Magnetism is not like electricity exactly. magnets come only in +/- or N/S pairs.
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Transcript of Magnetism is not like electricity exactly. magnets come only in +/- or N/S pairs.
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magnetism is not like electricity exactly.
magnets come only in +/- or N/S pairs
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unlike polesmeet head on
like polesmeet head on
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θ
v
B
+q
FB
B
v+
FB
v
FB-
(a) (b)
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how to concentrate the magnetic field? bend the wire
in a loop
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almost like a bar magnet (cases where they behave differently are
oddballs)
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Want more? Superposition.
Many loops = many little B’s working together.“solenoid”
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practical application: speakers
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back to torque ...
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loop plane parallel to B - torque loop plane perpendicular to B - no force or torque
or ... magnetic moment (perp to area) aligns with field
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dc motor : current loop wants to rotate in fieldtries to align area perpendicular along field
either oscillate current, or push it hard enough to “flip”
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linear motor - current pushes bar out (rail gun)
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coils of wire or bar magnets - want to align with field
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another type of moment - spin
•rough analogue is a spinning ball of charge
• looks like a tiny circulating current
•really, intrinsic angular momentum of particles
•more significant than orbital effect, but moment behavior is qualitatively similar
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spin of electron provides another magnetic moment
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magnetism in real materials
• like dielectrics are electrically polarizable ...
•... some materials are magnetically polarizable
•can be positive or negative
•B inside material is not the same as outside
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moments try to align with the field - your fridge
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moments try to align opposite in the field
degree of alignment per applied B = susceptibility
negative for diamagnetspositive for paramagnets
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total moment = diamagnetic GaAs (huge volume) + ferromagnetic Co (small volume)
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magnetic field amplifier - i.e., an electromagnet
B inside iron is HUGE ...
just need tiny B to align it
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how does this relate to NMR/MRI?
what happens to a moment in B?
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minimizing U means parallel to field
making torque zero means parallel to field
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so what?• local value of B depends on environment
•so precession frequency depends on environment
- electron density, electronegativity, induction
• local environment is a function of bonding
• if you can measure the precession frequency, you can ID atom + environment
•(cannot really explain this without quantum)
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H atoms in diff environments
givediff frequencies
ratio of peaks ... ratio of number of atoms
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http://www.cis.rit.edu/htbooks/nmr/inside.htm
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there is current through S1,but not through S2.
violates Ampere’s law?
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