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    Magnetic Levitation Experiments

    Magnetic Levitation Background Information

    Definition

    Magnetic levitation (maglev) is a method by which an object is suspended with no support other than magnetic fields.

    Basics

    See also: Maglev Train

    In magnetic levitation or magnetic suspension the electromagnetic force is used to counteract the effects of the gravitational force.

    Magnetic levitation transport, or maglev, is a form of transportation that suspends, guides and propels vehicles ( especially trains ) via electromagnetic force. This method can be faster than wheeled mass transit systems, potentially reaching velocities comparable to turboprop and jet aircraft ( 900km/h, 559 mph ). The maximum recorded speed of a maglev train is 581km/h ( 361 mph ), achieved in Japan in 2003.

    Earnshaw's theorem proved conclusively that it is not possible to levitate stably using only static, macroscopic, "classical" electromagnetic fields. The forces acting on an object in any combination of gravitational, electrostatic, and magnetostatic fields will make the object's position unstable. However, several possibilities exist to make levitation viable, by violating the assumptions of the theorem — for example, the use of electronic stabilization or diamagnetic materials.

    There are several methods to obtain magnetic levitation. The primary ones used in maglev trains are servo-stabilized electromagnetic suspension (EMS), electrodynamic suspension (EDS), and (in the future) Inductrack.

    Stability methods

    If two magnets are mechanically constrained along a single vertical axis (a piece of string, for example), and arranged to repel each other strongly, this will act to levitate one of the magnets above the other. This is considered pseudo-levitation.

    Direct diamagnetic levitation: A live frog levitates inside a 32 mm diameter vertical bore of a Bitter solenoid in a magnetic field of about 16 teslas at the Nijmegen High Field Magnet Laboratory. Direct link to videoA substance which is diamagnetic repels a magnetic field. Earnshaw's theorem does not apply to diamagnets; they behave in the opposite manner of a typical magnet due to their relative permeability of μr < 1. All materials have diamagnetic properties, but the effect is very weak, and usually overcome by the object's paramagnetic or ferromagnetic properties, which act in the opposite manner. Any material in which the diamagnetic component is strongest will be repelled by a magnet, though this force is not usually very large. Diamagnetic levitation can be used to levitate very light pieces of pyrolytic graphite or bismuth above a moderately strong permanent magnet. As water is predominantly diamagnetic, this technique has been used to levitate water droplets and even live animals, such as a grasshopper and a frog; however, the magnetic fields required for this are very high, typically in the range of 16 teslas, and therefore create significant problems if ferromagnetic materials are nearby.

    Superconductors may be considered perfect diamagnets (μr = 0), completely expelling magnetic fields due to the Meissner effect. The levitation of the magnet is stabilized due to flux pinning within the superconductor. This principle is exploited by EDS (electrodynamic suspension) magnetic levitation trains.

    In trains where the weight of the large electromagnet is a major design issue (a very strong magnetic field is required to levitate a massive train) superconductors are used for the electromagnet, since they can produce a stronger magnetic field for the same weight.

    Diamagnetically-stabilized levitation: A permagnet can be stably suspended by various configurations of strong permanent magnets and strong diamagnets. When using superconducting magnets, the levitation of a permanent magnet can even be stabilized by the small diamagnetism of water in human fingers.

    Rotational stabilization: A magnet can be stabilized by spinning it in a field created by a ring of other magnets. However, it will only remain stable until the rate of precession slows below a critical threshold — the region of stability is quite narrow both spatially and in the required rate of precession. The first discovery of this phenomenon was by Roy Harrigan, a Vermont inventor who patented a levitation device in 1983 based upon it. Several devices using rotational stabilization (such as the popular Levitron toy) have been developed citing this patent. Non-commercial devices have been created for university research laboratories, generally using magnets too powerful for safe public interaction.

    Servo stabilization of electromagnetic attraction: Dynamically-stabilized magnetic levitation can be achieved by measuring the position and trajectory of the magnet being levitated, and continuously adjusting the local magnetic field to compensate for its motion.

    This is the principle in place behind common tabletop levitation demonstrations, which use a beam of light to measure the position and velocity of an object. In simple systems, an electromagnet is above the object being levitated upwards; the electromagnet is turned off whenever the object gets too close, and turned back on when it falls further away. Such a simple system is not very robust; much more complicated and effective measurement, magnetic, and control systems are, however, possible.

    This is also the principle upon which electromagnetic suspension (EMS) magnetic levitation trains are based: The train wraps around the track, and is pulled upwards from below. The servo controls keep it at a constant distance from the track.

    Difficulties

    Most of the levitation techniques have various complexities.

    • Many of the active suspension techniques have a fairly narrow region of stability.
    • Magnetic fields are conservative forces and therefore in principle have no built-in damping. This can permit vibration modes to exist that can cause the item to leave the stable region. Eddy currents can be stabilizing if a suitably shaped conductor is present in the field, and other mechanical or electronic damping techniques have been used in some cases.
    • The power requirements of electromagnets increase rapidly with load-bearing capacity, which also necessitates relative increases in conductor and cooling equipment mass and volume.
    • Superconductors require very low temperatures to operate, often helium cooling is employed.

    Uses

    Maglev, or magnetic levitation, is a system of transportation that suspends, guides and propels vehicles, predominantly trains, using magnetic levitation from a very large number of magnets for lift and propulsion. This method has the potential to be faster, quieter and smoother than wheeled mass transit systems. The technology has the potential to exceed 6,400 km/h (4,000 mi/h) if deployed in an evacuated tunnel. If not deployed in an evacuated tube the power needed for levitation is usually not a particularly large percentage and most of the power needed is used to overcome air drag, as with any other high speed train.

    The highest recorded speed of a maglev train is 581 kilometres per hour (361 mph), achieved in Japan in 2003, 6 km/h faster than the conventional TGV speed record. This is slower than many aircraft, since aircraft can fly at far higher altitudes where air drag is lower, thus high speeds are more readily attained.

    Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)

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