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Magnetism
Legend has it that magnetism was first discovered some 2000 years ago
when an elderly shepherd named Magnes was herding his sheep across a
rocky valley when the iron nails in the soles of his sandles stuck to the rocks
there. We have learned a lot since then.
Magnetic fields

B
Out from north pole,
into south pole
Try breaking a magnet. What happens?
Do a quick magnet lab – show like poles repel, opposites attract
http://phet.colorado.edu/en/simulation/magnets-and-electromagnets
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Suppose you have two iron bars that look alike, but
only one of them is a magnet. How can you
determine which is the magnet only by investigating
their interaction with each other?
•
Suppose you have two iron bars that look alike, but
only one of them is a magnet. How can you
determine which is the magnet only by investigating
their interaction with each other?
The true magnet will stick to any part of the iron bar. How to
tell which is which? Make a T shape with the bars. Only the
magnet placed at the midpoint of the non-magnet will stick.
Can you think of other ways to tell which one is the magnet?
Magnetism
Iron filings lie along magnetic field lines. All magnets have a dipolar (two lobed)
field. That means that magnets have north poles and south poles. If you cut a
magnet in half, each half becomes a magnet with two poles. In the language of
physics, we say that the magnetic field is dipolar. There is not such a thing as a
magnetic monopole, at least that we have found. Electric monopoles do exist. A
charged particle is an example of an electric monopole.
Magnetic monopoles?
•
Field lines would start on a magnetic charge (a magnetic
monopole). Does not seem to exist.
Notice the pole of the
earth’s magnetic field
that is in the northern
hemisphere
The magnetic field of
an electron is
ultimately what is
responsible for
permanent magnets
Some types of magnetism
Ferromagnetic materials are strongly attracted to magnets. The atoms contain
electrons which are all spinning in the same direction, giving the atoms strong
polarity (like little magnets). Atoms tend to line up parallel to each other within
domains of the material. A magnetic field can force the domains to line up, and
the material itself can become magnetic. (Ex: iron, nickel, cobalt, steel)
Paramagnetic materials are weakly attracted to magnets. The atoms of these
substances contain electrons most of which spin in the same direction, but not
all. These metals can become very weak magnets. (Ex: aluminum, gold,
copper)
Diamagnetic materials are magnetically neutral, because the electrons in the
atoms are spinning in equal numbers in opposite directions. Individual atoms
have some polarity (more electrons spinning around one side than the other)
and will move in response to a magnetic field. This effect is used in Magnetic
Resonance Imaging (MRI).
http://www.worsleyschool.net/science/files/magnet/types/ofmagnetism.html
Magnetic domains - ferromagnetism
•
•
High temperature will
destroy macroscopic
alignment of domains
http://www.magnet.fsu.edu/educ
ation/tutorials/java/domains/index
.html
Magnetic domains
A magnetized surface changes the reflectivity depending on the magnetic domains.
The light also changes in polarity. These effects are called the Magneto Optical
Kerr Effect (MOKE) and they allow us to photograph the magnetic domains using a
special microscope called a Kerr-effect microscope.
Ferromagnetism
What does it take to demagnetize a ferromagnet?
Hit it or heat it: Either one randomizes orientation of domains.
Hitting: creates shock waves that disrupt atom alignment
Heating: increases thermal fluctuations, making orientation of atoms
more random
The lowest energy configuration of a ferromagnet is when the atoms
are all lined up. If you add energy to the system, the atoms can
realize higher energy configurations, and don’t have to all line up.
Above a certain temperature, there is no magnetization.
Ferromagnetism
Geological example:
Molten rock is magnetized as it cools in the Earth’s magnetic field.
The discovery of magnetized moon rock suggests that the moon once
had magnetic poles.
http://blogs.discovermagazine.com/80beats/2009/01/20/moon-rock-suggests-the-young-moon-had-a-fiery-core-and-a-magnetic-field/