force, there is no distance. Compare the magnet to the case where the Earth holds up buildings - the Earth is not doing any work, it won't "wear out".

On the other hand, I don't really understand why it's recommended that magnets be stored with a "keeper", so I may be missing something.

 

[CharlesP]

Magnets suffer from a "reverse H field" which tries to turn the domains around. Thermal vibration can do this too but not near 0 degrees K.
If you wrap a coil around a U shaped magnet and vibrate the keeper from the keeper position to infinity and back, you can get ac power from the coil without any change to the magnet. Using a Meissner plane shield instead of a keeper will have a similer though lesser effect.

 

[pervect]

Magnets suffer from a "reverse H field" which tries to turn the domains around. Thermal vibration can do this too but not near 0 degrees K.
If you wrap a coil around a U shaped magnet and vibrate the keeper from the keeper position to infinity and back, you can get ac power from the coil without any change to the magnet. Using a Meissner plane shield instead of a keeper will have a similer though lesser effect.

Can you provide more details of the "reverse H field"? All I can think of at the moment is B=uH, I don't see how there would be a reversal.

 

[CharlesP]

Let me get out my old Corson and Lorrain. I see three diagrams. In the first one (Fig. 7-19) Magnetization lines are all parallel and end at the end faces. H goes in the opposite direction and curves outward.

In Figure 7-23 B and H are shown. They are the same outside the metal. Inside the material B is curved inward and H is curved outward.

A right end pole face is shown in Figure 7-22. Hr=M/2 to the left inside and to the right outside. HL due to the left pole is continuous across the boundary but can be quite small. This Hr=M/2 inside in the direction opposite to M is what I called the reverse H field and it tries to demagnetize the magnet.

I might be able to take or draw pictures.