Jump to content

Magnetic levitation: Difference between revisions

From Wikipedia, the free encyclopedia
Browse history interactively
← Previous editNext edit →
Content deleted Content added
VisualWikitext
LR1998 (talk | contribs)
11 edits
No edit summary
LR1998 (talk | contribs)
11 edits
←Replaced content with 'my name is greedy and me comprado the englishit'
Line 1: Line 1:
my name is greedy and me comprado the englishit
{{About|all forms of magnetic levitation|trains based on this effect|Maglev}}
[[File:Diamagnetic graphite levitation.jpg|200px|thumb|right|Levitating [[pyrolytic carbon]]]]
'''Magnetic levitation''', '''maglev''', or '''magnetic suspension''' is a method by which an object is [[levitation|suspended]] with no support other than [[magnetic field]]s. [[Magnetic pressure]] is used to counteract the effects of the [[gravitational acceleration|gravitational]] and any other accelerations.

[[Earnshaw's theorem]] proves that using only [[ferromagnetism|ferromagnetic]] or [[Paramagnetism|paramagnetic materials]] it is impossible to stably levitate against gravity, but [[servomechanism]]s, the use of [[diamagnetic]] materials, [[superconduction]], or systems involving [[eddy current]]s permit this to occur.

In some cases the lifting force is provided by magnetic levitation, but there is a mechanical support bearing little load that provides stability. This is termed '''pseudo-levitation'''. Given this some car companys like volkswagon are developing hover cars in china that will have magnets that will interact with the magnetic feilds in the ground.

Magnetic levitation is used for [[maglev train]]s, [[magnetic bearing]]s and for product display purposes.

==Lift==
Magnetic materials and systems are able to attract or press each other apart or together with a force dependent on the magnetic field and the area of the magnets, and a [[magnetic pressure]] can then be defined.

The magnetic pressure of a magnetic field on a superconductor can be calculated by:

:<math>P_{mag} = \frac {B^2} {2 \mu_0}</math>

where <math>P_{mag}</math> is the force per unit area in [[pascal (unit)|pascals]], <math>B</math> is the [[magnetic field]] just above the superconductor in [[Tesla (unit)|tesla]]s, and <math>\mu_0</math> = 4π×10<sup>−7</sup>&nbsp;N·A<sup>−2</sup> is the [[permeability (electromagnetism)|permeability]] of the vacuum.<ref>Lecture 19 MIT 8.02 Electricity and Magnetism, Spring 2002</ref>

== Stability ==

===Static===
Static stability means that any small displacement away from a stable equilibrium causes a net force to push it back to the equilibrium point.

[[Earnshaw's theorem]] proved conclusively that it is not possible to levitate stably using only static, macroscopic, paramagnetic fields. The forces acting on any paramagnetic object in any combinations of [[gravitational field|gravitational]], [[electrostatic field|electrostatic]], and [[magnetostatic field]]s will make the object's position, at best, unstable along at least one axis, and it can be [[unstable equilibrium]] along all axes. However, several possibilities exist to make levitation viable, for example, the use of electronic stabilization or [[diamagnetic]] materials (since [[relative magnetic permeability]] is less than one<ref>Braunbeck, W. Free suspension of bodies in electric and magnetic fields, Zeitschrift für Physik, 112, 11, pp753-763 (1939)</ref>); it can be shown that diamagnetic materials are stable along at least one axis, and can be stable along all axes. Conductors can have a relative permeability to alternating magnetic fields of below one, so some configurations using simple AC driven electromagnets are self stable.

=== Dynamic stability ===
Dynamic stability occurs when the levitation system is able to damp out any vibration-like motion that may occur.

Magnetic fields are [[conservative force]]s and therefore in principle have no built-in damping, and in practice many of the levitation schemes are under-damped and in some cases negatively damped.<ref>A Review of Dynamic Stability of Repulsive-Force Maglev Suspension Systems- Y. Cai and D.M.Rote</ref> This can permit vibration modes to exist that can cause the item to leave the stable region.

Damping of motion is done in a number of ways:

* external mechanical damping (in the support), such as [[dashpot]]s, [[air drag]] etc.
* eddy current damping (conductive metal influenced by field)
* [[tuned mass damper]]s in the levitated object
* electromagnets controlled by electronics

== Methods ==
[[File:Levitation.JPG|thumb|right|Mechanical constraint (in this case the lateral restrictions created by a box) can permit pseudo-levitation of permanent magnets]]
For successful levitation and control of all 6 axes (3 spatial and 3 rotational) a combination of permanent magnets and electromagnets or diamagnets or superconductors as well as attractive and repulsive fields can be used. From Earnshaw's theorem at least one stable axis must be present for the system to levitate successfully, but the other axes can be stabilised using ferromagnetism.

The primary ones used in [[maglev train]]s are servo-stabilized electromagnetic suspension (EMS), electrodynamic suspension (EDS).

=== Mechanical constraint (pseudo-levitation)===

With a small amount of mechanical constraint for stability, achieving pseudo-levitation is a relatively straightforward process.

If two [[magnet]]s are mechanically constrained along a single vertical axis, for example, and arranged to repel each other strongly, this will act to levitate one of the magnets above the other.

Another geometry is where the magnets are attracted, but constrained from touching by a tensile member, such as a string or cable.

Another example is the [[Zippe-type centrifuge]] where a cylinder is suspended under an attractive magnet, and stabilized by a needle bearing from below.

=== Servomechanisms ===
[[File:Maglev june2005.jpg|thumb|The [[Transrapid]] system uses servomechanisms to pull the train up from underneath the track and maintains a constant gap while travelling at high speed]]
{{Main|Electromagnetic suspension}}
The attraction from a fixed strength magnet decreases with increased distance, and increases at closer distances. This is unstable. For a stable system, the opposite is needed, variations from a stable position should push it back to the target position.

Stable magnetic levitation can be achieved by measuring the position and [[speed]] of the object being levitated, and using a [[feedback loop]] which continuously adjusts one or more electromagnets to correct the object's motion, thus forming a [[servomechanism]].

Many systems use magnetic attraction pulling upwards against gravity for these kinds of systems as this gives some inherent lateral stability, but some use a combination of magnetic attraction and magnetic repulsion to push upwards.

Either system represents examples of ElectroMagnetic Suspension (EMS). For a very simple example, some tabletop levitation demonstrations use this principle, and the object cuts a beam of light to measure the position of the object. The electromagnet is above the object being levitated; 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; far more effective control systems exist, but this illustrates the basic idea.

EMS [[magnetic levitation train]]s are based on this kind of levitation: The train wraps around the track, and is pulled upwards from below. The [[Servomechanism|servo]] controls keep it safely at a constant distance from the track.

===Induced currents===
{{main|electrodynamic suspension}}
These schemes work due to repulsion due to [[Lenz's law]]. When a conductor is presented with a time-varying magnetic field electrical currents in the conductor are set up which create a magnetic field that causes a repulsive effect.

==== Relative motion between conductors and magnets ====
If one moves a base made of a very good electrical conductor such as [[copper]], [[aluminium]] or [[silver]] close to a magnet, an ([[eddy current|eddy]]) current will be induced in the conductor that will oppose the changes in the field and create an opposite field that will repel the magnet ([[Lenz's law]]). At a sufficiently high rate of movement, a suspended magnet will levitate on the metal, or vice versa with suspended metal. [[Litz wire]] made of wire thinner than the [[skin depth]] for the frequencies seen by the metal works much more efficiently than solid conductors.

An especially technologically interesting case of this comes when one uses a [[Halbach array]] instead of a single pole permanent magnet, as this almost doubles the field strength, which in turn almost doubles the strength of the eddy currents. The net effect is to more than triple the lift force. Using two opposed Halbach arrays increases the field even further.<ref>[https://www.llnl.gov/str/November03/Post.html]</ref>

Halbach arrays are also well-suited to magnetic levitation and stabilisation of [[gyroscope]]s and [[electric motor]] and [[electrical generator|generator]] spindles.

==== Oscillating electromagnetic fields ====
A [[electrical conductor|conductor]] can be levitated above an electromagnet (or vice versa) with an [[alternating current]] flowing through it. This causes any regular conductor to behave like a diamagnet, due to the [[eddy current]]s generated in the conductor.<ref>[http://www.classictesla.com/download/ieee_potentials_2000.pdf Eddy current magnetic levitation, models and experiments by Marc T. Thompson]</ref><ref>[http://sprott.physics.wisc.edu/demobook/chapter5.htm Levitated Ball-Levitating a 1 cm aluminum sphere]</ref> Since the eddy currents create their own fields which oppose the magnetic field, the conductive object is repelled from the electromagnet, and most of the field lines of the magnetic field will no longer penetrate the conductive object.

This effect requires non-ferromagnetic but highly conductive materials like aluminium or copper, as the ferromagnetic ones are also strongly attracted to the electromagnet (although at high frequencies the field can still be expelled) and tend to have a higher resistivity giving lower eddy currents. Again, litz wire gives the best results.

The effect can be used for stunts such as levitating a telephone book by concealing an aluminium plate within it.

At high frequencies (a few tens of kilohertz or so) and kilowatt powers small quantities of metals can be levitated and melted using [[levitation melting]] without the risk of the metal being contaminated by the crucible.<ref>"Magnetic levitation of liquid metals", Journal of Fluid Mechanics 117, pages 27-43, by A. J. Mestel</ref>

One source of oscillating magnetic field that is used is the [[linear induction motor]]. This can be used to levitate as well as provide propulsion.

===Diamagnetism===
Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the magnetic dipole moment. According to Lenz's law, this opposes the external field. Diamagnets are materials with a magnetic permeability less than μ<sub>0</sub> (a relative permeability less than 1).
Consequently, diamagnetism is a form of magnetism that is only exhibited by a substance in the presence of an externally applied magnetic field. It is generally quite a weak effect in most materials, although superconductors exhibit a strong effect.
Diamagnetic materials cause lines of magnetic flux to curve away from the material, and superconductors can exclude them completely (except for a very thin layer at the surface).

==== Direct diamagnetic levitation ====
[[File:Frog diamagnetic levitation.jpg|right|thumb|200px|A live frog levitates inside a 32 [[millimetre|mm]] [[diameter]] vertical bore of a [[Bitter solenoid]] in a magnetic field of about 16 [[tesla (unit)|teslas]]]]

A substance that is [[diamagnetic]] repels a magnetic field. All materials have diamagnetic properties, but the effect is very weak, and is 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.

[[Earnshaw's theorem]] does not apply to diamagnets. These behave in the opposite manner to normal magnets owing to their relative [[Permeability (electromagnetism)|permeability]] of ''μ''<sub>''r''</sub>&nbsp;< 1 (i.e. negative [[magnetic susceptibility]]).

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, frog and a mouse.<ref name=geim>[http://www.ru.nl/hfml/research/levitation/diamagnetic/ "The Frog That Learned to Fly"]. [[Radboud University Nijmegen]]. Retrieved 19 October 2010. For Geim's account of diamagnetic levitation, see Geim, Andrey. {{PDFlink|[http://www.ru.nl/publish/pages/561854/everyonesmagnetism.pdf "Everyone's Magnetism]|688&nbsp;KB}}. ''[[Physics Today]]''. September 1998. pp.&nbsp;36–39. Retrieved 19 October 2010. For the experiment with Berry, see [[Michael Berry (physicist)|Berry, M. V.]]; Geim, Andre. (1997). {{PDFlink|[http://www.ru.nl/publish/pages/561854/frog-ejp.pdf "Of flying frogs and levitrons"]|228&nbsp;KB}}. ''[[European Journal of Physics]]'' '''18''': 307–313. Retrieved 19 October 2010.</ref> However, the magnetic fields required for this are very high, typically in the range of 16 [[Tesla (unit)|tesla]]s, and therefore create significant problems if [[ferromagnetic]] materials are nearby.

The minimum criterion for diamagnetic levitation is <math> B \frac{dB}{dz} = \mu_0 \, \rho \, \frac{g}{\chi} </math>, where:
* <math> \chi </math> is the [[magnetic susceptibility]]
* <math> \rho </math> is the [[density]] of the material
* <math> g </math> is the local [[gravitational acceleration]] (−9.8 [[metre|m]]/[[second|s]]<sup>2</sup> on Earth)
* <math> \mu_0</math> is the [[permeability of free space]]
* <math> B </math> is the [[magnetic field]]
* <math> \frac{dB}{dz}</math> is the rate of change of the magnetic field along the vertical axis.

Assuming ideal conditions along the ''z''-direction of solenoid magnet:
* [[Water]] levitates at <math>B \frac{dB}{dz} \approx 1400\ \mathrm{T^2/m}</math>
* [[Graphite]] levitates at <math>B \frac{dB}{dz} \approx 375\ \mathrm{T^2/m}.</math>

==== Diamagnetically stabilized levitation ====

A permanent magnet 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.<!-- needs expansion --><ref>[http://netti.nic.fi/~054028/images/LeviTheory.pdf Diamagnetically stabilized magnet levitation]</ref>

[[File:Levitation superconductivity.JPG|thumb|right|A superconductor levitating a permanent magnet]]

=== Superconductors ===
{{main|Superdiamagnetism}}
[[Superconductors]] may be considered '''perfect diamagnets''' (''μ''<sub>''r''</sub>&nbsp;= 0), and have the additional property of completely expelling magnetic fields due to the [[Meissner effect]] when the superconductivity initially forms. The levitation of the magnet is further stabilized due to [[flux pinning]] within the superconductor; this tends to stop the superconductor leaving the magnetic field, even if the levitated system is inverted.

These principles are exploited by EDS (Electrodynamic Suspension), superconducting [[Magnetic bearing|bearings]], [[flywheel]]s, etc.

In trains, a very strong magnetic field is required to levitate a massive train. The [[JR–Maglev]] trains have superconducting magnetic coils, but the JR–Maglev levitation is not by the Meissner effect.

{{clear}}
[[File:Levitron-levitating-top-demonstrating-Roy-M-Harrigans-spin-stabilized-magnetic-levitation.ogg|thumb|right|Levitron is an example of spin stabilized magnetic levitation]]

=== Rotational stabilization ===
{{Main|Spin stabilized magnetic levitation}}

A magnet with a toroidal field can be levitated against gravity when [[gyroscope|gyroscopically]] stabilized by spinning it in a second toroidal field created by a base ring of magnet(s). However, it will only remain stable while the rate of [[precession]] is between both upper and lower critical thresholds—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 M. Harrigan, a Vermont inventor who patented a levitation device in 1983 based upon it.<ref>{{US patent reference|number= 4382245|y=1983|m=05|d=03| inventor= Roy M. Harrigan| title= Levitation device}}</ref> Several devices using rotational stabilization (such as the popular ''[[Levitron]]'' branded levitating top 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.
{{clear}}

===Strong focusing===
{{Main|Strong focusing}}
Earnshaw's theory strictly only applies to static fields. Alternating magnetic fields, even purely alternating attractive fields,<ref name="alt">[http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel1/20/1621/00042275.pdf?tp=&isnumber=1621&arnumber=42275&type=ref Attractive levitation for high-speed ground transport with largeguideway clearance and alternating-gradient stabilization Hull, J.R. Magnetics, IEEE Transactions on Volume 25, Issue 5, Sep 1989 Page(s):3272 - 3274 Digital Object Identifier 10.1109/20.42275]</ref> can induce stability and confine a trajectory through a magnetic field to give a levitation effect.

This is used in particle accelerators to confine and lift charged particles, and has been proposed for maglev trains also.<ref name="alt"/>

==Uses==

===Maglev transportation===
{{Main|Maglev (transport)}}
'''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&nbsp;km/h (4,000&nbsp;mi/h) if deployed in an [[vacuum|evacuated]] tunnel.<ref>http://www.popsci.com/scitech/article/2004-04/trans-atlantic-maglev</ref> 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 (physics)|drag]], as with any other high speed train.

The highest recorded speed of a maglev train is 581 kilometers per hour (361&nbsp;mph), achieved in Japan in 2003,<ref>http://www.guardian.co.uk/world/2003/dec/03/japan.justinmccurry</ref> 6&nbsp;km/h faster than the conventional [[TGV]] speed record.

===Magnetic bearings===
*[[Magnetic bearings]]
*[[Flywheel]]s
*[[Centrifuge]]s
*[[Magnetic ring spinning]]

===Levitation melting===
{{main|Levitation melting}}
'''Electromagnetic levitation''' (EML), patented by Muck in 1923,<ref>O. Muck. German patent no. 42204 (Oct. 30, 1923)</ref> is one of the oldest levitation techniques used for containerless experiments.<ref>{{citation|author=Paul C. Nordine, J. K. Richard Weber, and Johan G. Abadie|journal=Pure and Applied Chemistry|title=Properties of high-temperature melts using levitation|year=2000|volume=72|pages=2127–2136|doi=10.1351/pac200072112127}}</ref> The technique enables the [[levitation]] of an object using [[electromagnet]]s. A typical EML coil has reversed winding of upper and lower sections energized by a [[radio frequency]] power supply.

==History==
*1839 Earnshaw's theorem showed electrostatic levitation was impossible, later theorem was extended to magnetostatic levitation by others
*1912 [[Emile Bachelet]] awarded a patent in March 1912 for his “levitating transmitting apparatus” (patent no. 1,020,942) for electromagnetic suspension system
*1933 Superdiamagnetism [[Walter Meissner]] and [[Robert Ochsenfeld]] (the [[Meissner effect]])
*1934 [[Hermann Kemper]] “monorail vehicle with no wheels attached.” Reich Patent number 643316
*1939 [[Werner Braunbeck|Braunbeck]]’s extension showed that magnetic levitation is possible with diamagnetic materials
*1939 Bedford, Peer, and Tonks aluminum plate placed on two concentric cylindrical coils shows 6-axis stable levitation.<ref name=personalView>linear Electric Machines- A Personal View ERIC R. LAITHWAITE, FELLOW, IEEE, PROCEEDINGS OF THE IEEE, VOL. 63, NO. 2, FEBRUARY 1975</ref>
*1961 [[James R. Powell]] and BNL colleague [[Gordon Danby]] electrodynamic levitation using superconducting magnets
*1970s [[Spin stabilized magnetic levitation]] Roy M. Harrigan
*1974 [[Magnetic river]] [[Eric Laithwaite]] and others
*1979 [[transrapid]] train carried passengers
*1984 Low speed maglev shuttle in Birmingham Eric Laithwaite and others
*1997 Diamagnetically levitated live frog [[Andre Geim]]<ref name=geim/>
*1999 [[Inductrack]] permanent magnet electrodynamic levitation (General Atomics)
*2005 homopolar [[electrodynamic bearing]]<ref>[http://www.magnetal.se/Dokument/PhDThesis.pdf "Design and Analysis of a Novel Low Loss Homopolar Electrodynamic Bearing."] Lembke, Torbjörn. PhD Thesis. Stockholm: Universitetsservice US AB, 2005. Print. ISBN 91-7178-032-7</ref>

== See also ==
* [[Acoustic levitation]]
* [[Aerodynamic levitation]]
* [[Electrostatic levitation]]
* [[Optical levitation]]
* [[Cyclotron]]s levitate and circulate charged particles in a magnetic field
* [[Inductrack]] a particular system based on Halbach arrays and inductive track loops
* [[Launch loop]]
* [[Levitron]]
* [[Linear motor]]
* [[Linear motor#Rapid transits using linear motor propulsion|Rapid transits using linear motor propulsion]]
* [[Magnetic bearing]]
* [[Nagahori Tsurumi-ryokuchi Line]]
* [[StarTram]] is an extreme proposal for levitation via superconductors over multiple kilometers of distance
* [[Zippe-type centrifuge]] uses magnetic lift and a mechanical needle for stability
* [[Magnetic ring spinning]]

==References==
{{Reflist}}

== External links ==
{{div col|2}}
*[http://www.magnet.fsu.edu/education/community/slideshows/maglev/index.html Maglev Trains] Audio slideshow from the National High Magnetic Field Laboratory discusses magnetic levitation, the Meissner Effect, magnetic flux trapping and superconductivity
* [http://www.levitationfun.com/index.html Magnetic Levitation - Science is Fun]
* [http://www.youtube.com/watch?v=nWTSzBWEsms&feature=related Magnetic (superconducting) levitation experiment (YouTube)]
* [http://users.bigpond.net.au/com/maglevvideogallery/ Maglev video gallery]
* [http://my.execpc.com/~rhoadley/maglev.htm How can you magnetically levitate objects?]
* [http://sprott.physics.wisc.edu/demobook/chapter5.htm Levitated aluminum ball (oscillating field)]
* [http://www.coilgun.info/levitation/home.htm Instructions to build an optically triggered feedback maglev demonstration]
* [http://www.hfml.sci.kun.nl/levitation-movies.html Videos of diamagnetically levitated objects, including frogs and grasshoppers]
* [http://www.larryspring.com/class_motors.html Larry Spring's Mendocino Brushless Magnetic Levitation Solar Motor]
* [http://arxiv.org/abs/0803.3090 A Classroom Demonstration of Levitation...]
* [http://www.youtube.com/watch?v=Y_WG4YStMxs&feature=related 25kg MAGLEV suspension setup]
* [http://www.youtube.com/watch?v=kXodf7WKiFs 25kg MAGLEV suspension control via Classical control strategy]
* [http://www.youtube.com/watch?v=TsgoF13KvYk&feature=related 25kg MAGLEV suspension via State feedback control strategy]
* [http://www.physics.org/facts/frog-really.asp Frogs levitate in a strong enough magnetic field]
{{div col end}}

{{DEFAULTSORT:Magnetic Levitation}}
[[Category:Magnetism]]
[[Category:Magnetic levitation| ]]

Revision as of 12:15, 18 April 2013

my name is greedy and me comprado the englishit

Retrieved from "https://wiki.riteme.site/w/index.php?title=Magnetic_levitation&oldid=550965058"