Diamagnetism: Difference between revisions
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[[Image:Diamagnetic graphite levitation.jpg|200px|thumb|right|Levitating [[pyrolytic carbon]]]] |
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'''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]], these electrons will oppose the magnetic field ''changes'' provided by the applied field, preventing them from building up. The result is that lines of [[magnetic flux]] curve away from the material. In most materials, diamagnetism is a weak effect, but under conditions of [[superconductivity]], which only some materials may obtain, a strong [[quantum mechanics|quantum]] effect can emerge wherein the lines are completely blocked, excluding a very thin layer at the material's surface. Diamagnets are materials with a [[Permeability (electromagnetism)|magnetic permeability]] less than <math>\mu_0</math> (a relative permeability less than 1). |
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== History == |
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In 1778, [[Sebald Justinus Brugmans]] was the first individual to observe that [[bismuth]] and [[antimony]] were repelled by magnetic fields. However, the term ''diamagnetism'' was coined by [[Michael Faraday]] in September 1845, when he realized that all materials in nature possessed some form of diamagnetic response to an applied magnetic field. |
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== Diamagnetic materials == |
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{|class="wikitable sortable" style="float:left; clear:right; margin:20px" width="200px" |
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|+Notable diamagnetic materials<ref name="hyper">{{cite web|url=http://hyperphysics.phy-astr.gsu.edu/Hbase/tables/magprop.html|title=Magnetic Properties of Solids|last=Nave|first=Carl L.|work=Hyper Physics|accessdate=2008-11-09}}</ref> |
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!Material!! χ<sub>v</sub> (10<sup>−5</sub>) |
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|- |
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|Superconductor |
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| −10<sup>5</sup> |
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|- |
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|[[Pyrolytic carbon]] |
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| −40.0 |
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|- |
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|[[Bismuth]] |
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| −16.6 |
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|- |
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|[[Mercury (element)|Mercury]] |
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| −2.9 |
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|- |
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|[[Silver]] |
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| −2.6 |
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|- |
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|[[Diamond|Carbon (diamond)]] |
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| −2.1 |
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|- |
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|[[Lead]] |
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| −1.8 |
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|- |
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|[[Graphite|Carbon (graphite)]] |
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| −1.6 |
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|- |
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|[[Copper]] |
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| −1.0 |
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|- |
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|[[Water]] |
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| −0.91 |
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|} |
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Diamagnetism is a very general phenomenon, because all [[electrons]], including the electrons of an atom, will always make a weak contribution to the material's response. However, for materials that show some other form of magnetism (such as [[ferromagnetism]] or [[paramagnetism]]), the diamagnetism is completely overpowered. Substances that mostly display diamagnetic behaviour are termed diamagnetic materials, or diamagnets. Materials that are said to be diamagnetic are those that are usually considered by non-physicists to be ''non-magnetic'', and include [[Water (properties)|water]], [[wood]], most organic compounds such as petroleum and some plastics, and many metals including [[copper]], particularly the heavy ones with many [[core electrons]], such as [[mercury (element)|mercury]], [[gold]] and [[bismuth]]. The magnetic susceptibility of various molecular fragments are called [[Pascal's constants]]. |
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Diamagnetic materials have a relative magnetic permeability that is less than or equal to 1, and therefore a [[magnetic susceptibility]] which is less than 0 since susceptibility is defined as χ<sub>v</sub> = μ<sub>v</sub> − 1. This means that diamagnetic materials are repelled by magnetic fields. However, since diamagnetism is such a weak property its effects are not observable in everyday life. For example, the magnetic susceptibility of diamagnets such as water is χ<sub>v</sub> = −9.05×10<sup>−6</sup>. The most strongly diamagnetic material is [[bismuth]], χ<sub>v</sub> = −1.66×10<sup>−4</sup>, although [[pyrolytic carbon]] may have a susceptibility of χ<sub>v</sub> = −4.00×10<sup>−4</sup> in one plane. Nevertheless, these values are orders of magnitudes smaller than the magnetism exhibited by paramagnets and ferromagnets. Note that because χ<sub>v</sub> is derived from the ratio of the internal magnetic field to the applied field, it is a dimensionless value. |
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All conductors exhibit an effective diamagnetism when they experience a changing magnetic field. The [[Lorentz force]] on electrons causes them to circulate around forming [[eddy currents]]. The eddy currents then produce an induced magnetic field which opposes the applied field, resisting the conductor's motion. |
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[[Image:Superconductor.GIF|thumb|right|A [[superconductor]] acts as an essentially perfect diamagnetic material when placed in a magnetic field and it excludes the field, and the flux lines avoid the region]] |
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[[Superconductors]] may be considered to be perfect diamagnets (χ<sub>v</sub> = −1), since they expel all fields (except in a thin surface layer) due to the [[Meissner effect]]. However this effect is not due to eddy currents, as in ordinary diamagnetic materials (see the article on [[superconductivity]]). |
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== Demonstrations of diamagnetism == |
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=== Curving water surfaces === |
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If a powerful magnet (such as a [[supermagnet]]) is covered with a layer of water (that is thin compared to the diameter of the magnet) then the field of the magnet significantly repels the water. This causes a slight dimple in the water's surface that may be seen by its reflection.<ref>{{cite web|url=http://amasci.com/amateur/neodymium.html#water |title=Neodymium supermagnets: Some demonstrations—Diamagnetic water |last1=Beatty |first1=Bill |work=Science Hobbyist|year=2005|accessdate=September 2011}}</ref><ref>{{cite web|url=http://quit007.deviantart.com/gallery/23787987#/d2e4dmz |title=Magnetic Force |work=DeviantART |author=Quit007|year=2011|accessdate=September 2011}}</ref> |
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=== Diamagnetic levitation ===<!-- This section is linked from [[Magnetic levitation]] --> |
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{{main|Magnetic levitation#Diamagnetism}} |
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[[Image:Frog diamagnetic levitation.jpg|left|thumb|200px|A live frog levitates inside a 32 mm diameter vertical bore of a [[Bitter solenoid]] in a magnetic field of about 16 [[tesla (unit)|teslas]] at the Nijmegen High Field Magnet Laboratory.<ref>{{cite web|url=http://www.ru.nl/hfml/research/levitation/diamagnetic/ |title=The Frog That Learned to Fly |work=High Field Laboratory |publisher=[[Radboud University Nijmegen]] |year=2011 |accessdate=September 2011}}</ref> ]] |
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Diamagnets may be levitated in stable equilibrium in a magnetic field, with no power consumption. [[Earnshaw's theorem]] seems to preclude the possibility of static magnetic levitation. However, Earnshaw's theorem only applies to objects with positive moments, such as ferromagnets (which have a permanent positive moment) and paramagnets (which induce a positive moment). These are attracted to field maxima, which do not exist in free space. Diamagnets (which induce a negative moment) are attracted to field minima, and there can be a field minimum in free space. |
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A thin slice of [[pyrolytic graphite]], which is an unusually strong diamagnetic material, can be stably floated in a magnetic field, such as that from [[Rare-earth magnet|rare earth]] permanent magnets. This can be done with all components at room temperature, making a visually effective demonstration of diamagnetism. |
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The [[Radboud University Nijmegen]], the [[Netherlands]], has conducted experiments where water and other substances were successfully levitated. Most spectacularly, a live frog (see figure) was levitated.<ref>{{cite web|url=http://www.ru.nl/hfml/research/levitation/diamagnetic/ |title=The Real Levitation |work=High Field Laboratory |publisher=[[Radboud University Nijmegen]] |year=2011 |accessdate=September 2011}}</ref> |
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In September 2009, NASA's Jet Propulsion Laboratory in Pasadena, California announced they had successfully levitated mice using a [[superconducting magnet]],<ref>{{cite journal|last=Liu|first=Yuanming|last2=Zhu|first2=Da-Ming|last3=Strayer|first3=Donald M.|last4=Israelsson|first4=Ulf E.|title=Magnetic levitation of large water droplets and mice|journal=[[Advances in Space Research]]|volume=45|issue=1|pages=208–213|year=2010|doi=10.1016/j.asr.2009.08.033|bibcode = 2010AdSpR..45..208L }}</ref> an important step forward since mice are closer biologically to humans than frogs.<ref>{{cite web |url=http://www.livescience.com/animals/090909-mouse-levitation.html |title=Mice levitated in lab |last1=Choi |first1=Charles Q. |work=Live Science |date=09-09-2009 |accessdate=September 2011}}</ref> They hope to perform experiments regarding the effects of microgravity on bone and muscle mass. |
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Recent experiments studying the growth of protein crystals has led to a technique using powerful magnets to allow growth in ways that counteract Earth's gravity.<ref>{{cite web|last1=Kleiner |first1=Kurt |url=http://www.newscientist.com/article/dn12467-magnetic-gravity-trick-grows-perfect-crystals.html |title=Magnetic gravity trick grows perfect crystals |work=[[New Scientist]] |date=08-10-2007 |accessdate=September 2011}}</ref> |
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A simple homemade device for demonstration can be constructed out of bismuth plates and a few permanent magnets that will levitate a permanent magnet.<ref>{{cite web |url=http://web.archive.org/web/20080212011654/http://www.fieldlines.com/other/diamag1.html |title=Fun with diamagnetic levitation |publisher=ForceField |date=02-12-2008 |accessdate=September 2011}}</ref> |
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== Theory of diamagnetism == |
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The [[Bohr–van Leeuwen theorem]] proves that there cannot be any diamagnetism or paramagnetism in a purely classical system. Yet the classical theory for Langevin diamagnetism gives the same prediction as the quantum theory.<ref name=Kittel>{{cite book |last = Kittel |first = Charles |author-link=Charles Kittel |title = Introduction to Solid State Physics |publisher = [[John Wiley & Sons]] |edition = 6th |year = 1986 |pages= 299–302|isbn = 0-471-87474-4}}</ref> The classical theory is given below. |
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=== Langevin diamagnetism === |
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The Langevin theory of diamagnetism applies to materials containing atoms with closed shells (see [[dielectrics]]). A field with intensity <math>\scriptstyle B</math>, applied to an [[electron]] with charge <math>\scriptstyle e</math> and mass <math>\scriptstyle m</math>, gives rise to [[Larmor precession]] with frequency <math>\scriptstyle \omega \;=\; \frac{e B}{2 m}</math>. The number of revolutions per unit time is <math>\scriptstyle \frac{1}{2\pi}\omega</math>, so the current for an atom with <math>\scriptstyle Z</math> electrons is (in [[SI units]])<ref name=Kittel/> |
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:<math> I = -\frac{Ze^2B}{4 \pi m}.</math> |
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The [[magnetic moment]] of a current loop is equal to the current times the area of the loop. Suppose the field is aligned with the <math>\scriptstyle z</math> axis. The average loop area can be given as <math>\scriptstyle \pi\left\langle\rho^2\right\rangle</math>, where <math>\scriptstyle \left\langle\rho^2\right\rangle</math> is the mean square distance of the [[electrons]] perpendicular to the <math>\scriptstyle z</math> axis. The [[magnetic moment]] is therefore |
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:<math> \mu = -\frac{Ze^2B}{4 m}\langle\rho^2\rangle.</math> |
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If the distribution of charge is spherically symmetric, we can suppose that the distribution of <math>\scriptstyle x,\, y,\, z</math> coordinates are [[independent and identically distributed]]. Then <math>\scriptstyle \left\langle x^2 \right\rangle \;=\; \left\langle y^2 \right\rangle \;=\; \left\langle z^2 \right\rangle \;=\; \frac{1}{3}\left\langle r^2 \right\rangle</math>, where <math>\scriptstyle \left\langle r^2 \right\rangle</math> is the mean square distance of the electrons from the nucleus. Therefore <math>\scriptstyle \left\langle \rho^2 \right\rangle \;=\; \left\langle x^2\right\rangle \;+\; \left\langle y^2 \right\rangle \;=\; \frac{2}{3}\left\langle r^2 \right\rangle</math>. If <math>N</math> is the number of atoms per unit volume, the diamagnetic [[magnetic susceptibility|susceptibility]] is |
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:<math>\chi = \frac{\mu_0 N \mu}{B} = -\frac{\mu_0 N Z e^2}{6 m}\langle r^2\rangle.</math> |
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=== Diamagnetism in metals === |
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The Langevin theory does not apply to [[metals]] because they have non-localized electrons. The theory for the diamagnetism of a free electron gas is called Landau diamagnetism, and instead considers the weak counter-acting field that forms when their trajectories are curved due to the [[Lorentz force]]. Landau diamagnetism, however, should be contrasted with [[Paramagnetism#Delocalization|Pauli paramagnetism]], an effect associated with the polarization of delocalized electrons' spins.<ref name="ntnu">{{cite web|url=http://phy.ntnu.edu.tw/~changmc/Teach/SS/SS_note/chap11.pdf|title=Diamagnetism and paramagnetism|work=NTNU lecture notes|last=Chang|first=M. C.|accessdate=2011-02-24}}</ref> |
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== See also == |
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*[[Ferromagnetism]] |
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*[[Magnetochemistry]] |
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*[[Paramagnetism]] |
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== References == |
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{{reflist}} |
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== External links == |
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* [http://www.youtube.com/watch?v=8tFsrGRwOOM Video of a museum-style magnetic elevation train model which makes use of diamagnetism] |
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* [http://www.ru.nl/hfml/research/levitation/diamagnetic/ Videos of frogs and other diamagnets levitated in a strong magnetic field] |
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* [http://www.grand-illusions.com/images/articles/toyshop/diamagnetic_levitation_2/diamagnetic_levitation_2.wmv Video of levitating pyrolytic graphite] |
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* [http://www.science.tv/watch/e257e44aa9d5bade97ba/liquid-nitrogen-and-superconductor Video of Meissner-Ochsenfeld effect involving liquid nitrogen] |
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* [http://netti.nic.fi/~054028/images/LevitorMK1.0-1.mpg Video of a piece of neodymium magnet levitating between blocks of bismuth.] |
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** [http://netti.nic.fi/~054028/ Website about this device, with images (in Finnish).] |
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{{magnetic states}} |
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[[Category:Electric and magnetic fields in matter]] |
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[[Category:Levitation]] |
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[[Category:Magnetism]] |
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Revision as of 14:50, 10 January 2012
![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/c9/Diamagnetic_graphite_levitation.jpg/200px-Diamagnetic_graphite_levitation.jpg)
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, these electrons will oppose the magnetic field changes provided by the applied field, preventing them from building up. The result is that lines of magnetic flux curve away from the material. In most materials, diamagnetism is a weak effect, but under conditions of superconductivity, which only some materials may obtain, a strong quantum effect can emerge wherein the lines are completely blocked, excluding a very thin layer at the material's surface. Diamagnets are materials with a magnetic permeability less than (a relative permeability less than 1).
History
In 1778, Sebald Justinus Brugmans was the first individual to observe that bismuth and antimony were repelled by magnetic fields. However, the term diamagnetism was coined by Michael Faraday in September 1845, when he realized that all materials in nature possessed some form of diamagnetic response to an applied magnetic field.
Diamagnetic materials
Material | χv (10−5) |
---|---|
Superconductor | −105 |
Pyrolytic carbon | −40.0 |
Bismuth | −16.6 |
Mercury | −2.9 |
Silver | −2.6 |
Carbon (diamond) | −2.1 |
Lead | −1.8 |
Carbon (graphite) | −1.6 |
Copper | −1.0 |
Water | −0.91 |
Diamagnetism is a very general phenomenon, because all electrons, including the electrons of an atom, will always make a weak contribution to the material's response. However, for materials that show some other form of magnetism (such as ferromagnetism or paramagnetism), the diamagnetism is completely overpowered. Substances that mostly display diamagnetic behaviour are termed diamagnetic materials, or diamagnets. Materials that are said to be diamagnetic are those that are usually considered by non-physicists to be non-magnetic, and include water, wood, most organic compounds such as petroleum and some plastics, and many metals including copper, particularly the heavy ones with many core electrons, such as mercury, gold and bismuth. The magnetic susceptibility of various molecular fragments are called Pascal's constants.
Diamagnetic materials have a relative magnetic permeability that is less than or equal to 1, and therefore a magnetic susceptibility which is less than 0 since susceptibility is defined as χv = μv − 1. This means that diamagnetic materials are repelled by magnetic fields. However, since diamagnetism is such a weak property its effects are not observable in everyday life. For example, the magnetic susceptibility of diamagnets such as water is χv = −9.05×10−6. The most strongly diamagnetic material is bismuth, χv = −1.66×10−4, although pyrolytic carbon may have a susceptibility of χv = −4.00×10−4 in one plane. Nevertheless, these values are orders of magnitudes smaller than the magnetism exhibited by paramagnets and ferromagnets. Note that because χv is derived from the ratio of the internal magnetic field to the applied field, it is a dimensionless value.
All conductors exhibit an effective diamagnetism when they experience a changing magnetic field. The Lorentz force on electrons causes them to circulate around forming eddy currents. The eddy currents then produce an induced magnetic field which opposes the applied field, resisting the conductor's motion.
Superconductors may be considered to be perfect diamagnets (χv = −1), since they expel all fields (except in a thin surface layer) due to the Meissner effect. However this effect is not due to eddy currents, as in ordinary diamagnetic materials (see the article on superconductivity).
Demonstrations of diamagnetism
Curving water surfaces
If a powerful magnet (such as a supermagnet) is covered with a layer of water (that is thin compared to the diameter of the magnet) then the field of the magnet significantly repels the water. This causes a slight dimple in the water's surface that may be seen by its reflection.[2][3]
Diamagnetic levitation
![](http://upload.wikimedia.org/wikipedia/commons/thumb/7/7b/Frog_diamagnetic_levitation.jpg/200px-Frog_diamagnetic_levitation.jpg)
Diamagnets may be levitated in stable equilibrium in a magnetic field, with no power consumption. Earnshaw's theorem seems to preclude the possibility of static magnetic levitation. However, Earnshaw's theorem only applies to objects with positive moments, such as ferromagnets (which have a permanent positive moment) and paramagnets (which induce a positive moment). These are attracted to field maxima, which do not exist in free space. Diamagnets (which induce a negative moment) are attracted to field minima, and there can be a field minimum in free space.
A thin slice of pyrolytic graphite, which is an unusually strong diamagnetic material, can be stably floated in a magnetic field, such as that from rare earth permanent magnets. This can be done with all components at room temperature, making a visually effective demonstration of diamagnetism.
The Radboud University Nijmegen, the Netherlands, has conducted experiments where water and other substances were successfully levitated. Most spectacularly, a live frog (see figure) was levitated.[5]
In September 2009, NASA's Jet Propulsion Laboratory in Pasadena, California announced they had successfully levitated mice using a superconducting magnet,[6] an important step forward since mice are closer biologically to humans than frogs.[7] They hope to perform experiments regarding the effects of microgravity on bone and muscle mass.
Recent experiments studying the growth of protein crystals has led to a technique using powerful magnets to allow growth in ways that counteract Earth's gravity.[8]
A simple homemade device for demonstration can be constructed out of bismuth plates and a few permanent magnets that will levitate a permanent magnet.[9]
Theory of diamagnetism
The Bohr–van Leeuwen theorem proves that there cannot be any diamagnetism or paramagnetism in a purely classical system. Yet the classical theory for Langevin diamagnetism gives the same prediction as the quantum theory.[10] The classical theory is given below.
Langevin diamagnetism
The Langevin theory of diamagnetism applies to materials containing atoms with closed shells (see dielectrics). A field with intensity , applied to an electron with charge and mass , gives rise to Larmor precession with frequency . The number of revolutions per unit time is , so the current for an atom with electrons is (in SI units)[10]
The magnetic moment of a current loop is equal to the current times the area of the loop. Suppose the field is aligned with the axis. The average loop area can be given as , where is the mean square distance of the electrons perpendicular to the axis. The magnetic moment is therefore
If the distribution of charge is spherically symmetric, we can suppose that the distribution of coordinates are independent and identically distributed. Then , where is the mean square distance of the electrons from the nucleus. Therefore . If is the number of atoms per unit volume, the diamagnetic susceptibility is
Diamagnetism in metals
The Langevin theory does not apply to metals because they have non-localized electrons. The theory for the diamagnetism of a free electron gas is called Landau diamagnetism, and instead considers the weak counter-acting field that forms when their trajectories are curved due to the Lorentz force. Landau diamagnetism, however, should be contrasted with Pauli paramagnetism, an effect associated with the polarization of delocalized electrons' spins.[11]
See also
References
- ^ Nave, Carl L. "Magnetic Properties of Solids". Hyper Physics. Retrieved 2008-11-09.
- ^ Beatty, Bill (2005). "Neodymium supermagnets: Some demonstrations—Diamagnetic water". Science Hobbyist. Retrieved September 2011.
{{cite web}}
: Check date values in:|accessdate=
(help) - ^ Quit007 (2011). "Magnetic Force". DeviantART. Retrieved September 2011.
{{cite web}}
: Check date values in:|accessdate=
(help)CS1 maint: numeric names: authors list (link) - ^ "The Frog That Learned to Fly". High Field Laboratory. Radboud University Nijmegen. 2011. Retrieved September 2011.
{{cite web}}
: Check date values in:|accessdate=
(help) - ^ "The Real Levitation". High Field Laboratory. Radboud University Nijmegen. 2011. Retrieved September 2011.
{{cite web}}
: Check date values in:|accessdate=
(help) - ^ Liu, Yuanming; Zhu, Da-Ming; Strayer, Donald M.; Israelsson, Ulf E. (2010). "Magnetic levitation of large water droplets and mice". Advances in Space Research. 45 (1): 208–213. Bibcode:2010AdSpR..45..208L. doi:10.1016/j.asr.2009.08.033.
- ^ Choi, Charles Q. (09-09-2009). "Mice levitated in lab". Live Science. Retrieved September 2011.
{{cite web}}
: Check date values in:|accessdate=
and|date=
(help) - ^ Kleiner, Kurt (08-10-2007). "Magnetic gravity trick grows perfect crystals". New Scientist. Retrieved September 2011.
{{cite web}}
: Check date values in:|accessdate=
and|date=
(help) - ^ "Fun with diamagnetic levitation". ForceField. 02-12-2008. Retrieved September 2011.
{{cite web}}
: Check date values in:|accessdate=
and|date=
(help) - ^ a b Kittel, Charles (1986). Introduction to Solid State Physics (6th ed.). John Wiley & Sons. pp. 299–302. ISBN 0-471-87474-4.
- ^ Chang, M. C. "Diamagnetism and paramagnetism" (PDF). NTNU lecture notes. Retrieved 2011-02-24.
External links
- Video of a museum-style magnetic elevation train model which makes use of diamagnetism
- Videos of frogs and other diamagnets levitated in a strong magnetic field
- Video of levitating pyrolytic graphite
- Video of Meissner-Ochsenfeld effect involving liquid nitrogen
- Video of a piece of neodymium magnet levitating between blocks of bismuth.