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Magnetic fields are a fundamental concept in physics that describe the behavior of magnetic forces. They are produced by moving electric charges and intrinsic magnetic moments of elementary particles associated with a fundamental quantum property known as spin . Magnetic fields and electric fields are both interrelated and are components of the electromagnetic force, one of the four fundamental forces of nature .

The study of magnetic fields is called magnetism. It is a branch of physics that deals with the properties of magnets, magnetic fields, and the effects of magnetic fields on matter . The magnetic field is an invisible field of attraction force that surrounds a magnet. Whenever a subatomic particle with such a negative charge, such as electrons, moves about, it creates a magnetic field .

Electric current produces a magnetic field. This magnetic field can be visualized as a pattern of circular field lines surrounding a wire. One way to explore the direction of a magnetic field is with a compass . The direction of the magnetic field is perpendicular to the direction of the current flow and is determined by the right-hand rule . The magnitude of the magnetic field is proportional to the current and the distance from the wire .

The formal statement of the direction and magnitude of the field due to each segment is called the Biot-Savart law . Each segment of current produces a magnetic field like that of a long straight wire, and the total field of any shape current is the vector sum of the fields due to each segment . The equation for the magnetic field strength (magnitude) produced by a long straight current-carrying wire is:

$$\mathrm { B } = \dfrac { \mu _ { 0 } \mathrm { I } } { 2 \pi \mathrm { r } }$$

where B is the magnetic field strength, I is the current, r is the shortest distance to the wire, and the constant ?0=4?10?7 T?m/A is the permeability of free space .

Magnetic fields such as that of Earth cause magnetic compass needles and other permanent magnets to line up in the direction of the field. Magnetic fields force moving electrically charged particles in a circular or helical path. This force—exerted on electric currents in wires in a magnetic field—underlies the operation of electric motors .

In summary, magnetic fields are a fundamental concept in physics that describe the behavior of magnetic forces. They are produced by moving electric charges and intrinsic magnetic moments of elementary particles associated with a fundamental quantum property known as spin. The study of magnetic fields is called magnetism. Electric current produces a magnetic field, and the direction of the magnetic field is perpendicular to the direction of the current flow and is determined by the right-hand rule. The magnitude of the magnetic field is proportional to the current and the distance from the wire. Magnetic fields such as that of Earth cause magnetic compass needles and other permanent magnets to line up in the direction of the field. Magnetic fields force moving electrically charged particles in a circular or helical path, and this force underlies the operation of electric motors..