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phy142:lectures:20 [2011/03/30 23:26]
mdawber [Lecture 20 - Magnetic Materials]
phy142:lectures:20 [2014/03/28 11:35] (current)
mdawber [Video of lecture]
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 ====== Lecture 20 - Magnetic Materials ====== ====== Lecture 20 - Magnetic Materials ======
 +
 +----
 +If you need a pdf version of these notes you can get it [[http://​www.ic.sunysb.edu/​class/​phy141md/​lecturepdfs/​142lecture20S12.pdf|here]]
  
 ===== Video of lecture ===== ===== Video of lecture =====
  
-The video should play in any browser, but works best in anything that isn't Internet Explorer. If you are having trouble watching ​the video within the page you can [[http://​www.ic.sunysb.edu/​class/​phy141md/​lecturevids/​phy142lecture20.mp4|download the video]] and play it in [[http://​www.apple.com/​quicktime/​download/​|Quicktime]].+The 2014 recording got corrupted (maybe too many magnets around!) so here is the 2013 recording.
  
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 +
 ===== Orbital Magnetic Moment ===== ===== Orbital Magnetic Moment =====
  
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 $\vec{\mu}_{s}=-g\frac{e}{2m}\vec{S}$ $\vec{\mu}_{s}=-g\frac{e}{2m}\vec{S}$
  
-In the above the spin of an electron is either $\vec{S}=+1/2\hbar$ or $\vec{S}=-1/2\hbar$ and $g$ is a factor which is different for different particles. For an electron $g=2.0023193043622$. This factor is known to great precision, and the deviation from exactly ​to is due to quantum electrodynamics.+In the above the spin of an electron is either $\vec{S}=+\frac{1}{2}\hbar$ or $\vec{S}=-\frac{1}{2}\hbar$ and $g$ is a factor which is different for different particles. For an electron $g=2.0023193043622$. This factor is known to great precision, and the deviation from exactly ​two is due to quantum electrodynamics.
    
  
-The magnetic moment of the an atom is obtained from the sum of the orbital magnetic moments and spin magnetic moments from all of the electrons in the atom. Some atoms will have a total magnetic moment and others will not.+The magnetic moment of an atom is obtained from the sum of the orbital magnetic moments and spin magnetic moments from all of the electrons in the atom. Some atoms will have a total magnetic moment and others will not.
  
 ===== Magnetism inside a material ===== ===== Magnetism inside a material =====
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 [[wp>​Diamagnetism]] is the weakest form of magnetism, and is experienced by all materials, but is only significant in materials where the atoms have no total magnetic moment. When a magnetic field is applied the orbitals of the electrons are affected in such a way that an magnetic moment which opposes the field develops. [[wp>​Diamagnetism]] is the weakest form of magnetism, and is experienced by all materials, but is only significant in materials where the atoms have no total magnetic moment. When a magnetic field is applied the orbitals of the electrons are affected in such a way that an magnetic moment which opposes the field develops.
  
-The result is that diamagnets experience a repulsive force. We can see this effect on water using the strong [[wp>​Neodymium_magnet|neodymium iron boride]] magnets we used to make our motors.+The result is that diamagnets experience a repulsive force in an applied magnetic field
  
-sufficiently strong magnetic field can be used to levitate [[http://​www.ru.nl/​hfml/​research/​levitation/​diamagnetic/​|all sorts of diamagnetic objects, including frogs]].+As water is diamagnetic a sufficiently strong magnetic field can be used to levitate [[http://​www.ru.nl/​hfml/​research/​levitation/​diamagnetic/​|all sorts of diamagnetic objects, including frogs]].
  
-A superconductor,​ although it works on a macroscopic,​ rather than microscopic level, acts as a perfect diamagnet, ​an can be [[http://​outreach.phas.ubc.ca/​phys420/​p420_96/​bruce/​ybco.html|levitated]] with a much smaller magnetic field.+A superconductor,​ although it works on a macroscopic,​ rather than microscopic level, acts as a perfect diamagnet, ​and [[http://​outreach.phas.ubc.ca/​phys420/​p420_96/​bruce/​ybco.html|levitation]] can be achieved ​with a much smaller magnetic field. 
 + 
 +{{hitclev.jpg}}
  
 ===== Paramagnetism ===== ===== Paramagnetism =====
  
-If the atoms in a material have a magnetic moment then much stronger effects than diamagnetism occur. In a [[wp>​Paramagnetism|paramagnet]] the magnetic dipoles are normally randomly oriented due to thermal motion. ​ However, in a magnetic field the dipoles align themselves to produce a magnetization in the same direction as the field. ​+If the atoms in a material have a magnetic moment then much stronger effects than diamagnetism occur. In a [[wp>​Paramagnetism|paramagnet]] the magnetic dipoles are normally randomly oriented due to thermal motion. ​ However, in a magnetic field the dipoles align themselves to produce a magnetization in the same direction as the field. 
 + 
 +===== Magnetic Shielding ===== 
 + 
  
 An example of a very good paramagnetic material, is [[wp>​Mu-metal|mu-metal]],​ this is a nickel-iron alloy that can be used as magnetic shielding because it redirects the magnetic field lines. We can consider paramagnets as focusing the lines of magnetic flux, and diamagnets as excluding them. An example of a very good paramagnetic material, is [[wp>​Mu-metal|mu-metal]],​ this is a nickel-iron alloy that can be used as magnetic shielding because it redirects the magnetic field lines. We can consider paramagnets as focusing the lines of magnetic flux, and diamagnets as excluding them.
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 ===== Domains ===== ===== Domains =====
  
-In a ferromagnet the mangetic dipoles would always like to all point in the same direction, but frequently we see that a ferromagnetic material does not appear to be magnetized. This occurs because the material can split into ferromagnetic [[wp>​Magnetic_domains|domains]],​ and while in each domain all the dipoles point in the same direction, the overall direction of each domain can be different. When a ferroelectric material which initially has zero net magnetization,​ due to domains, is placed in a magnetic field it shows a different $M-H$ curve until it is fully magnetized after which hysteresis is observed.+In a ferromagnet the mangetic dipoles would always like to all point in the same direction, but frequently we see that a ferromagnetic material does not appear to be magnetized. This occurs because the material can split into ferromagnetic [[wp>​Magnetic_domains|domains]],​ and while in each domain all the dipoles point in the same direction, the overall direction of each domain can be different. When a ferroelectric material which initially has zero net magnetization,​ due to domains, is placed in a magnetic field it shows a different $M-H$ curve until it is fully magnetized after which hysteresis is observed. The driving force behind the formation of the domains is the energetic drive to minimize the external magnetic field around the sample
  
 {{hysteresisdomains.png}} {{hysteresisdomains.png}}
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 For more on magnetic materials, see the [[http://​www.irm.umn.edu/​hg2m/​hg2m_index.html|Hitchhiker'​s guide to magnetism]]. For more on magnetic materials, see the [[http://​www.irm.umn.edu/​hg2m/​hg2m_index.html|Hitchhiker'​s guide to magnetism]].
 +
  
phy142/lectures/20.1301541968.txt · Last modified: 2011/03/30 23:26 by mdawber
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