A magnetic field

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A magnetic field is a field of force produced by a magnetic object or particle, or by a changing electric field[1] and is detected by the force it exerts on other magnetic materials and moving electric charges. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field.[nb 1]

The complex mathematics underlying the magnetic field of an object is usually illustrated using magnetic field lines. These lines are strictly a mathematical concept and do not exist physically. Nonetheless, certain physical phenomena, such as the alignment of iron filings in a magnetic field, produces lines in a similar pattern to the imaginary magnetic field lines of the object.

Magnets exert forces and torques on each other through the magnetic fields they create. Electric currents and moving electric charges produce magnetic fields. Even the magnetic field of a magnetic material can be modeled as being due to moving electric charges.[nb 2] Magnetic fields also exert forces on moving electric charges.

The magnetic fields within and due to magnetic materials can be quite complicated and is described using two separate fields which can be both called a magnetic field: a magnetic B field and a magnetic H field. Energy is needed to create a magnetic field. This energy can be reclaimed when the field is destroyed and, therefore, can be considered as being "stored" in the magnetic field. The value of this energy depends on the values of both B and H.

An electric field is a field created by an electric charge and such fields are intimately related to magnetic fields; a changing magnetic field generates an electric field and a changing electric field produces a magnetic field. (See electromagnetism.) The full relationship between the electric and magnetic fields, and the currents and charges that create them, is described by the set of Maxwell's equations. In view of special relativity, electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic field. A pure electric field in one reference frame is observed as a combination of both an electric field and a magnetic field in a moving reference frame. In quantum physics, this electromagnetic field is understood to be caused by virtual photons. Most often this quantum description is not needed because the simpler classical theory is sufficient.

Magnetic fields have had many uses in ancient and modern society. The Earth produces its own magnetic field, which is important in navigation since the north pole of a compass points toward the south pole of Earth's magnetic field, located near the Earth's geographical north. Rotating magnetic fields are utilized in both electric motors and generators. Magnetic forces give information about the charge carriers in a material through the Hall effect. The interaction of magnetic fields in electric devices such as transformers is studied in the discipline of magnetic circuits.