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GG 450
Magnetism
February 7, 2008
QUIZ
Like gravity, magnetic fields are
POTENTIAL fields, and we expect that we
will be able to trade vectors for scalars at
some point again.
Unlike gravitational fields, we can easily
generate large magnetic fields that
overpower the earth's field, and we can
effectively block out the earth's field if we
want to.
Like gravity, magnetic fields are
POTENTIAL fields, and we expect that we
will be able to trade vectors for scalars at
some point again.
Unlike gravitational fields, we can easily
generate large magnetic fields that
overpower the earth's field, and we can
effectively block out the earth's field if we
want to.
Unlike gravitational fields, which always attract (as
far as we know), magnetic fields generate both
attractive and repulsive forces. Electrical and
magnetic fields are impossible to separate, as one is
created by the other.
As far as we know, all magnetic fields are generated
by moving electrical charge. Another way to think
about it is that moving electrical charges exert
"magnetic" forces on each other independent of the
electrical forces.
What is a magnetic field?
A magnetic field exists if a moving electric charge feels a nonelectrostatic force at that point. (an electrostatic force is
one that the charge would feel if it weren’t moving). A
particle carrying an electrical charge passing through a
magnetic field feels a force proportional to the amount of its
charge and its velocity:
F  qvH sin    qv  H
where F is a vector force acting on the particle, H is
the strength of the magnetic field, v is its velocity, q
is its charge, and  is the angle between the
magnetic field and the direction of motion of the
particle.
DEFINITIONS:
B: magnetic flux density, or magnetic induction, flux
per unit area
S.I. units are Tesla= 1 weber/m2. A weber is a
unit of magnetic pole strength.
H: magnetic field strength (magnetic intensity), = B/m,
S.I. units: nanoTesla (nT) = gamma. m is the
magnetic permeability,
So, remember that H  B for small magnetic
fields.
Can you think of a common instrument in SOEST that makes
direct use of this force?
What direction does the charge feel the force in?
Right hand rule: If your right thumb is the direction of the
electric current (+ charges), then the magnetic field is in the
direction that your fingers point, and the force on the electrical
charge is out from your palm.
A charge moving in the direction of the magnetic field feels
no magnetic force.
Which way is the particle above
going to move? Is it accelerating?
Alternatively, if a wire is moved through a magnetic
field, a current is generated in the wire.
In this figure, the wire
loop is forced to spin in
the magnetic field.
What happens when the
wire loop continues
half way around?
What kind of device is
this?
What if we vary the
magnetic field instead of
forcing the wire to move?
What makes a material “magnetic”?
Consider an electron revolving around the nucleus of an atom
or around a molecule? The current generated as the electron
revolves is shown below as the arrowed circle, generating a
magnetic field shown by the arrow:
Indeed, this is the way many of the magnetic fields we are
familiar with are formed. The strength of a magnet will
depend on how many of these molecules are lined up in the
same direction:
unmagnetized material no external field
weakly magnetized
material
Completely "saturated"
ferromagnetic material nearly all "domains"
aligned. Very strong
external field.
How do we quantify these fields?
First: the force between two magnetic poles:
1 m1m2
F
m r2
where F = the force between the poles, m1 and m2 are
the POLE STRENGTHs, r is the distance between the
poles, and µ is called the MAGNETIC
PERMEABILITY.
This formula should look VERY familiar.
Every magnet has two poles - a plus and a
minus, or N and S. Each pole generates a
MAGNETIC FIELD whose strength is: H=m/r2,
where H is the magnetic field strength and m
is the pole strength. H is what we measure in
the field, and its units are nanoTesla or nT, (or
gammas). In CGS units, one Oersted= 1
dyne/[unit pole strength] or 105 nT.
Permeability, μ, also called magnetic
permeability, is a constant of proportionality
that exists between magnetic induction and
magnetic field intensity. This constant is equal
to approximately 1.257 x 10-6 henry per meter
(H/m) in free space (a vacuum). In other
materials it can be much different, often
substantially greater than the free-space value,
which is symbolized µo.
Materials that cause the lines of
flux to move farther apart, resulting
in a decrease in magnetic flux
density compared with a vacuum,
are called diamagnetic.
Materials that concentrate magnetic flux by a
factor of more than 1 but less than or equal
to 10 are called paramagnetic.
materials that concentrate the flux by a factor of
more than 10 are called ferromagnetic.
The permeability factors of some substances change
with rising or falling temperature, or with the
intensity of the applied magnetic field.
In engineering applications, permeability is
often expressed in relative, rather than in
absolute, terms. If µo represents the
permeability of free space (that is, 1.257 x 106 H/m) and µ represents the permeability of
the substance in question (also specified in
henrys per meter), then the relative
permeability, µr, is given by:
µr = µ / µo
= µ (7.958 x 105)
Diamagnetic materials have µr less than 1, but no
known substance has relative permeability much less
than 1.
Magnetic moment is a measure of the torque
generated by a pair of magnetic poles, called a
magnetic couple:
C=2 (m l) H sin
where C is a torque (force *
distance) m is the pole strength and l is the distance
between the poles. The magnetic moment is defined
as M=ml.
-m

+
l
+m
H
This torque causes a compass needle to turn to the north.
Magnetic intensity (I) is the moment per unit
volume, or poles per unit area. As I increases,
more of a body is magnetized and the field per unit
area increases. Lines of Force: Lines of force
external to a magnet go from + to -, or from the
magnetic N pole to the S pole. Lines of force are
perpendicular to surfaces of equal field
strength. They are equivalent to “rays” in
seismology and the direction of acceleration in
gravity. They show the direction a magnetized
body will feel a force in.
Magnetic susceptibility: When a material is placed
in a magnetic field, some of its molecules flip to align
with or against the "inducing" field. This generates
another magnetic field called the induced field. The
strength of the induced field relative to the inducing
field for a given volume of material is a measure of its
Magnetic susceptibility. I=kH, where I is the
intensity of the induced field, k is the susceptibility,
and H is the strength of the inducing field.
Susceptibility is the fundamental parameter in
magnetic prospecting.
Ferromagnetic materials - like iron - have very
high susceptibilities.
Diamagnetic materials - like quartz, have very
small negative susceptibilities. paramagnetic
materials - like pyroxene and olivine - have weak
susceptibilities ferrimagnetic materials - like
magnetite - have relatively high susceptibilities.
Magnetite accounts for a large fraction of the
susceptibility of rocks and for magnetic anomalies
in the earth’s crust.
I
ferromagnetic
ferrimagnetic
paramagnetic
diamegnetic
H
Induction of magnetic fields: Nearly all
magnetic anomalies in shallow exploration are
INDUCED. That is, they would go away if the
earth's field went away. But some very
important anomalies are REMANENT
magnetizations - locked into the rock. These
anomalies would NOT go away if the earth's
field were switched off.
Do you know any examples of remanent
anomalies?
As we increase the strength of the inducing field H,
more and more domains flip in response to this field,
increasing the induced field I. As more and more
magnetic domains are oriented the material becomes
saturated, and the induce field reaches its highest
possible value.
This is called a
hysteresis curve.
When the inducing field is removed, part of the
induced field remains - termed the REMANENT
magnetization. Some materials will hold a remanent
magnetization when they cool from a melt (such as
magnetite). This is the way that magnetic anomalies
are generated on the seafloor.
As the ocean crust cools below about 700°C, the
material passes through its CURIE POINT, where
the magnetic field is locked in. If the rock is ever
heated above the Curie point again, the remanent
magnetic field will be lost, and when the rock cools
again, the remanent field will change to the
direction of the current field.
Another type of remanent magnetization can
be caused by slow deposition of sediments.
Laths of magnetite will tend to settle to the
bottom pointing like little magnets with their
north pole pointing north. This trend will
magnetize the sediment depending on how
much magnetic material is present.
Simple dipole magnetic fields: All magnets have
two poles, a positive and negative pole. Lines of
force go from the positive to the negative pole.
N
S
A simple bar magnet
showing lines of force.
The closer the lines of
force, the larger the
magnetic field.
N
S
N
S
Two magnets with opposite poles near each
other will be drawn together. When in contact,
or close proximity, the two magnets will
appear to be one large magnet.
S
N
N
S
Two magnets with the same pole towards each other will
repel.