Superconductor Levitation

[labview1958]

I want to levitate a type 2 superconducting disk between two permanent

magnets. One magnet is below the superconductor and one is above the

superconductor. Here is a sketch.

 

S

magnet

N

 

 

N

superconductor disk

S

 

 

S

magnet

N

 

Is it possible? If the lower magnet is glued to a table and the upper magnet

moved further up, would the superconductor move up or down?

[calbiterol]

Well, first off, it's much easier to levitate a magnet - 'cause it requires less force; it's probably the lighter of the two - and because the magnet will induce a current in the superconductor, creating a field, levitating the magnet. I believe this is called the Meissner Effect. Got to Wikipedia's superconductor page for more information.

 

Now, second off, how on God's green earth are you cooling the superconductor? Do you realize that the superconductors with the highest critical temperature only become superconductors at about 130 K? And they are made of special compounds designed for the specific purpose of superconducting, and as such are very, very, very expensive. For cheaper materials much, much lower temperatures are required, like lead, for example, don't superconduct until 7.2 K, and they require even lower temperatures for things such as levitation. Note that lead is a type-1 superconductor, though, and so a field can destroy it's superconductivity faster than a type-2. Do you have access to liquid nitrogen and these crazy compounds, like carbon nanotubes or Yttrium barium copper oxide?

 

If so, where are you getting these, and why aren't you sharing the wealth?

[BigMoosie]

Sorry to derail you but I have a question. I was thinkin the other day that it would be cool to have electromagnetic plates (literally dinner plates) and magnetic cutlery that hovers above your plate, is this feasible?

[5614]

It is feasilbe, but you'd have to have a seriously powerful electromagnetic plate (cutlery is heavy) and then you'd need a magnetic cutlery, metal would just be attracted.

 

And I soo replied to this thread already!!! Placing magnetic fields around superconductors rips apart the paired electrons (cooper pairs) and thus the superconductivity is lost.

 

Meissner Effect is the effect of a superconductor repelling the magnetic field outside of the wire/superconducting disc. So the magnetic field for a superconductor is only around the outside, not inside.

[labview1958]

I bought the superconductor type 2 disc from an internet site. I cannot recall the name. However they are not expensive. For liquid nitrogen you can buy from the factory for USD 1.00 per litre. You can cool it and make it levitate but when I try to move the magnet the superconductor runs out and falls. Sometimes for some unkown reason the bottom superconductor will jump up and stick with the superconductor. Can anyone explain?

[calbiterol]

I had no idea any of this was so cheap! Awesome.

[5614]

Sometimes for some unkown reason the bottom superconductor will jump up and stick with the superconductor.

I thought you only had one superconductor???

 

when I try to move the magnet the superconductor runs out and falls

What calbiterol said about critical temperature is quite cruicial, also magnets do destroy superconductivity, if your magnets are very powerful this may be a factor to consider.

 

How are you cooling your liquid nitrogen to suitable temperatures? Or if you buy it pre-cooled how long does it stay at that temperature for?

[calbiterol]

Well, liquid nitrogen is normally in flasks, filled to the brim with LN₂ such that there is no way for it to become a gas in the flask - there's no room for the liquid to expand into a gas. If you let it out into an open environment through an expansion valve at atmospheric pressure, it will boil. But it will stay at it's atmospheric boiling point (about 77K, I believe) until it boils - meaning that the liquid is at 77k as soon as you let it out of the tank. I think that's how it works, at least. In other words, I think it stays at that temperature pretty much indefinitely, until it boils. http://home.howstuffworks.com/refrigerator.htm has a pretty good description of the refrigeration process. I assume they fill the flasks (tanks) with nitrogen at cryrogenic temps at atmospheric pressure.

[labview1958]

Does anyone know how much liquid nitrogen is required to change the temperature of a copper sphere weighing 100 grams from room temperature to 77K? Once there how much more liquid nitrogen is required to keep the copper sphere at 77K for 1 hour? Anyway how to measure the temperature of the copper sphere?

[calbiterol]

It depends on the system you have set up. Is it in an air-tight box with only liquid nitrogen and the superconductor?

[DQW]

The superconductor you bought is likely Yttrium Barium Copper Oxide , YBa(2)Cu(3)O(7), popularly known as YBCO. These are cheap and have a Tc of about 90K. If you cool a YBCO disk down with LN2 (77K), it should be able to stably levitate a suitable permanent magnet. Commonly used magnets are made from Nd(2)Fe(14)B (popularly called NdFeB magnets), and a little piece of one of these will levitate stably (not slip off) as long as the size of the magnet is smaller than that of the SC disc.

 

If you want to levitate the SC over the magnet (and this is a little harder because you want the SC to be continuously immersed in LN2), make sure the SC is smaller than the magnet.

[labview1958]

Any comments about the arrangement below?

 

S

magnet

N

 

 

N

superconductor disk

S

 

 

S

magnet

N

 

 

Does the presence of the superconductor reduces the attraction between the magnets? Is the superconductor acting like a shield?

[DQW]

Comments on above geometry :

 

1. The advantage of the geometry is that it will increase stability intrinsically (ie: if it were in air)

 

2. The drawback (over a two piece system with the SC on the bottom) is that you now have to have liquid N2 all the way up to the SC (submerging it completely). The vigorously boiling liquid will definitely introduce instabilities which may knock the SC out.

 

3. The geometry is dependent on the dimensions of the parts. What are the shapes and sizes of the SC and the magnets ?

 

 

No, there won't be a shielding effect. See attached pic.

[labview1958]

If the magnets are half the size of the superconductor, would the presence of the superconductor increase/decrease the magnetic force repulsion/attraction between the two magnets as compared when there is no superconductor? My hunch is that there would be a shielding effect that reduces the magnetic force. From your diagram it is clear that the field lines have changed, thus the value of the attractive force would be reduced.

[DQW]

By "size", do you mean 'volume' or 'diameter' or something else ?

 

In any case, the force between the magnets in the new configuration can be seen as the superposition of three different forces :

(i) the force between magnets in the absence of the SC,

(ii) the force between the lower magnet and the SC (imagine no upper magnet exists) and

(iii) the force between the upper magnet and the SC (imagine no lower magnet exists)

 

While (i) < 0, (ii), (iii) > 0, so there must be a reduction in the attractive force in the superposed case {(i)+(ii)+(iii)} compared to just (i).

[labview1958]

Let's assume that my superconductor is in a plastic tube held in place and cannot move. I record the repulsive force at room temperature. I pour in the plastic tube 100ml of liquid nitrogen. As the SC reaches 77K would the force reading increase or decrease or reamains the same?

[D_Noonian]

If I understand you correctly, what you are attempting to do is disprove Earnshaw's Law by creating a diamagnetic field in which you can vertically trap/suspend (a.k.a. levitate) a superconducting (SC) magnet (although horiz. positioning will be intrinsicly unstable).

 

In fact, it is possible to disprove Earnshaw's Law by using "diamagnetics" (e.g. bismuth, carbon, pyrolytics) by replacing your SC magent and LN2 tube with a simple non-superconducting NIB magnet. In doing so, the geometry of your experiment is greatly reduced as follows (note: adjustment is very tricky and tedious):

 

wooden vertical & upper overhead support (non-metal)

6 or 8 Donut-shaped Rare Earth Magnets (suspended from above support by bolt/nut)

adjustable air-space (approx. 4-8 inches)

diamagnetic material (bismuth, carbon, pyrolytic)

(nylon spacer) -- NIB magnet levitates here -- (nylon spacer)

diamagnetic material (bismuth, carbon, pyrolytic)

wooden base (non-metal)

 

Levitation of non-SC magnets is that simple, and total cost should not exceed more than $25 USD! Goto: http://www.forcefield.com , and check-out their experiments section, under "diamagentics". They supply all types of magnets, and also have small kits for building a diamagnetic (pyrolytic) levitating train!!!

 

Problems with using SC to duplicate the above levitation experiment:

First, type-2 SC magnets can be very costly (depending on specifications) and LN2 maybe low cost but is very dangerous to handle (special equipment req'd; Dewars Flask, low-temp gloves, etc.) even if LN2 is safely contained.

Secondly, Type-2 SC require near zero temp. and the flux pinning effect may cause both unstable vert. and horiz. levitation -- unless SC is made to rotate about its axis and/or an electromagnetic control circuitry is used to stablize (see similar device used on non-SC toy called Levitron Anti-Gravity Top; available from Edmund Scientific's Spring 2005 catalog, p.89, or goto http://www.scientificsonline.com).

 

Does your application absolutely require using SC with LN2? Are you opposed to using non-SC diamagnetics at room temp. for levitation?, or are you just unfamiliar with these non-SC materials (bismuth, carbon, etc), and diamagnetic levitation procedures?

 

What is the nature of your work? Is it antigravity craft?, transportation levitation?, or alternative energy prime mover related?

 

Perhaps, I can be of some help to you.

D-Noonian :)'

[calbiterol]

In fact, it is possible to disprove Earnshaw's Law by using "diamagnetics" (e.g. bismuth, carbon, pyrolytics) by replacing your SC magent and LN2 tube [/b'] with a simple non-superconducting NIB magnet.

 

IIRC, this is untrue. You can levitate a magnet, yes, but this doesn't disprove Earnshaw's Law. IIRC. This is because the law is based on a bunch of assumptions, and that breaks some of the assumptions of the law. If the conditions upop which the law is based are false, then the law does not necessarily apply. This is one of those cases. Again, emphasis on the "if I remember correctly" part. But I'm pretty sure that's the case.

[labview1958]

I am a school teacher and a science hobbyist. Right now I am dabbling in superconductivity. However I am quite busy and I have not yet gone around to do my thought experiment. My assumption is that as the SC becomes a shield, thus moving the top magnet downwards have no effect upon the force reading. That is the superconductor shields the attraction between the upper and lower magnet. If the SC is fixed and the upper magnet is moved there is no change in the force reading. Is it a valid assumption. The magnets are much smaller than the SC.

[DQW]

No, it is not valid. You are assuming that the force on the upper magnet is independent of its position above the (fixed) SC ? What is the basis for such an assumption ?

[labview1958]

As the upper magnet is moved down, the SC prevents the magnetic field from penetrating it as the small magnet has a very weak field. Thus the upper part of the SC repels ALL the upper field lines. Thus the lower magnet is not affected by the movement of the upper magnet. Both magnets are small and weak.

[labview1958]

The basic queistion is: Can the superconductor shield the weak lower magnet from the weak upper magnet! Both magnets are much smaller then the superconductor.

[DQW]

The short answer : "yes"

[labview1958]

Can the difference register on a weighing machine which has an accuracy of 0.0001g ?

[labview1958]

The strength of the upper magnet cannot be changed by the superconductor. The imagne of the magnet becomes stronger in the superconductor as it moves towards it. As the superconductor is fixed it cannot run away. As the upper part of the SC becomes stronger the lower part of the SC remains the same as the separation distance between the SC and the lower magnet is the same. I am a bit confused. Can someone recommend a website or book that deal with my 'problem"