Brief History of Magnetic Levitation

In the early 1900s, Emile Bache let first conceived of a magnetic suspension using repulsive forces generated by alternating currents. Bache let’s ideas for EDS remained dormant until the 1960s when superconducting magnets became available, because his concept used too much power for conventional conductors. In 1922, Hermann Kemper in Germany pioneered attractive-mode (EMS) Maglev and received a patent for magnetic levitation of trains in 1934.

In 1939-43, the Germans first worked on a real train at the ATE in Goettingen. The basic design for practical attractive-mode maglev was presented by Kemper in 1953. The Tran rapid (TR01) was built in 1969.Maglev development in the U. S. began as a result of the the High-Speed Ground Transportation (HSGT) Act of 1965.

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This act authorized Federal funding for HSGT projects, including rail, air cushion vehicles, and Maglev. This government largesse gave the U. S. researchers an early advantage over their foreign counterparts. Americans pioneered the concept of superconducting magnetic levitation (EDS,) and they dominated early experimental research. As early as 1963, James Powell and Gordon Danby of Brookhaven National Laboratory realized that superconductivity could get around the problems of Bache let’s earlier concepts.

In 1966, Powell and Danby presented their Maglev concept of using superconducting magnets in a vehicle and discrete coils on a guide way. Powell and Danby were awarded a patent in 1968, and their work was eventually adopted by the Japanese for use in their system. Powell and Danby were awarded the 2000 Benjamin Franklin Medal in Engineering by the Franklin Institute for their work on EDS Maglev.In 1969, groups from Stanford, Atomics International and Sandia developed a continuous-sheet guide way (CSG) concept.

In this system, the moving magnetic fields of the vehicle magnets induce currents in a continuous sheet of conducting material such as aluminum. Several groups, including MIT (Kolm and Thornton, MIT, 1972,) built 1/25th scale models and tested them at speeds up to 27 m/s (97.2 km/h.) The CSG concept is alive and well in 2001 with the Magplane.

If a small magnet is brought near a superconductor, it will be repelled because induced super currents will produce mirror images of each pole. If a small permanent magnet is placed above a superconductor, it can be levitated by this repulsive force. The black ceramic material is a sample of the yttrium based superconductor.By tapping with a sharp instrument, the suspended magnet can be caused to oscillate or rotate.

This motion is found to be damped, and will come to rest in a few seconds.The Meissner effect in superconductors like this black ceramic yttrium based superconductor acts to exclude magnetic fields from the material. Since the electrical resistance is zero, super currents are generated in the material to exclude the magnetic fields from a magnet brought near it. The currents which cancel the external field produce magnetic poles which mirror the poles of the permanent magnet, repelling them to provide the lift to levitate the magnet.

The levitation process is quite remarkable. Since the levitating currents in the superconductor meet no resistance, they can adjust almost instantly to maintain the levitation. The suspended magnet can be moved, put into oscillation, or even spun rapidly and the levitation currents will adjust to keep it in suspension.

Levitating pyrolytic graphiteThere are some materials that are more diamagnetic than bismuth. These include superconductors (which at this time require cryogenic temperatures to work), and similar materials that exhibit “giant diamagnetism” (also at very low temperatures).But there is one material that is more diamagnetic than bismuth at room temperature, at least in one direction. That material is called pyrolytic graphite.

Pyrolytic graphite is a synthetic material, made by a process called chemical vapor deposition. To make pyrolytic graphite, methane gas at low pressure (about 1 Torr) is heated to 2000 degrees Celsius. Very slowly, (one thousandth of an inch per hour) a layer of graphite grows. The graphite made this way is very highly ordered, and the layers of carbon atoms form like a crystal of hexagonal sheets.

These sheets lie on top of one another like sheets of mica. You can separate the layers with a sharp knife to make thinner sheets. Pyrolytic graphite is more diamagnetic than bismuth, but only in the direction perpendicular to the sheets of carbon. In other directions, it is still diamagnetic, but not as good as bismuth.

With a piece half a millimeter thick, using neodymium-iron-boron super magnets, you can see from the photos that the piece is levitating about a millimeter above the magnets. To make the pyrolytic graphite plate sit still above the magnet, we need a way to force it towards the center. We can do that by using four magnets. The poles of the magnets push on the diamagnetic material more strongly than other parts of the magnet.

With four magnets, the four edges of the square of pyrolytic graphite will be pushed away from the four poles. If the square is slightly smaller than half the width of the four magnets (a little smaller than one magnet), then we can place it in the center, and it will be pushed to the middle and stay. Since diamagnetic materials are repelled by either pole, we can place the magnets with alternating north and south poles, and they will stick nicely to one another. I like to sit the whole array on a piece of sheet steel, so the magnets stay put.

The pyrolytic graphite plate floats above the magnets and springs back when you push it down with a finger. Since pyrolytic graphite is a little more diamagnetic than bismuth, it makes a great substitute for bismuth in the levitating magnet project.Place the blade carefully in the middle of the edge of the graphite. Slowly push the blade in with a slight rocking motion.

The graphite will make a nice clean sound as it starts to split. Sometimes you will get one thin piece and one thicker piece after they are split. You can often split the thicker piece again, giving you three pieces. If you are very skilled, you can get four pieces, but you will break a few gaining that skill.

Lastly, once the slices are very thin, you can cut them in half by rocking the sharp knife over the middle of each one. The pieces will snap and may fly some distance unless you put a finger over them to hold them down. The thick graphite is too heavy to float on the magnets. The nice thin sheets you split it into will float, and the thinnest ones will float highest.

How can you magnetically levitate objects?Magnetism is fascinating, especially when it is used to cause objects to levitate or float or be suspended in the air, defying the gravity which keeps us on the ground.  How can this be done?  There are 10 ways to magnetically levitate objects:

  • Repulsion between like poles of permanent magnets or electromagnets.   However, there needs to be a way to constrain the magnets so they don’t flip over and become attracted to each other. For example, floating donut magnets have the dowel rod in the center to keep them from flipping over.
  • Repulsion between a magnet and a metallic conductor induced by relative motion.  However, the magnet needs to be restrained from moving in the same direction as the conductor, otherwise it will travel with the conductor.
  • Repulsion between a metallic conductor and an AC electromagnet.  It is possible to shape the magnetic field to keep the conductor constrained in its motions; otherwise, a mechanical means is needed to keep the conductor in place.
  • Repulsion between a magnetic field and a diamagnetic substance.  This is the case of the floating frog, and the floating magnet between two diamagnetic disks.
  • Repulsion between a magnet and a superconductor. No mechanical constraints are needed for this.
  • Attraction between unlike poles of permanent magnets or electromagnets.  This will work as long as there is a mechanical method to constrain the magnets so they don’t touch.
  • Attraction between the open core of an electromagnetic solenoid and a piece of iron or a magnet.  The iron or magnet will touch the inside surface of the solenoid.
  • Attraction between a permanent magnet or electromagnet and a piece of iron.  Again, the iron needs to be constrained.
  • Attraction between an electromagnet and a piece of iron or a magnet, with sensors and active control of the current to the electromagnet used to maintain some distance between them.
  • Repulsion between and electromagnet and a magnet, with sensors and active control of the current to the electromagnet used to maintain some distance between them.

The stable levitation of magnets is forbidden by Earn Shaw’s theorem, which statesthat no stationary object made of magnets in a fixed configuration can be held instable equilibrium by any combination of static magnetic or gravitational forces,.Earn Shaw’s theorem can be viewed as a consequence of the Maxwell equations, whichdo not allow the magnitude of a magnetic field in a free space to possess a maximum,as required for stable equilibrium.

Diamagnetism (which respond to magnetic fields withmild repulsion) are known to flout the theorem, as their negative susceptibility resultsin the requirement of a minimum rather than a maximum in the field’s magnitude,Nevertheless, levitation of a magnet without using superconductors is widely thoughtto be impossible. We find that the stable levitation of a magnet can be achieved usingthe feeble diamagnetism of materials that are normally perceived as beingnon-magnetic, so that even human fingers can keep a magnet hovering in mid-airwithout touching it.

Earn Shaw TheoremThe proof of Earn Shaw’s theorem is very simple if you understand some basic vector calculus. The static force as a function of position F(x) acting on any body in vacuum due to gravitation, electrostatic and magneto static fields will always be divergence less.

Because of the small distances, quantum effects are significant but Earn Shaw’s theorem assumes that only classical physics is relevant.Feedback: If you can detect the position of an object in space and feed it into a control system which can vary the strength of electromagnets which are acting on the object, it is not difficult to keep it levitated. You just have to program the system to weaken the strength of the magnet whenever the object approaches it and strengthen when it moves away. You could even do it with movable permanent magnets.

These methods violate the assumption of Earn Shaw’s theorem that the magnets are fixed. Electromagnetic suspension is one system used in magnetic levitation trains (maglev) such as the one at Birmingham airport, England. It is also possible to buy gadgets which levitate objects in this way.Diamagnetism: It is possible to levitate superconductors and other diamagnetic materials.

This is also used in maglev trains. It has become common place to see the new high temperature superconducting materials levitated in this way. A superconductor is perfectly diamagnetic which means it expels a magnetic field. Other diamagnetic materials are common place and can also be levitated in a magnetic field if it is strong enough.

Water droplets and even frogs have been levitated in this way at a magnetic laboratory in the Netherlands (Physics World, April 1997).Earn Shaw’s theorem does not apply to diamagnetic as they behave like “anti-magnets”: they align ANTI-parallel to magnetic lines while the magnets meant in the theorem always try to align in parallel. In diamagnetic, electrons adjust their trajectories to compensate the influence of the external magnetic field and these results in an induced magnetic field which is directed in the opposite direction. It means that the induced magnetic moment is ant parallel to the external field.

Superconductors are diamagnetic with the macroscopic change in trajectories (screening current at the surface). The frog is another example but the electron orbits are changed in every molecule of its body.MethodsThere are several methods to obtain magnetic levitation. The primary ones used in maglev trains are servo-stabilized electromagnetic suspension (EMS), electrodynamics suspension (EDS), and Induct rack.

At some critical velocity the induced magnetic field is strong enough to induce levitation over a series of such loops. The Halbach arrays can be placed in a stable configuration and installed in, for example, a train cart.The Induct rack maglev train system avoids the problems inherent in both the EMS and EDS systems, especially failsafe suspension. It uses only permanent magnets — in a Halbach array mounted in the train cart — and empowered conductive loops installed in the track to provide levitation.

The only requirement for levitation is that the train must already be moving at a few kilometers per hour (roughly the same as walking speed) to keep levitating.The electric current induced in the loop conductors in the track drains energy from the motion of the train (called “magnetic drag”), but efficiency is still good, and no active electronics or cryogenics for superconductors are needed.

Thermodynamics Thermodynamics subtype is electromagnetism and its further subtype leading to Magneto dynamics. As can be seen from this example: The interaction of the electromagnetic fields with various media (liquid and solid metals, liquid semiconductors, plasmas, electrolytes, Ferro fluids) occurs by means of various forces, including Lorentz, Kelvin, and diamagnetic forces.

This allows to control, process, manipulate materials, and to affect their microstructure. Examples of the action of various forces include magnetic levitation of electrically conducting and non-conducting fluids, melting, stirring, pumping, stabilization of melts, free surfaces and interfaces, etc. EPM is involved in the production of metals and alloys (e.g. aluminum, steel, titanium and magnesium alloys), ceramics and glasses of highest purity, semiconductors (Si, GaAs, CdTe), and in efficient control of production of nano-scale metallic and ceramic powders, Ferro fluids for medical and engineering applications, laser welding, etc. Solidification occurs in a wide range of industrial applications, including crystal growth and casting.

The understanding of the solidification processes heavily relies on thermodynamics for describing heat transfer and phase transition phenomena as well as on magneto hydrodynamics for accounting for fluid flows and the appearance of convective instabilities and turbulence. The goal is to better understand the parameters that affect solidification, in particular in relation to external electromagnetic fields or mechanical perturbations, the formation of the mushy zone, and its effect on the microstructure of materials.

Fundamental studies on solidification include model experiments and numerical simulation. The goal is to improve the understanding of free surface and interface instabilities with the aim of controlling the behavior of surfaces of electrically conducting fluids. This includes modeling and experimental work on the stabilization of interfaces using external fields. The numerical description of levitation melting, in which a piece of metal is being simultaneously levitated and melted by the magnetic field generated by a high frequency current is also of particular interest.

References

  1. Moon, Francis C. (1994). Superconducting Levitation Applications to Bearings and Magnetic Transportation.
  2. Braun beck, W. Free suspension of bodies in electric and magnetic fields, Zeitschrift für Physik, 112, 11, pp753-763 (1939)Brandt, Science, Jan 1989·

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