Main Page | See live article | Alphabetical index

In physics, magnetism is a phenomenon by which materials exert an attractive or repulsive force on other materials. Magnetism arises whenever electrically charged particles are in motion. Some well known materials that exhibit magnetic properties are iron, some steels, and the mineral lodestone. All materials are influenced to one degree or another by the presence of a magnetic field, although in some cases the influence is too small to detect without special equipment.

Magnetic forces are fundamental forces that arise due to the movement of electrically charged particles. Maxwell's equations describe the origin and behavior of the fields that govern these forces (see also Biot-Savart's Law).

For the case of electric current moving through a wire, the resulting field is directed according to the "right hand rule". If the right hand is used as a model, and the thumb of the right hand points along the wire from positive towards the negative side, then the magnetic field will wrap around the wire in the direction indicated by the fingers of the right hand. If a loop is formed, such that the charged particles are traveling in a circle, then all of the field lines in the center of the loop are directed in the same direction. The result is called a magnetic dipole. When placed in a magnetic field, a magnetic dipole will tend to align itself with that field. For the case of a loop, if the fingers of the right hand are directed in the direction of current flow, the thumb will point in the direction corresponding to the North pole of the dipole.

volt
tesla
gauss
oersted
weber
ampere
maxwell

## Magnetic Dipoles

In an atom the orbital magnetic moments of some electron pairs cancel each other. The same is true for the spin magnetic moments. The overall magnetic moment of the atom is thus the sum of all of the magnetic moments of the individual electrons, accounting for moment cancellation between properly paired electrons. For the case of a completely filled electron shell or subshell, the magnetic moments completely cancel each other out. Thus only atoms with partially filled electron shells have a magnetic moment. The magnetic properties of materials are in large part determined by the nature and magnitude of the atomic magnetic moments.

Several forms of magnetic behavior have been observed including:

Highly magnetic stars called magnetars are also believed to exist.

## Models of Magnetic Material

Magnetic material may be modelled by a system of spins located at positions in a lattice, where the interaction of neighboring spins contributes to the total energy of the system and the states of the spins change according to some non-deterministic (probabalistic) rule (the dynamics of the system). In the Ising model spins have only two possible states (up and down), whereas in the Potts model they may have more than two possible states. This is discussed in detail in Spin Models, particularly in the section Modelling Magnetic Material and subsequent sections.