History

1. Ancient Greeks notice that certain stones from Asia Minor were capable of attracting iron
2. Stones became known as “magnetite” with the property of “magnetism”
3. Chinese and Italian inventors capitalize on the property of north/south orientation to develop the compass (pre-1500’s)

Compass

One end of a compass needle points north and the other south.

Although first believed to be a fundamental force, magnetism results from the motion of electric charges.

Therefore, magnetism is part of a larger phenomenon called “electromagnetism“.

Magnetism results when electrons spin on their axes or flow through conductors.

1. Magnetic fields can be cancelled when some electrons spin clockwise and their partner in the pair spins counterclockwise.
2. Aluminum and copper pair in this way and therefore these metals show no permanent magnetic properties.

Permanent magnets

1. Iron, for example, keeps two electrons unpaired and aligned in the same direction.
2. Because there is no cancellation of field, iron sets up a magnetic field.
3. This is what occurs in a permanent magnet.

Domain Theory of Magnetism

1. All ferromagnetic substances (materials that are strongly attracted to magnets) consist of many small regions of atoms known as magnetic domains.
2. If the domains are randomly arranged, the fields cancel, leaving no net magnetic field.
3. If the domains are aligned, the magnetic fields add, creating a magnetic field.

Three methods of magnetism

Method One: Heating and Cooling Within a Strong Magnetic Field

1. Heat a permanent magnet above the critical temperature (called the Curie temp), domains disappear and the magnet loses its ferromagnetic properties.
2. Upon cooling, place the material on a permanent magnet and new domains will align with the permanent magnet and produce a stronger magnet than the original.
3. Stroke a nail 30 times in the same direction with a strong bar magnet, removing the nail between each stroke

Source

Method Two: Exposure to the Earth’s Magnetic Field

1. Heat a ferromagnetic substance to its Curie temp.
2. Allow the substance to cool aligned with the N-S orientation of the Earth.
3. Also, strike a ferromagnetic substance with a hammer to disrupt the original domains and they will align themselves with the Earth’s magnetic field.

Method Three: Stroking with a Permanent Magnet

1. Heat a ferromagnetic substance to its Curie temp.
2. Allow the substance to cool aligned with the N-S orientation of the Earth.
3. Also, strike a ferromagnetic substance with a hammer to disrupt the original domains and they will align themselves with the Earth’s magnetic field.
4. Stroke a nail 30 times in the same direction with a strong bar magnet, removing the nail between each stroke.

Source

Which method works best?

1. If the three aforementioned methods were all tried, method one would prove to be the most effective.
2. Reason: the mobility of the magnetic domains and the strong magnetic field surrounding the permanent magnet.

Magnetic Field Designation

1. The letter “B” designates magnetic field
2. The direction is defined as outward from the “north seeking” pole and inward towards the “south seeking” pole
3. Strength of field is associated with the number of field lines just like electric field lines

Earth’s Magnetic Field

1. Magnetic north is at the geographic south pole and vice versa
2. The cause of the Earth’s Magnetic field is probably due to moving ions within the liquid interior
3. Magnetic field has reversed direction during the last million years – evidenced from solidified lava after volcanic eruptions

Magnetic Declination

1. True north is indicated on a compass on a line from the Great Lakes to South Carolina.  At other locations, the compass does not indicate true north.

Units for Magnetic Fields

1. The unit used for magnetic field ( B ) is the Tesla (T) , which is also equal to a Weber/m² .
2. Common lab magnets are generally ~2.5 T
3. A superconducting magnet is ~30 T.
4. The Earth’s magnetic field at the surface is ~0.00005 T
5. Another common unit used for magnetic field (although not SI) is the Gauss (G) (1 T = 10000 G) [Ever degauss your computer screen?]

 

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