magnetism in materials · magnetism in materials lwhat causes magnetism in materials, like this bar...
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Magnetism in materials
l What causes magnetismin materials, like this barmagnet?
l Can imagine it hassomething to do withcurrent loops
l Each electron inside anatom circles the atom inabout 1.6X10-3 s
u so it creates a magneticfield of 20 T at the centerof the atom
u if every electron did this,then very strong magneticfields would be createdinside each atom andinside every material
Copyright (c) Grolier Interactive Inc.
Rutherford model
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Not so fastl Magnetic field created by one
electron travelling in onedirection is most oftencancelled out by an electrontravelling in the other direction
l Magnetic properties of mostmaterials explained by the factthat the electron is not onlymoving in a circle whileorbiting the nucleus, but isalso spinning on its axis
l This spin is sort of a currentloop as well
l But for most types of atoms,electrons usually pair up withspins opposite each other
l So again, a cancellation ofmagnetic fields
Copyright (c) Grolier Interactive Inc.
Rutherford model
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Fig. 19.37a, p.609
Ferromagnetism
l In some materials, suchas iron, cobalt, andnickel, the magneticfields produced byelectron spins do notcancel completely
l These materials arecalled ferromagnetic
l Groups of nearby atomstend to have spinsaligned in same direction
u domains
domains are from 10-4 cm to 0.1 cm in size
The material does not produce a noticeable magnetic field because the domains are all pointing in random directions
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Applying an external magnetic field
l Suppose I apply anexternal magnetic field tomy ferromagneticmaterial
l Domains aligned withrespect to external fieldtend to grow at theexpense of those not
l i.e. the material becomesmagnetized
l In “hard” magneticmaterials, themagnetization persistseven after the externalfield is removed
u a permanent magnet
but I can change the direction of magnetization if I try hard enough
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Iron
l In “soft” magneticmaterials such as iron,the domains tend torandomize after theexternal field is removedbecause of thermalagitation (or by bangingwith a hammer)
l There are ways ofcausing a lot of thermalagitation and I can causeeven a permanentmagnet to lose itsmagnetization
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Diamagnetism
l Besides ferromagnetism,there are two other typesof magnetism
u paramagnetism (forgetabout it)
u diamagnetism
l Superconductors arediamagnetic
u this means they really,really hate magnetic fields
u …and will do anything theycan to get rid of them
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Fig. 20.p621, p.621
Magnetic induction
l This next chapter ismostly about MichaelFaraday
u 1791-1867
l Little formaleducation; almostentire self-taught
u he was apprenticedas a book-binder andended up readingmost of the books hewas supposed to bind
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Royal Lectures
l He attended all of thepublic lectures given bythe Royal Academy ofSciencies
l Kept a notebook from thelectures of Sir HumpreyDavies, bound it anpresented it to him
u Davies made him a labassistant
l We’ve alreadyencountered a unitnamed after him (theFarad) and some of hismost useful ideas
u electric and magnetic fieldlines
because of his lack of formalmathematical training, most ofFaraday’s thinking was intuitive
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Fig. 20.1, p.621
An important experiment
l This is his most famousexperiment
u and he thought of itwhile…
l In the early 1800’s hewas where we are now inthis course
u strong electric fields createmagnetic fields (bycreating currents)
u from symmetry it seemedthat strong magnetic fieldsshould be able to createelectric fields (andcurrents)
sitting in his laboratory
this is the experiment thathe set up
what did he find?let’s try our version.
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What he found
l A strong magnetic fielddoes not create anelectric current
l But we did notice acurrent in the meterwhen we first closed theswitch and just after weopened it again
l So it’s not a magneticfield that creates anelectric current; it’s achanging magnetic field
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Magnetic flux
…and it’s not the changing magnetic field per se but the changing magnetic flux that creates the current
define the magnetic flux asFB = BTA = BAcosq
think of it as countingthe # of field lines passing thrua surface
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Fig. 20.3, p.622
Magnetic flux
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Fig. 20.4, p.623
Faraday’s law of induction
l The emf e equals thetime rate of changeof the magnetic fluxu e = - N DFB/Dt
lWe saw one way ofchanging themagnetic flux
u here’s another
this - sign is so important we’regoing to give it a name all toitself: Lenz’s law
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Notice the direction of the current flow
l That’s the - signu the direction of the
induced emf is suchthat it tries to producea current whosemagnetic fieldopposes the changein flux thru the loop
u Lenz’s law (the “IdahoRepublican law”)
s any change is resistedno matter what thedirection
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Notice the direction of the current flow
I push the magnetin; the coil is pushing back
I pull the magnetout; the coil ispulling back
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Lenz’s law
l Suppose I have aconducting bar sliding onconducting rails, in amagnetic field pointinginto the page
l If it slides to the right,what is the direction ofthe current
u what is the direction of theforce?
l What if the bar weresliding to the left?
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Quiz1. What is the magnitude of the
force on the proton?
a) 1.6 X 10-19 Nb) 5 X 105 Nc) 15.0 Nd) 4.4 X 10-4 Ne) .031 N
2. What is the direction of theforce on the proton?a) to the rightb) to the leftc) up towards the top of the
paged) down towards the bottom of
the pagee) out of the plane of the page
I v
.1 m
A current of 1 A (downward)
creates a magnetic field at the
position marked by a smiley face.
A proton at that position
(charge =1.6 X 10-19 C) is
travelling with v = 5 X 105 m/s
in the direction indicated.