About magnetism

The branch of science that describes the effects of the interactions between charges due to their motion and spin. These may appear in various forms, including electric currents and permanent magnets. The interactions are described in terms of the magnetic field, although the field hypothesis cannot be tested independently of the electrokinetic effects by which it is defined.

The magnetic field complements the concept of the electrostatic field used to describe the potential energy between charges due to their relative positions. Special relativity theory relates the two, showing that magnetism is a relativistic modification of the electrostatic forces. The two together form the electromagnetic interactions which are propagated as electromagnetic waves, including light. They control the structure of materials at distances between the long-range gravitational actions and the short-range “strong” and “weak” forces most evident within the atomic nucleus.

The term “magnetism” originates in the material magnetite, an iron ore, which producesweak natural magnets in the formof lodestones, exerting forces on each other and on pieces of iron. Peregrinus showed in 1269 that the behavior can be described in terms of magnetic poles on opposite end surfaces. The analogy between magnetic poles and electric charge greatly influenced the later development of the subject. The Earth provides an example of the subsequent explanation of magnetic behavior in terms of the flow of current and movement of charge, including the quantum state described as spin.

This leaves open the question of whether or not isolated magnetic poles, or monopoles, exist as separate physical entities. The magnetic field can be visualized as a set of lines illustrated by iron filings scattered on a suitable surface. The intensity of the field is indicated by the line spacing, and the direction by arrows pointing along the lines. The sign convention is chosen so that the Earth’smagnetic field is directed from the north magnetic pole toward the south magnetic pole.