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generates a electric field (see electromagnetism). In special relativity, the electric field and magnetic field 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.
B and HSee also: MagnetizationAlternative names for B and H B name magnetic flux density magnetic induction magnetic field used by electrical engineers electrical engineers physicists
Magnetic field lines shown by iron filings. The high permeability of individual iron filings causes the magnetic field to be larger at the ends of the filings. This causes individual filings to attract each other, forming elongated clusters that trace out the appearance of lines. It would not be expected that these "lines" be precisely accurate field lines for this magnet; rather, the magnetization of the iron itself would be expected to alter the field somewhat. A magnetic field is a vector field which surrounds magnets and electric currents, and is detected by the force it exerts on moving electric charges and on magnetic materials. When placed in a magnetic field, magnetic dipoles tend to align their axes parallel to the magnetic field. Magnetic fields also have their own energy with an energy density proportional to the square of the field intensity. For the physics of magnetic materials, see magnetism and magnet, and more specifically ferromagnetism, paramagnetism, and diamagnetism. For constant magnetic fields, such as are generated by magnetic materials and steady currents, see magnetostatics. A changing electric field results in a magnetic field, and a changing magnetic field also
H name magnetic field intensity magnetic field strength auxiliary magnetic field magnetizing field used by electrical engineers electrical engineers physicists physicists
The term magnetic field is used for two different vector fields, denoted B and H, although there are many alternative names for both (see sidebar). To avoid confusion, this article uses B-field and H-field for these fields, and uses magnetic field where either or both fields apply. The B-field can be defined in many equivalent ways based on the effects it has on its environment. For instance, a particle having an electric charge, q, and moving in a B-field with a velocity, v, experiences a force, F, called the Lorentz force (see below). In SI units, the Lorentz force equation is
where is the vector cross product. The Bfield is measured in teslas in SI units and in gauss in cgs units.
From Wikipedia, the free encyclopediaAlthough views have shifted over the years, B is now understood as being the fundamental quantity, while H is a derived field. It is defined as a modification of B due to material media such that (in SI):
Magnetic fieldplaced in a non-uniform external magnetic field. In this model, each magnetic pole is a source of a magnetic field that is stronger near the pole. Further, an external magnetic field exerts a force in the direction of the magnetic field for a north pole and in the opposite direction for the south pole. In a nonuniform magnetic field, each pole sees a different field and consequently is subject to a different force. The difference in the two forces moves the magnet in the direction of increasing magnetic field. (There may also be a net torque.) In contrast, a magnet in a uniform magnetic field experiences at most a torque, and no net magnetic force, no matter how strong the field is. Unfortunately, the idea of "poles" does not accurately reflect what happens inside a magnet (see ferromagnetism). For instance, a small magnet placed inside of a larger magnet will feel a force in the opposite direction. The more physically correct description of magnetism involves atomic sized loops of current distributed throughout the magnet. Mathematically, the force on a magnet having a magnetic moment m is: . The force on a magnet due to a non-uniform magnetic field can be determined by summing up all of the forces on the elementary magnets that make up the entire magnet. The ability of a nonuniform magnetic field to sort differently oriented dipoles is the basis of the Stern-Gerlach experiment, which established the quantum mechanical nature of the magnetic dipoles associated with atoms and electrons.
(definition of H ) where M is the magnetization of the material and 0 is the magnetic constant. The Hfield is measured in amperes per meter (A/m) in SI units and in oersteds (Oe) in cgs units. In materials for which M is proportional to B the relationship between B and H can be cast into the simpler form: H = B , where is a material dependent parameter called the permeability. In free space, there is no magnetization M so that H = B 0 (free space). For many materials, though, there is no simple relationship between B and M. For example, ferromagnetic materials and superconductors have a magnetization that is a multiple-valued function of B due to hysteresis. See History of B and H below for further discussion.
The magnetic field and permanent magnetsPermanent magnets are objects that produce their own persistent magnetic fields. All permanent magnets have both a north and a south pole. Like poles repel and opposite poles attract. Permanent magnets are made of ferromagnetic materials such as iron and nickel that have been magnetized. For more details about magnets see magnetization below and the article ferromagnetism.
Force on a magnet due to a nonuniform BSee also: Magnet#Two models for magnets: magnetic poles and atomic currents and Magnetic moment The most commonly experienced effect of the magnetic field is the force between two magnets. This force is often described as like poles repel while opposites attract. A more general description, that also applies to magnetic fields that have no poles (such as that due to the current through a straight wire), is that a magnet experiences a force, when
Torque on a magnet due to a BfieldSee also: Faradays law of induction A magnet placed in a magnetic field will feel a torque that will try to align the magnet with the magnetic field. The torque on a magnet due to an external magnetic field is easy to observe by placing two magnets near each other while allowing one to rotate. The alignment of a magnet with the magnetic field of the Earth is how compasses work. It is used to determine the direction of a local magnetic field as well (see below). A small magnet is mounted such that it is free to turn (in a given plane) and its north pole is
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Magnetic fieldthe shaft. The inverse process, changing mechanical motion to electrical energy, is accomplished by the inverse of the above mechanism in the electric generator. See Rotating magnetic fields below for an example using this effect with electromagnets.
Visualizing the magnetic fieldMapping out the strength and direction of the magnetic field is simple in principle. First, measure the strength and direction of the magnetic field at a large number of locations. Then mark each location with an arrow (called a vector) pointing in the direction of the local magnetic field with a length proportional to the strength of the magnetic field. An alternative method of visualizing the magnetic field which greatly simplifies the diagram while containing the same information is to connect the arrows to form "magnetic field lines". A compass placed near the north pole of a magnet will point away from that polelike poles repel. The opposite occurs for a compass placed near a magnets south pole. The magnetic field points away from a magnet near its north pole and towards a magnet near its south pole. Not all magnetic fields are describable in terms of poles, though. A straight current-carrying wire, for instance, produces a magnetic field that points neither towards nor away from the wire, but encircles it instead.
The direction of the magnetic field near the poles of a magnet is revealed by placing compasses nearby. As seen here, the magnetic field points towards a magnets south pole and away from its north pole. marked. By definition, the direction of the local magnetic field is the direction that the north pole of a compass (or of any magnet) tends to point. The magnetic torque also provides the driving torque for simple electric motors. An electric motor changes electrical energy into mechanical energy (motion). In a motor, a magnet is fixed to a shaft free to rotate (forming a rotor). This magnet is subjected to a magnetic field from an array of electromagnets called the stator. The polarity of each individual electromagnet in the stator easily can be flipped by switching the direction of the current through its coils. By flipping component electromagnet polarities in sequence, the field of the stator continuously changes to place like poles next to the rotor, subjecting the rotor to a torque that is transferred to
B-field linesVarious physical phenomena have the effect of displaying magnetic field lines. For example, iron filings placed in a magnetic field will line up in such a way as to visually show the orientation of the magnetic field (see figure at top). Another place where magnetic fields are visually displayed is in the polar auroras, in which visible streaks of light line up with the local direction of Earths magnetic field (due to plasma particle dipole interactions). In these phenomena, lines or curves appear that follow along the direction of the local magnetic field. These field lines provide a simple way to depict or draw the magnetic field (or any other vector field).  The magnetic fi