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MagnetismFirst Principles

I. The Big Picture

What is Magnetism?http://www.htlstp.ac.at/~inetwrk1/Magnet_e/anzieh.gif

Bar Magnets & Shavingshttp://www.htlstp.ac.at/~inetwrk1/Magnet_e/anzieh.gif

Poles, Charges, Forces & Attractionshttp://www.htlstp.ac.at/~inetwrk1/Magnet_e/anzieh.gif

What is the Earth’s Magnetic Field?

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Earth’s Magnetic Fieldhttp://antwrp.gsfc.nasa.gov/apod/ap021125.html

The Earth's Magnetic Field Credit & Copyright: Gary A. Glatzmaier (UCSC

http://www.es.ucsc.edu/~glatz/geodynamo.html)

Total Magnetic Field Strength - 2002Views of Earth's Magnetic Field Based on the International Geomagnetic

Reference Field (IGRF)http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/igrfpg.pl

What is the Source of the Earth’s Magnetic Field?http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/igrfpg.pl Earth’s Interior

Magnetic Field of Currenthttp://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magcur.html#c1

Dipole Model - Lines of Force

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Compasshttp://ths.sps.lane.edu/benchmarks/force%20and%20motion/pages/magnet_compasses-t.htm

National Geophysical Data Center (NGDC)http://www.sciam.com/askexpert_question.cfm?articleID=00010D87-BB04-1C71-

9EB7809EC588F2D7

• 90% of Earth's magnetic field looks like a dipolar magnetic source - strength generally increasing from equator to pole

•10% of the magnetic field – non-dipolar

Variations Indicating Non-dipolar ComponentViews of Earth's Magnetic Field Based on the International Geomagnetic Reference

Field (IGRF)http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/igrfpg.pl

Why Can’t the Earth’s Magnetic Field be caused by a Permanent Magnet?

Earth’s Interior

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Earth’s Magnetic Fieldhttp://antwrp.gsfc.nasa.gov/apod/ap020818.html Tracking the Magnetic North Pole

http://www.geolab.nrcan.gc.ca/geomag/northpole_e.shtml

Declination - 2002Views of Earth's Magnetic Field Based on the International Geomagnetic

Reference Field (IGRF)http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/igrfpg.pl

II. The Details

Magnetics & Gravity

� Passive method(like gravity)

� Potential fields(like gravity)

� Figure 4. Map showing the residual gravity field of the Santa Clara Valley and vicinity. The gravity field, in conjunction with mapped geology, drill hole data, seismic profiling, and other geophysical interpretations, is used to infer the location and shape of the contact between the Cenozoic deposits and the underlying Mesozoic bedrock. This interface, which tends to be a strong density contrast in the Santa Clara Valley, shows considerable relief, especially in the Cupertino and Evergreen basins. Heavy rectangle is map view outline of

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� Figure 5. Map showing the residual magnetic field of the Santa Clara Valley and vicinity. The magnetic field is used to define the shape and location of the buried Logan Gabbro southwest of the San Andreas fault, and various serpentinite (designated by "S") and metavolcanic bodies northeast of the fault. Heavy rectangle is map view outline of the 3D geologic map of the Santa Clara Valley. Faults after Jennings (1994). Contour interval, 25 nanoTesla. (From Jachens et al., 2001)

� Gravity => point mass� Magnetics => fundamental magnetic element is called a magnetic monopole

� p1 is a positive pole� Magnetic force is always directed away from p1 (the force is repulsive).

� p2 is a negative pole � Magnetic force is directed towards p2 (attractive)

� Combine to examine dipole field (in red). � Simply the force generated by one monopole added to the force generated by a second monopole

� Negative monopole at x=-1, labeled S, and a positive monopole at x=1, labeled N

� Vector addition, we get the green arrows, showing the Magnetic Force

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� Field lines are everywhere parallel to the direction of the magnetic force (“lines of force”)

Direction of Field On a Larger Scale� Coulomb’s Law (1785)

force of attraction or repulsion between electrically charged bodies and between magnetic poles also obeys an inverse square law like that derived for gravity by Newton

Magnitude of the Force

µ = magnetic permeability� p1 and p2 = strengths of the two magnetic monopoles (m1 and

m2 in book)� r = distance between the two poles.� p1 and p2 can be either positive or negative in sign (repulsive

or attractive)

p1 p2r

• Magnetic force is given in Newtons,N, where a Newton is a kg-m/s2.

• We also know that distance has the units of meters, m. Permeability, µ, is defined to be a unitless constant.

•Magnetic field, F, has some strength and points in some direction.

•Declination •Inclination

• The magnetic field strength, H, is defined as the force per unit pole strength exerted by a magnetic monopole, p1. (magnetizing force in book)

• H is the magnetic analog to the gravitational acceleration, g.

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• Units of magnetic field strength are Newtons per Ampere-meter, N / (Amp-m), or tesla (T)

• Units of nanoteslas (nT), (1 billionth of a tesla)

•Avg. Earth's magnetic field is about 50,000 nT (range between 69,000 and 25,000 nT)

• nanotesla (nT) = gamma (γ).

The strength of the magnetic field induced by the magnetic material is called the intensity of magnetization, I.

•Like density for gravity, the fundamental rock parameter controlling the magnetic field variations is magnetic susceptibility (k)

•Wide variations in susceptibility also occur within a given rock type, making it difficult as a characteristic property

I = intensity of magnetization (J in book)H = strength of the inducing magnetic fieldk = constant of proportionality,k, known as the magnetic

susceptibility

unitless constant that is determined by the physical properties of the magnetic material

Magnetic Susceptibility x 10^3 (SI)* •Air ~0 •Quartz -0.01 •Rock Salt -0.01 •Calcite -0.001 - 0.01•Hematite 0.5 - 35•Illmenite 300 - 3500 •Magnetite 1200 - 19,200•Sandstones 0 - 20•Gneiss 0.1 - 25 •Slate 0 - 35 •Granite 0 - 50•Gabbro 1 - 90 •Basalt 0.2 - 175•Peridotite 90 - 200

� Crustal magnetic field - at least two possible sources.

� Induced magnetization� Remnant magnetization. � Both mechanisms contribute to the observed field

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1. Main Field - caused by electrical currents in the Earth's fluid outer core.

2. External Magnetic Field - Earth's ionosphere with the solar wind. Temporal variations - solar activity.

3. Crustal Field - Magnetism of crustal rocks due to induction from the Earth's main magnetic field and from remnant magnetization.

So What are We Measuring?

•Summary

� Nature of the Earth’s magnetic field (Dipole and dynamic)� Magnetic Poles and Forces� Magnetic Force - (Direction, D & I, and Magnitude, F)� Magnetic force (F) leads to magnetic field (H)� Magnetic field (strength) induces an intensity as a function

of the magnetic susceptibility (k) � We measure total field intensity, which includes Earth’s

field, external solar field, induced field and remnant field