This website was developed by Geno Jezek,
owner of the online magnet store, www.Custom-Magnets.com. Custom-Magnets has been supplying
magnetic material and custom fridge magnets since 1999.
Important Uses and Examples of Magnetic Fields
Earth's Magnetic Field
Convection currents in the outer liquid of Earth's Core, according to the Dynamo Theory, move and produce electric currents which, in turn, produce magnetic fields. This is exemplified by what happens with a compass -- the needle rotates so the North Pole of the compass magnet points north.
In the above sketch, Earth's geographic North is near the top and the South pole near the bottom. The south pole of that particular magnet is deep in Earth's interior, beneath Earth's North Magnetic Pole. Confusion arises when one considers that if Earth is a magnet, the poles are labeled backwards, as the south pole of that magnet would be the one nearer the North magnetic pole, and vice versa. (Opposite poles attract. North compass pole pulls toward Earth's South pole.)
Therefore, the North magnetic pole is not given its name because of polarity of the field, but because of its geographical location. Conversely, on a magnet, the North and South poles are named because they are "north-seeking" and "south-seeking".
Finally, earth's magnetic field isn't constant. The strength of the field, and the precise location of Earth's poles vary. Further, periodically the poles reverse their orientation (geomagnetic reversal), the most recent of which occurred 780,000 years ago.
Rotating Magnetic Fields
Rotating magnetic fields are key to operating alternating-current motors. In this process, a permanent magnet rotates within the field so it maintains its alignment with the external field. Nikola Tesla used this theory in his early AC electric motors.
Using two orthogonal coils with 90 degree phase difference in their AC currents, a rotating magnetic field can be constructed. This is practically applied through a three-wire arrangement with unequal currents.
To overcome the problems of standardizing the conductor given this inequality, three-phase systems are used where there are three currents in equal magnitude with 120 degrees of phase difference. Three similar coils, each with geometrical angles of 120 degrees create the rotating magnetic field. This is a key proponent of electric motors, and is one of the main reasons three-phase systems dominate the world's electrical power supply designs.
Magnets degrade over time, losing the force and strength of their magnetic pull and fiends. Because of this, synchronous motors use DC voltage-fed rotor windings, allowing for the excitation of the machine to be controlled. Induction motors use short-circuited rotors (instead of a magnet) following the rotating magnetic field of a multicoiled stator. The rotors short-circuited turns develop currents in the rotating field of the stator, and these currents, in turn, move the rotor according to Lorentz Force.
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