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A magnetic field (sometimes called B-field[1]) is a physical field that describes the magnetic influence on moving electric charges, electric currents,[2]: ch1 [3] and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the magnetic field.[2]: ch13 [4]: 278 A permanent magnet's magnetic field pulls on ferromagnetic materials such as iron, and attracts or repels other magnets. In addition, a nonuniform magnetic field exerts minuscule forces on "nonmagnetic" materials by three other magnetic effects: paramagnetism, diamagnetism, and antiferromagnetism, although these forces are usually so small they can only be detected by laboratory equipment. Magnetic fields surround magnetized materials, electric currents, and electric fields varying in time. Since both strength and direction of a magnetic field may vary with location, it is described mathematically by a function assigning a vector to each point of space, called a vector field (more precisely, a pseudovector field).
In electromagnetics, the term magnetic field is used for two distinct but closely related vector fields denoted by the symbols B and H. In the International System of Units, the unit of B, magnetic flux density, is the tesla (in SI base units: kilogram per second2 per ampere),[5]: 21 which is equivalent to newton per meter per ampere. The unit of H, magnetic field strength, is ampere per meter (A/m).[5]: 22 B and H differ in how they take the medium and/or magnetization into account. In vacuum, the two fields are related through the vacuum permeability, ; in a magnetized material, the quantities on each side of this equation differ by the magnetization field of the material.
Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin.[6][2]: ch1 Magnetic fields and electric fields are interrelated and are both components of the electromagnetic force, one of the four fundamental forces of nature.
Magnetic fields are used throughout modern technology, particularly in electrical engineering and electromechanics. Rotating magnetic fields are used in both electric motors and generators. The interaction of magnetic fields in electric devices such as transformers is conceptualized and investigated as magnetic circuits. Magnetic forces give information about the charge carriers in a material through the Hall effect. The Earth produces its own magnetic field, which shields the Earth's ozone layer from the solar wind and is important in navigation using a compass.
Description
The force on an electric charge depends on its location, speed, and direction; two vector fields are used to describe this force.[2]: ch1 The first is the electric field, which describes the force acting on a stationary charge and gives the component of the force that is independent of motion. The magnetic field, in contrast, describes the component of the force that is proportional to both the speed and direction of charged particles.[2]: ch13 The field is defined by the Lorentz force law and is, at each instant, perpendicular to both the motion of the charge and the force it experiences.
There are two different, but closely related vector fields which are both sometimes called the "magnetic field" written B and H.[note 1] While both the best names for these fields and exact interpretation of what these fields represent has been the subject of long running debate, there is wide agreement about how the underlying physics work.[7] Historically, the term "magnetic field" was reserved for H while using other terms for B, but many recent textbooks use the term "magnetic field" to describe B as well as or in place of H.[note 2] There are many alternative names for both (see sidebars).
The B-field
| Alternative names for B[8] |
|---|
The magnetic field vector B at any point can be defined as the vector that, when used in the Lorentz force law, correctly predicts the force on a charged particle at that point:[10][11]: 204
Here F is the force on the particle, q is the particle's electric charge, v, is the particle's velocity, and × denotes the cross product. The direction of force on the charge can be determined by a mnemonic known as the right-hand rule (see the figure).[note 3] Using the right hand, pointing the thumb in the direction of the current, and the fingers in the direction of the magnetic field, the resulting force on the charge points outwards from the palm. The force on a negatively charged particle is in the opposite direction. If both the speed and the charge are reversed then the direction of the force remains the same. For that reason a magnetic field measurement (by itself) cannot distinguish whether there is a positive charge moving to the right or a negative charge moving to the left. (Both of these cases produce the same current.) On the other hand, a magnetic field combined with an electric field can distinguish between these, see Hall effect below.
The first term in the Lorentz equation is from the theory of electrostatics, and says that a particle of charge q in an electric field E experiences an electric force:
The second term is the magnetic force:[11]
Using the definition of the cross product, the magnetic force can also be written as a scalar equation:[10]: 357
he command, "Measure the direction and magnitude of the vector B at such and such a place," calls for the following operations: Take a particle of known charge q. Measure the force on q at rest, to determine E. Then measure the force on the particle when its velocity is v; repeat with v in some other direction. Now find a B that makes the Lorentz force law fit all these results—that is the magnetic field at the place in question.
The B field can also be defined by the torque on a magnetic dipole, m.[12]: 174
The SI unit of B is tesla (symbol: T).[note 4] The Gaussian-cgs unit of B is the gauss (symbol: G). (The conversion is 1 T ≘ 10000 G.[13][14]) One nanotesla corresponds to 1 gamma (symbol: γ).[14]