Magnetism (part 9) - Lecture 04 Chapter 32 F x B F = AI = B...

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Physics 113, Dr. Renat Letfullin 1 Lecture 04: Chapter 32 B x . F F θ θ μ B U - = μ B × = μ τ AI = μ
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Physics 113, Dr. Renat Letfullin 2 Today.. . Atomic Magnets The Electron Spin Ferromagnetism Nuclear Magnetism S N
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Physics 113, Dr. Renat Letfullin 3 A plausible explanation for the magnetic properties of materials is the orbital motion of the atomic electrons. The figureshows a classical model of an atom in which a negativeelectron orbits a positive nucleus. The electron's motion is that of a current loop. Consequently, an orbiting electron acts as a tiny magnetic dipole, with a north pole and a south pole. However, theatoms of most elements contain many electrons. Unlikethe solar system, where all of the planets orbit in the same direction, electron orbits arearranged to oppose each other: one electron moves counterclockwise for each electron that moves clockwise. Thus the magnetic moments of individual orbits tend to cancel each other and the net magnetic moment is either zero or very small. Atomic Magnets
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Physics 113, Dr. Renat Letfullin 4 The Electron Spin An electron's inherent magnetic moment is often called the electron spin , because in a classical picture, a spinning ball of charge would have a magnetic moment. This classical picture is not a realistic portrayal of how the electron really behaves, but its inherent magnetic moment makes it seem as if the electron were spinning. An electron also has an intrinsic angular momentum. However, its magnetic moment is twice as large as a spinning sphere of charge with that angular momentum should have. This is due to quantum effects. The key to understanding atomic magnetism was the 1922 discovery that electrons have an inherent magnetic moment. Perhaps this shouldn't be surprising. An electron has a mass , which allows it to interact with gravitational fields, and a charge , which allows it to interact with electric fields. There's no reason an electron shouldn't also interact with magnetic fields, and to do so, it comes with a built-in magnetic moment. Q=-e e μ ε = - 9.274x10 -24 J/T
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Physics 113, Dr. Renat Letfullin 5 Ferromagnetism (1) It happens that in iron (and other elements nearby in the periodic table, e.g., Co and Ni) the spins interact with each other in such a way that atomic magnetic moments tend to all line up in the same direction. Materials that behave in this fashion are called ferromagnetic . The figures show how the spin magnetic moments are aligned for the atoms making up a ferromagnetic solid.
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  • Spring '08
  • JOENATHAN
  • Physics, Magnetism, Magnetic Field, Magnetic moment, Magnet, Dr. Renat Letfullin

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