Introduction to Magnetic Fields

Magnetic fields are phenomena that result from electric currents and magnetic materials. Because a magnetic force has both direction and magnitude, magnetic fields are represented as vector fields.

Permanent Magnets
In permanent magnets, the magnetic field remains constant, and is generated by the simple existence of magnetic materials. All permanent magnets have a north pole and a south pole, which each have their own magnetic field. If two fields are of the same pole, they will repel each other. If they are of opposite poles, the objects will be attracted to one another. As of this writing, no permanent magnet exists as a "magnetic monopole," and all permanent magnets--without fail--have both a north and a south pole.

Direction of a Magnetic Field
A magnetic field's direction is represented with the symbol B. The magnitude of B is indicated by the spacing of field lines. The direction is determined by the direction in which the north pole of a compass points at a given location on the magnetic field. The field will always take its shape as loops that expand from the magnet's center, some in one direction, others in the opposite.

Earth's Magnetic Field
Earth's magnetic field, known as the geomagnetic field, is reminiscent of a gigantic permanent bar magnet. Logically, this theoretical bar magnet would align with the Earth's rotational axis, but that is not the case. In fact, although the magnetic poles remain at opposite end of the Earth, they sit at an angle to the rotational axis that changes over time. Even more counterintuitively is the fact that it seems what we geographically refer to as Earth's north pole is more accurately called its magnetic south pole, and vice versa.

Sources and Further Research
Wakler, James S. Physics: Fourth Edition: AP Edition. Pearson Education, Inc., 2010.

Khan Academy

Crash Course