History
- Ancient Greeks notice that certain stones from Asia Minor were capable of attracting iron
- Stones became known as “magnetite” with the property of “magnetism”
- 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.
- Magnetic fields can be cancelled when some electrons spin clockwise and their partner in the pair spins counterclockwise.
- Aluminum and copper pair in this way and therefore these metals show no permanent magnetic properties.
Permanent magnets
- Iron, for example, keeps two electrons unpaired and aligned in the same direction.
- Because there is no cancellation of field, iron sets up a magnetic field.
- This is what occurs in a permanent magnet.
Domain Theory of Magnetism
- All ferromagnetic substances (materials that are strongly attracted to magnets) consist of many small regions of atoms known as magnetic domains.
- If the domains are randomly arranged, the fields cancel, leaving no net magnetic field.
- 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
- Heat a permanent magnet above the critical temperature (called the Curie temp), domains disappear and the magnet loses its ferromagnetic properties.
- 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.
- Stroke a nail 30 times in the same direction with a strong bar magnet, removing the nail between each stroke
Method Two: Exposure to the Earth’s Magnetic Field
- Heat a ferromagnetic substance to its Curie temp.
- Allow the substance to cool aligned with the N-S orientation of the Earth.
- 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
- Heat a ferromagnetic substance to its Curie temp.
- Allow the substance to cool aligned with the N-S orientation of the Earth.
- Also, strike a ferromagnetic substance with a hammer to disrupt the original domains and they will align themselves with the Earth’s magnetic field.
- Stroke a nail 30 times in the same direction with a strong bar magnet, removing the nail between each stroke.
Which method works best?
- If the three aforementioned methods were all tried, method one would prove to be the most effective.
- Reason: the mobility of the magnetic domains and the strong magnetic field surrounding the permanent magnet.
Magnetic Field Designation
- The letter “B” designates magnetic field
- The direction is defined as outward from the “north seeking” pole and inward towards the “south seeking” pole
- Strength of field is associated with the number of field lines just like electric field lines
Earth’s Magnetic Field
- Magnetic north is at the geographic south pole and vice versa
- The cause of the Earth’s Magnetic field is probably due to moving ions within the liquid interior
- Magnetic field has reversed direction during the last million years – evidenced from solidified lava after volcanic eruptions
Magnetic Declination
- 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
- The unit used for magnetic field ( B ) is the Tesla (T) , which is also equal to a Weber/m2 .
- Common lab magnets are generally ~2.5 T
- A superconducting magnet is ~30 T.
- The Earth’s magnetic field at the surface is ~0.00005 T
- 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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