Magnets

A magnet is a material or object that produces a magnetic field. A "hard" or "permanent" magnet is one which stays magnetized for a long time, such as magnets often used on refrigerator doors. more...

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Permanent magnets occur naturally in some rocks, particularly lodestone, but are now more commonly manufactured. A "soft" or "impermanent" magnet is one which loses its memory of previous magnetizations. "Soft" magnetic materials are often used in electromagnets to enhance (often hundreds or thousands of times) the magnetic field of a wire that carries an electrical current and is wrapped around the magnet; the field of the "soft" magnet increases with the current.

Two measures of a material's magnetic properties are its magnetic moment and its magnetization. A material without a permanent magnetic moment can, in the presence of magnetic fields, be attracted (paramagnetic), or repelled (diamagnetic). Liquid oxygen is paramagnetic; graphite is diamagnetic. Paramagnets tend to intensify the magnetic field in their vicinity, whereas diamagnets tend to weaken it. "Soft" magnets, which are strongly attracted to magnetic fields, can be thought of as strongly paramagnetic; superconductors, which are strongly repelled by magnetic fields, can be thought of as strongly diamagnetic.

Background on the physics of magnetism and magnets

Magnetic field

The magnetic field (usually denoted B) is a vector field (that is, some vector at every point of space and time), with SI units of teslas. The magnetic field at given time and place is a vector, so it has a magnitude and a direction. The direction is defined as the direction that a compass needle would point if it were held there, and the magnitude (also called strength) is defined to be proportional to how strongly the compass needle gets pushed in that direction.

Magnetic moment

The magnetic moment (also called magnetic dipole moment, and usually denoted μ) of a magnet is a vector which (loosely speaking) characterizes how magnetic it is. The direction of the magnetic moment points from the magnet's south pole to its north pole, and the magnitude relates to how strong and how far apart these poles are.

To be more exact, the magnetic moment manifests itself in two ways. First, the magnet creates a certain magnetic field, and the strength of that field at any given point is proportional to the magnitude of the magnet's magnetic moment. Second, when the magnet is put into an "external" magnetic field created by a different source, it will respond by feeling a torque pushing it so that the magnetic moment points parallel to the field (see above). The amount of torque felt in a given magnetic field is proportional to the magnetic moment.

Read more at Wikipedia.org