Challenge Questions Blog

### Challenge Questions

Stop in and exercise your brain. Talk about this month's Challenge from Specs & Techs or similar puzzles.

So do you have a Challenge Question that could stump the community? Then submit the question with the "correct" answer and we'll post it. If it's really good, we may even roll it up to Specs & Techs. You'll be famous!

Answers to Challenge Questions appear by the last Tuesday of the month.

 Previous in Blog: Magnetic Monopole: Newsletter Challenge (August 2011) Next in Blog: Paramagnetic Behavior: Newsletter Challenge (October 2011)

# The Floating Superconductor: Newsletter Challenge (September 2011)

Posted August 31, 2011 5:01 PM

This month's Challenge Question:

If you immerse a rectangular piece of superconductor in liquid nitrogen and then place it in the air above a permanent magnet, the superconductor will float and spin without touching the magnet. Why?

And the Answer is:

The magnet induces a constant eddy current on the surface of the zero-resistivity superconductor. The eddy current direction follows Lenz's Law, which means that the direction is such that it produces a magnetic field opposing the magnetic field of the permanent magnet. This creates a repulsive force between the magnet and the superconductor.

Interested in this topic? By joining CR4 you can "subscribe" to

### Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

### Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru

Join Date: Sep 2007
Location: Japan
Posts: 2703
#1

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

08/31/2011 9:45 PM

I think because the generated magnetic field in the superconductor is not perfectly homogeneous?

__________________
From the Movie "The Big Lebowski" Don't pee on the carpet man!
Guru

Join Date: Dec 2008
Location: Cd. Juarez, Chihuahua, Mexico.
Posts: 1023
#2

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/01/2011 9:08 AM

Because it becomes a magnetic mirror of the P.M.

__________________
No hay conocimiento ni herramienta que sustituya al sentido comun.
Score 1 for Good Answer
Guru

Join Date: Apr 2009
Location: Yellowstone Valley, in Big Sky Country
Posts: 6963
#3

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/01/2011 9:57 AM

"In a weak applied field, a superconductor "expels" nearly all magnetic flux. It does this by setting up electric currents near its surface. The magnetic field of these surface currents cancels the applied magnetic field within the bulk of the superconductor. As the field expulsion, or cancellation, does not change with time, the currents producing this effect (called persistent currents) do not decay with time. Therefore the conductivity can be thought of as infinite: a superconductor.

"Persistent electric current flows on the surface of the superconductor, acting to exclude the magnetic field of the magnet (Faraday's law of induction). This current effectively forms an electromagnet that repels the magnet."

This well worded explanation is compliments of Wiki; Meissner Effect.

__________________
When you come to a fork in the road, take it. (Yogiism)
3
Guru

Join Date: Sep 2009
Posts: 1049
#4

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/01/2011 7:04 PM

The trick word is "place". To "place" the superconductive piece i.e move it towards the magnetic field you will induce some voltage during this movement, this voltage will produce concentric shaped currents, which due to superconductivity will not be damped when "placing" stops. That current's polarity will give an equivalent of a same polarity magnet over the real one, so repulsion between them. This repulsion if stronger than piece's weight, could make floating possible. The marginal spinning tendency is because of the actual (very small) electron mass that was forced to accelerate to the speed that corresponds to actual induced current, and this force will stop when "placing" stops, i.e if you stop it from spinning it will not start again. But spinning itself due to flywheel effect can also make the system less mechanically unstable. S.M.

__________________
Life is complex. It has a real part and an imaginary part.
Good Answer (Score 3)
Power-User

Join Date: Jun 2009
Location: Australia
Posts: 277
#5
In reply to #4

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/01/2011 11:57 PM

A GA for getting it right - but to add.

The conventional emf induction and force in a current carrying conductor laws apply - if the superconductor is placed over a magnetic pole then as SimpleMind says, "placing it in the magnetic filed induces a circular current" - fairly obvious if you imagine you were on the superconductor and looking down at magnetic flux lines radiating outwards and "upwards" from the magnetic pole as "you" moved down into the field.

Once the current has been developed in the superconductor it does not stop, because there is no electrical resistance AND this new circular current has its own circular magnetic field (circular around the current path) that interacts with the polar magnetic field (essentially according to F=BIL) the same as in an electric motor, and in this case in a direction that opposes the motion into the polar magnetic field - which is UP.

Those with a creative mind might wonder what would happen if the magnet and superconductor were held close together prior to cooling the superconductor and it were lifted up and away from the magnetic pole - it would be drawn DOWN!!

Guru

Join Date: Mar 2007
Location: Greece / Athens
Posts: 722
#6
In reply to #5

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/02/2011 3:17 AM

I suppose that the direction of the superconductor's rotation will be reversed in relation with the electrons' rotation inside it. Am I right?.....

__________________
George
Guru

Join Date: Mar 2007
Location: Greece / Athens
Posts: 722
#15
In reply to #5

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/12/2011 3:28 AM

__________________
George
Off Topic (Score 5)
Associate

Join Date: Feb 2011
Posts: 37
#7
In reply to #4

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/02/2011 7:55 AM

I get that the acceleration of the non-zero mass electrons causes the superconductor to spin via conservation of momentum, and that there is zero electrical resistance slowing the electrons down. But will the electrons ever slow down due to damping by some other means? Gravitational resistance, resistance due to electrons colliding with other particles in the conductor, etc? Will drag on the spinning conductor slow it down, thereby slowing the electrons down once again by conservation of momentum? Or is this akin to a flywheel on frictionless magnetic bearings in an evacuated chamber that will never slow down once spun up?

Associate

Join Date: Dec 2005
Location: South Central Wisconsin
Posts: 35
#8

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/02/2011 8:21 AM

Interesting comments/answers, but I always thought it was for the same reason water creates a vortex going down the drain...Earth rotation.

__________________
That which is watched, is optimized.........
Power-User

Join Date: Jun 2009
Location: Australia
Posts: 277
#9
In reply to #8

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/02/2011 8:45 AM

I doubt that its like the vortex down the plug hole thing - which is a result of Coriolis acceleration. i.e. lateral acceleration resulting from moving radially outward on a rotating disc and so acquiring increased lateral velocity.

My educated guess is that any spinning is a consequence of less than perfectly symmetrical placement in the magnetic field or any rotational component in the placement of the super conductor. Such things are likely to show up as a rotation - either through conservation o energy/momentum in the first case or as a way of equalising the initial current forces on entry. Stopping the rotation will stop the mass movement, but the circulating current will continue to circulate exactly as it was albeit on now constant physical path in the mass.

Active Contributor

Join Date: Sep 2011
Posts: 19
#18
In reply to #8

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

10/02/2011 5:11 AM

Does it rotate allways in one direction?

Off Topic (Score 5)
Power-User

Join Date: Sep 2011
Posts: 163
#10

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/04/2011 2:14 PM

I'd have to say what we are seeing is gravity itself being used as a force here, in addition to what someone else said about our placement of the material into the magnetic field. In other words, the seems to be suspended because as gravity is pulling it down into the fields it's actually generating opposing magnetic fields in the superconductor. If you were to try and push it down further, you would only generate stronger fields in the material that would further repel it from the magnet.

As far as the spinning, I think its more like a frictionless bearing spinning under the momentum of the force we applied to it. Which eventually slows down do to friction with air. In the case of a cube shaped superconductor being spun on a horizontal axis (like a ferris wheel), the side of the cube that has to overcome the resistance to getting closer to the magnet is equalized by the oppisite side that is being repelled.

I've never worked with superconducting materials, but a fun experiment to try would be to place two identical pieces of superconducting materials connected together by some non ferris (plastic) material, and suspend them over a magnet. The idea being to try and create a kind of balance type scale. Then set it into a rocking motion. This might require an upright support of some kind for the pivot point.

Guru

Join Date: Aug 2008
Location: City of destiny, INDIA
Posts: 775
#11

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/05/2011 11:55 AM

No one is talking about effect of liquid nitrogen. Liquid nitrogen has to do some thing here, otherwise OP would have not said it. I see two possibilities of its effect, which may be responsible for floating and spinning. (1) Conductivity of super conductor piece will be highest when it is taken out from liquid nitrogen at -196 degC, and then as its body temperature goes up conductivity is reducing. (2) Increase in temperature also creates turbulence in air between super conductor and magnet.

The rectangular shape also has to do some thing for this effect. The polarity of induced EMF may be changing continuously due to this even shape.

Score 1 for Good Answer
Power-User

Join Date: May 2011
Location: Chapel Hill, NC (sometimes Otago NZ)
Posts: 120
#12

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/06/2011 3:51 PM

Superconductors are perfect diamagnets. Bismuth and graphite are really strong conventional ones but the effect is so weak that levitation in a gravitational field as strong as the Earth's is difficult. You can make it effectively stronger by immersing the body in a paramagnetic fluid. This creates a magnetically induced effective buoyancy. These bodies can spin but air resistance will damp them as well. They only STABLY levitate if other magnets or charges act on them as well or they rotate to stabilize themselves (c.f. Levitron).

Stable magnetic levitation is forbidden by Earnshaw's theorem for any collection of charges an multipoles. It is the nonclassical nature of diamagnetism in all cases that make this possible.

The superconducting phase transition only occurs at low temperatures. This causes the electrons to form a new phase akin to solid and liquid phase transitions. Just as there is a latent heat cost for solids and liquids, there is an "energy gap" that this phase requires. The makes it very expensive for a magnetic field to enter the body. The further you push it into the field the more it repels. This alone does not explain why the superconductor does not slide across the magnet and fall off. The STABLE aspect of levitation is only for "Type II" superconductors. These phase separate into a collection of vortices that let magnetic field penetrate the body and "pin" it in place. This same effect ultimately means these superconductors typically have finite (but small) resistance. The vortices get dragged across the sample and dissipate some energy at the wall where they die out.

Guru

Join Date: Mar 2009
Posts: 507
#13

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/07/2011 12:14 AM

can't brake a strong magnetic field the superconduction?

Guru

Join Date: Mar 2009
Posts: 507
#17
In reply to #13

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/20/2011 11:47 PM

while placing the superconductor ahead the permanent magent, the magnetic fiel changes and induces a current inside the superconductor; this creates an inner magnetic field that lets the superconductor rotate; this rotation of the superdonductor is the mirror to the changing magnetic field of a static superconductor - same like generate electrical power in every power plant!

Participant

Join Date: Sep 2011
Posts: 1
#14

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/09/2011 5:03 PM

The permanent magnet's field induces an electrcal current within the superconductor, which in turn produces a magnatic field around the superconductor causing it to float.

Spinning I would guess is the result of changing polariity within the superconductor such that there is always a lateral force causing the rotation.

Once the temperature of the superconductor rises it will loose that ability and will fall.

Power-User

Join Date: Jun 2007
Location: ether
Posts: 340
#16

### Re: The Floating Superconductor: Newsletter Challenge (September 2011)

09/13/2011 10:48 AM

the superconductor as he remains in that state creates without electrical field a inner electrical current, and when you apply the magnetic you get a force as in a motor, for some time at least I guess!

__________________
“For no man can forbid the spark nor tell whence it may come.” ? Francis Bacon
Interested in this topic? By joining CR4 you can "subscribe" to

### Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

### Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!

### Users who posted comments:

cchaf (1); CRBuff (1); Doorman (1); electronick (1); Epke (1); G.K. (2); jklepatch (1); MrM (1); pritam (1); SimpleMind (1); snatr (1); TrevorM (2); Tritium17.6 (1); WilhelmHKoen (2); Yahlasit (1)

 Previous in Blog: Magnetic Monopole: Newsletter Challenge (August 2011) Next in Blog: Paramagnetic Behavior: Newsletter Challenge (October 2011)