Magnetic circuit 1115085 225519561 2008-07-14T03:09:40Z Thijs!bot 1392310 robot Adding: [[it:Circuito magnetico]] A '''magnetic circuit''' is a closed path containing a [[magnetic flux]]. It generally contains [[magnetic]] elements such as [[permanent magnet]]s, [[ferromagnetic]] materials, and [[electromagnet]]s, but may also contain air gaps and other materials. Some examples of magnetic circuits are: * [[horseshoe]] [[magnet]] with iron [[magnet keeper|keeper]] (low-[[reluctance]] circuit) * horseshoe magnet with no keeper (high-reluctance circuit) * [[electric motor]] (variable-reluctance circuit) ==Magnetic circuit laws== If <math>\Phi</math> is the magnetic flux in the circuit, <math>\Theta</math> is the [[magnetomotive force]] ''F'' applied to the circuit, and <math>R_m</math> is the [[reluctance]] of the circuit, then it follows from [[Ampère's law]] that: :<math>\Phi = \frac F R</math> This is analogous to [[Ohm's law]] in electrical circuits, where the [[current (electricity)|current]] is equal to the [[voltage]] (sometimes called ''electromotive force'') divided by the [[electrical resistance|resistance]] of the circuit. Here, magnetic flux, magnetomotive force and reluctance are analogous to current, voltage and resistance respectively. If ''A'' is the area, ''&mu;'' is the [[Permeability (electromagnetism)|permeability]] of the material, and ''l'' is the length :<math>R_m = \frac{l}{\mu A}</math> This is similar to the equation for electrical resistance in materials, with permeability being analogous to conductivity. Longer, thinner geometries with low permeabilities lead to higher reluctance. Low reluctance, like low resistance in electric circuits, is generally preferred. Magnetic circuits obey other laws that are similar to electrical circuit laws. For example, the total reluctance <math>R_T</math> of reluctances <math>R_1,\ R_2,\ \dots</math> in series is: :<math>R_T = R_1 + R_2 + \dots</math> (this also follows from [[Ampère's law]] and is analogous to [[Kirchhoff's circuit laws|Kirchhoff's voltage law]] for adding resistances in series). Also, the sum of magnetic fluxes <math>\Phi_1,\ \Phi_2,\ \dots</math> into any node is always zero: :<math>\Phi_1 + \Phi_2 + \dots = 0</math>. This follows from [[Gauss's law]] and is analogous to [[Kirchhoff's circuit laws|Kirchhoff's current law]] for analysing electrical circuits. Together, the three laws above form a complete system for analysing magnetic circuits, in a manner similar to electric circuits. Comparing the two types of circuits shows that: * The equivalent to resistance ''R'' is the ''reluctance'' ''R''<sub>m</sub> * The equivalent to current ''I'' is the ''magnetic flux'' ''&Phi;'' * The equivalent to voltage ''V'' is the ''magnetomotive Force'' ''F'' Magnetic circuits can be solved for the flux in each branch by application of the magnetic equivalent of [[Kirchhoff's circuit laws|Kirchhoff's Voltage Law]] ([[KVL]]) for pure source/resistance circuits. Specifically, whereas KVL states that the voltage excitation applied to a loop is equal to the sum of the voltage drops (resistance times current) around the loop, the magnetic analogue states that the magnetomotive force (achieved from ampere-turn excitation) is equal to the sum of MMF drops (product of flux and reluctance) across the rest of the loop. (If there are multiple loops, the current in each branch can be solved through a matrix equation--much as a matrix solution for mesh circuit branch currents is obtained in loop analysis--after which the individual branch currents are obtained by adding and/or subtracting the constituent loop currents as indicated by the adopted sign convention and loop orientations.) Per [[Ampère's law]], the excitation is the product of the current and the number of complete loops made and is measured in ampere-turns. Stated more generally: ''<math>M = N\,I = \oint \vec{H} \cdot d\vec{l}</math>'' (Note that, per Stokes's theorem, the closed [[line integral]] of H dot dl around a contour is equal to the open [[surface integral]] of curl H dot dA across the surface bounded by the closed countour. Since, from [[Maxwell's equations]], [[Curl (mathematics)|curl]] H = J, the closed line integral of H dot dA evaluates to the total current passing through the surface. This is equal to the excitation, NI, which also measures current passing through the surface, thereby verifying that the net current flow through a surface is zero ampere-turns in a closed system that conserves energy.) More complex magnetic systems, where the flux is not confined to a simple loop, must be analysed from first principles by using [[Maxwell's equations]]. ==References== * [http://www.analogzone.com/col_0909.pdf ''Magnetic-Electric Analogs''] by Dennis L. Feucht, Innovatia Laboratories (PDF) ==External links== * [http://www.magnet.fsu.edu/education/tutorials/java/magneticshunt/ Interactive Java Tutorial on Magnetic Shunts] National High Magnetic Field Laboratory ==See also== * [[Magnetic core]] * [[Tokamak]] [[Category:Electromagnetism]] [[ca:Circuit magnètic]] [[cs:Magnetický obvod]] [[de:Magnetischer Kreis]] [[el:Μαγνητικό κύκλωμα]] [[es:Circuito magnético]] [[fr:Circuit magnétique]] [[it:Circuito magnetico]] [[zh:磁路]]