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Lecture 12.1 :!
Magnetic Properties of Matter, and
intro to Induction.
Lecture Outline:!
Forces on Current Loops!
Magnetic Properties of Matter!
Induced Current!
Motional emf!
Textbook !Reading:!
Ch. 32.9! - 33.2
March 31, 2015
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Announcements
!
•Quiz #5 in class on Thursday.
Will cover Ch. 32.1-32.8!
•Homework #9 due on Monday, April 7 in Mastering Physics.
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Last Lecture…
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Last Lecture…
Recall the current loop and its magnetic field.
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Last Lecture…
Two neighboring current loops will attract/repel each
other depending on the orientation of their current.
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Forces on Current Loops
A current loop placed in an external magnetic field will rotate.
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Forces on Current Loops
This is the basis for a simple electric motor.
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Magnetic Properties of Matter
We now understand how moving charges create magnetic fields,
but what about materials that seem to have inherent magnetism?
Atoms, like hydrogen, can have a magnetic
moment due to the orbiting electron “current”.
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Electrons have their own intrinsic magnetic
moment! Unpaired electrons in an atom
contribute to the atoms magnetic moment.
Magnetic Properties of Matter
In the semiclassical Bohr model of the hydrogen atom, the
electron moves in a circular orbit of radius 0.53x10-10 m with
speed = 2.2x106 m/s. According to this model, what is the
magnetic field at the center of a hydrogen atom? (Problem 32.73)
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Magnetic Properties of Matter
Even though atoms may have a magnetic moment, the atoms in a
material can be randomly arranged, canceling each other out.
Ferromagnets are materials
where the atomic magnetic
moments line up.
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Magnetic Properties of Matter
Magnetic domains in ferromagnets can be altered by external field.
Moving domain walls in a grain of silicon steel caused by an increasing external magnetic field in
the "downward" direction. White areas are domains with magnetization directed up, dark areas are
domains with magnetization directed down. - Source: Wikipedia
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Magnetic Properties of Matter
http://web.mit.edu/newsoffice/2011/squid-geomagnetism-0304.html
A slice of ferromanganese crust from the Pacific Ocean was analyzed using a SQUID microscope at Vanderbilt University. The top image shows a portion of the
image of the slice taken with an electron microprobe. The second image shows the magnetized regions in the slice, with red areas showing one direction of
magnetization and blue the opposite direction.
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Magnetic Properties of Matter
Probably all of you are carrying an example of a magnetic
material where the arrangement of domains matters.
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Curie Temperature
Heating a ferromagnet above its “Curie Temperature” can cause
the magnetic domains to become unaligned, rendering the
material un-magnetized until it is cooled back below that
temperature.
Above Curie Temperature
Below Curie Temperature
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Superconductors
Superconductors cooled below their critical temperature (Tc)
don’t allow external magnetic fields to penetrate into their bulk.
This leads to some interesting phenomena, like levitation…
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Induced Current
Faraday observed that a changing magnetic field
creates an induced current.
Closing the switch creates a current in top loop, which
causes a changing magnetic field in the other loop.
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Induced Current
Faraday observed that a changing magnetic field
creates an induced current.
Changing magnetic field can also be due to relative motion.
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Motional emf
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Motional emf
An electric field builds up in the moving conductor.
It reaches maximum strength when the force due to
the electric field exactly balances the magnetic force.
This moving conductor is like a battery that stays charged only as long as it keeps moving.
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Clicker Question #1
A metal bar moves through a magnetic field. The
induced charges on the bar are
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Motional emf
Imagine sliding a bar over a conducting rail located in a
magnetic field. A current is induced.
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Motional emf
Force is required to maintain a constant speed of the moving
wire. We can study the energy in the circuit.
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Clicker Question #2
An induced current flows
clockwise as the metal bar is pushed to the right. The
magnetic field points
!
A.
B.
C.
D.
E.
Up.
Down.
Into the screen.
Out of the screen.
To the right.
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Motional emf
Recall what happens to the bar if we add a potential
(battery) across the two rails, as shown.
It would appear such a scheme will accelerate the bar
arbitrarily fast...making a “rail gun”! There are limitations
to the speed, which we will discuss soon.
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Reminders
!
•Quiz #5 on Thursday. Covers Ch. 32.1-32.8!
•Homework #9 is due Tuesday (April 7).!
•Start reading Ch. 33.!
!
!
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