In
electromagnetismElectromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
and
electronicsElectronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
,
inductance is the ability of an
inductorAn inductor is a passive twoterminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
to store
energyIn physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
in a
magnetic fieldA magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
. Inductors generate an opposing
voltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
proportional to the rate of change in
current Science and Mathematics :* Electric current* Current , including** Ocean currents** Air currents** Current  currents in rivers and streams* Current density, mathematical concept unifying electric current, fluid current, and others...
in a circuit. This property also is called
selfinductance to discriminate it from
mutual inductance, describing the voltage induced in one electrical circuit by the rate of change of the electric current in another circuit.
The quantitative definition of the self inductance
L of an electrical circuit in
SISi, si, or SI may refer to : Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
unitsA unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...
(
webersIn physics, the weber is the SI unit of magnetic flux. A flux density of one Wb/m2 is one tesla.The weber is named for the German physicist Wilhelm Eduard Weber . Definition :...
per
ampereThe ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after AndréMarie Ampère , French mathematician and physicist, considered the father of electrodynamics...
, known as henries) is
where
v denotes the voltage in volts and
i the current in amperes. The simplest solutions of this equation are a constant current with no voltage or a current changing linearly in time with a constant voltage.
The term 'inductance' was coined by
Oliver HeavisideOliver Heaviside was a selftaught English electrical engineer, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, invented mathematical techniques to the solution of differential equations , reformulated Maxwell's field equations in terms of electric and...
in February 1886. It is customary to use the symbol
L for inductance, possibly in honour of the physicist
Heinrich LenzHeinrich Friedrich Emil Lenz was a Russian physicist of Baltic German ethnicity. He is most noted for formulating Lenz's law in electrodynamics in 1833....
.
The
SIThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
unit of inductance is the
henry (H), named after American scientist and magnetic researcher
Joseph HenryJoseph Henry was an American scientist who served as the first Secretary of the Smithsonian Institution, as well as a founding member of the National Institute for the Promotion of Science, a precursor of the Smithsonian Institution. During his lifetime, he was highly regarded...
. 1 H = 1
WbIn physics, the weber is the SI unit of magnetic flux. A flux density of one Wb/m2 is one tesla.The weber is named for the German physicist Wilhelm Eduard Weber . Definition :...
/
AThe ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after AndréMarie Ampère , French mathematician and physicist, considered the father of electrodynamics...
.
Inductance is caused by the magnetic field generated by electric currents according to
Ampere's lawIn classical electromagnetism, Ampère's circuital law, discovered by AndréMarie Ampère in 1826, relates the integrated magnetic field around a closed loop to the electric current passing through the loop...
. To add inductance to a circuit,
electronic componentAn electronic component is a basic electronic element and may be available in a discrete form having two or more electrical terminals . These are intended to be connected together, usually by soldering to a printed circuit board, in order to create an electronic circuit with a particular function...
s called
inductorAn inductor is a passive twoterminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s are used, typically consisting of coils of wire to concentrate the magnetic field and to collect the induced voltage. This is analogous to adding
capacitanceIn electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallelplate capacitor...
to a circuit by adding
capacitorA capacitor is a passive twoterminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s. Capacitance is caused by the electric field generated by electric charge according to
Gauss's lawIn physics, Gauss's law, also known as Gauss's flux theorem, is a law relating the distribution of electric charge to the resulting electric field. Gauss's law states that:...
.
The generalization to the case of
K electrical circuits with currents
i_{m} and voltages
v_{m} reads
Inductance here is a symmetric matrix. The diagonal coefficients
L_{m,m} are called coefficients of self inductance, the offdiagonal elements are called coefficients of mutual inductance. The coefficients of inductance are constant as long as no magnetizable material with nonlinear characteristics is involved. This is a direct consequence of the linearity of
Maxwell's equationsMaxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.Maxwell's equations...
in the fields and the current density. The coefficients of inductance become functions of the currents in the nonlinear case, see nonlinear inductance.
Derivation from Faraday's law of inductance
The inductance equations above are a consequence of
Maxwell's equationsMaxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.Maxwell's equations...
. There is a straightforward derivation in the important
case of electrical circuits consisting of thin wires.
Consider a system of
K wire loops, each with one or several wire turns. The
flux linkageFlux linkage is a property of a coil of conducting wire and the magnetic field through which it passes. It is determined by the number of turns in the coil and the flux of the magnetic field....
of loop
m is given by
Here
N_{m} denotes the number of turns in loop
m, Φ
_{m} the
magnetic fluxMagnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...
through this loop, and
L_{m,n} are some constants. This equation follows from
Ampere's lawIn classical electromagnetism, Ampère's circuital law, discovered by AndréMarie Ampère in 1826, relates the integrated magnetic field around a closed loop to the electric current passing through the loop...
 magnetic fields and fluxes are linear functions of the currents. By
Faraday's lawFaraday's law of induction dates from the 1830s, and is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators...
of induction we have
where
v_{m} denotes the voltage induced in circuit
m. This agrees with the definition of inductance above if the coefficients
L_{m,n} are identified with the coefficients of inductance. Because the total currents
N_{n}i_{n} contribute to Φ
_{m} it also follows that
L_{m,n} is proportional to the product of turns
N_{m}N_{n}.
Inductance and magnetic field energy
Multiplying the equation for
v_{m} above with
i_{m}dt and summing over
m gives the energy transferred to the system in the time interval
dt,
This must agree with the change of the magnetic field energy
W caused by the currents. The integrability condition
requires
L_{m,n}=L_{n,m}. The inductance matrix
L_{m,n} thus is symmetric. The integral of the energy transfer is the magnetic field energy as a function of the currents,
This equation also is a direct consequence of the linearity of Maxwell's equations. It is helpful to associate changing electric currents with a buildup or decrease of magnet field energy. The corresponding energy transfer requires or generates a voltage. A mechanical analogy in the
K=1 case with magnetic field energy (1/2)
Li^{2} is a body with mass
M, velocity
u and kinetic energy (1/2)
Mu^{2}. The rate of change of velocity (current) multiplied with mass (inductance) requires or generates a force (an electrical voltage).
Coupled inductors
Mutual inductance occurs when the change in current in one inductor induces a voltage in another nearby inductor. It is important as the mechanism by which
transformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s work, but it can also cause unwanted coupling between conductors in a circuit.
The mutual inductance,
M, is also a measure of the coupling between two inductors. The mutual inductance by circuit
i on circuit
j is given by the double integral
NeumannFranz Ernst Neumann was a German mineralogist, physicist and mathematician.Biography:Neumann was born in Joachimsthal, Margraviate of Brandenburg, located not far from Berlin. In 1815 he interrupted his studies at Berlin to serve as a volunteer in the Hundred Days against Napoleon, and was...
formula, see calculation techniques
The mutual inductance also has the relationship:
where
is the mutual inductance, and the subscript specifies the relationship of the voltage induced in coil 2 due to the current in coil 1.
 N_{1} is the number of turns in coil 1,
 N_{2} is the number of turns in coil 2,
 P_{21} is the permeance of the space occupied by the flux.
The mutual inductance also has a relationship with the coupling coefficient. The coupling coefficient is always between 1 and 0, and is a convenient way to specify the relationship between a certain orientation of inductor with arbitrary inductance:
where
 k is the coupling coefficient and 0 ≤ k ≤ 1,
 L_{1} is the inductance of the first coil, and
 L_{2} is the inductance of the second coil.
Once the mutual inductance,
M, is determined from this factor, it can be used to predict the behavior of a circuit:
where
 V_{1} is the voltage across the inductor of interest,
 L_{1} is the inductance of the inductor of interest,
 dI_{1}/dt is the derivative, with respect to time, of the current through the inductor of interest,
 dI_{2}/dt is the derivative, with respect to time, of the current through the inductor that is coupled to the first inductor, and
 M is the mutual inductance.
The minus sign arises because of the sense the current
I_{2} has been defined in the diagram. With both currents defined going into the dots the sign of M will be positive.
When one inductor is closely coupled to another inductor through mutual inductance, such as in a
transformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
, the voltages, currents, and number of turns can be related in the following way:
where
 V_{s} is the voltage across the secondary inductor,
 V_{p} is the voltage across the primary inductor (the one connected to a power source),
 N_{s} is the number of turns in the secondary inductor, and
 N_{p} is the number of turns in the primary inductor.
Conversely the current:
where
 I_{s} is the current through the secondary inductor,
 I_{p} is the current through the primary inductor (the one connected to a power source),
 N_{s} is the number of turns in the secondary inductor, and
 N_{p} is the number of turns in the primary inductor.
Note that the power through one inductor is the same as the power through the other. Also note that these equations don't work if both transformers are forced (with power sources).
When either side of the transformer is a tuned circuit, the amount of mutual inductance between the two windings determines the shape of the frequency response curve. Although no boundaries are defined, this is often referred to as loose, critical, and overcoupling. When two tuned circuits are loosely coupled through mutual inductance, the bandwidth will be narrow. As the amount of mutual inductance increases, the bandwidth continues to grow. When the mutual inductance is increased beyond a critical point, the peak in the response curve begins to drop, and the center frequency will be attenuated more strongly than its direct sidebands. This is known as overcoupling.
Calculation techniques
In the most general case, inductance can be calculated from Maxwell's equations. Many important cases can be solved using simplifications. Where high frequency currents are considered, with
skin effectSkin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...
, the surface current densities and magnetic field may be obtained by solving the Laplace equation. Where the conductors are thin wires, self inductance still depends on the wire radius and the distribution of the current in the wire. This current distribution is approximately constant (on the surface or in the volume of the wire) for a wire radius much smaller than other length scales.
Mutual inductance
The mutual inductance by a filamentary circuit
i on a filamentary circuit
j is given by the double integral
NeumannFranz Ernst Neumann was a German mineralogist, physicist and mathematician.Biography:Neumann was born in Joachimsthal, Margraviate of Brandenburg, located not far from Berlin. In 1815 he interrupted his studies at Berlin to serve as a volunteer in the Hundred Days against Napoleon, and was...
formula
The symbol μ
_{0} denotes the magnetic constant (4π×10
^{−7} H/m),
C_{i} and
C_{j} are the curves spanned by the wires,
R_{ij} is the distance between two points. See a
derivation of this equation.
Selfinductance
Formally the selfinductance of a wire loop would be given by the above equation with
i =
j. The problem, however, is that
1/R now becomes infinite, making it necessary to take the finite wire radius
a and the distribution of the current in the wire into account. There remain the contribution from the integral over all points with R ≥
a/2 and a correction term,
Here
a and
l denote radius and length of the wire, and
Y is a constant that depends on the distribution of the current in the wire:
Y = 0 when the current flows in the surface of the wire (
skin effectSkin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...
),
Y = 1/4 when the current is homogeneous across the wire. This approximation is accurate when the wires are long compared to their crosssectional dimensions. Here is a
derivation of this equation.
Method of images
In some cases different current distributions generate the same magnetic field in some section of space. This fact may be used to relate self inductances (
method of imagesSee also Method of image charges for applications in electrostatics and magnetostaticsMethod of images is a mathematical tool for solving differential equations in which the domain of the sought function is extended by the addition of its mirror image with respect to a symmetry hyperplane, with...
). As an example consider the two systems:
 A wire at distance d/2 in front of a perfectly conducting wall (which is the return)
 Two parallel wires at distance d, with opposite current
The magnetic field of the two systems coincides (in a half space). The magnetic field energy and the inductance of the second system thus are twice as large as that of the first system.
Relation between inductance and capacitance
Inductance per length L' and
capacitanceIn electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallelplate capacitor...
per length C' are related to each other in the special case of transmission lines consisting of two parallel perfect conductors of arbitrary but constant cross section,
Here ε and µ denote dielectric constant and magnetic permeability of the medium the conductors are embedded in. There is no electric and no magnetic field inside the conductors (complete
skin effectSkin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...
, high frequency). Current flows down on one line and returns on the other. Signals will propagate along the transmission line at the speed of electromagnetic radiation in the nonconductive medium enveloping the conductors.
Selfinductance of simple electrical circuits in air
The selfinductance of many types of electrical circuits can be given in closed form. Examples are listed in the table.
Inductance of simple electrical circuits in air
Type  Inductance /  Comment 

Single layer solenoid 
for w << 1
for w >> 1 
: Number of turns r: Radius l: Length w = r/l : Elliptic integralIn integral calculus, elliptic integrals originally arose in connection with the problem of giving the arc length of an ellipse. They were first studied by Giulio Fagnano and Leonhard Euler... s 
Coaxial cable, high frequency 

a_{1}: Outer radius a: Inner radius l: Length 
Circular loop 

r: Loop radius a: Wire radius 
Rectangle 

b, d: Border length d >> a, b >> a a: Wire radius 
Pair of parallel wires 

a: Wire radius d: Distance, d ≥ 2a l: Length of pair 
Pair of parallel wires, high frequency 

a: Wire radius d: Distance, d ≥ 2a l: Length of pair 
Wire parallel to perfectly conducting wall 

a: Wire radius d: Distance, d ≥ a l: Length 
Wire parallel to conducting wall, high frequency 

a: Wire radius d: Distance, d ≥ a l: Length 
The symbol μ
_{0} denotes the magnetic constant (4π×10
^{−7} H/m). For high frequencies the electric current flows in the conductor surface
(
skin effectSkin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...
), and depending on the geometry it sometimes is necessary to distinguish
low and high frequency inductances. This is the purpose of the constant
Y:
Y = 0 when the current is uniformly distributed over the surface of the wire (skin effect),
Y = 1/4 when the current is uniformly distributed over the cross section of the wire. In the high frequency case, if conductors approach each other, an additional screening current flows in their surface, and expressions containing Y become invalid. Details for some circuit types are available on another page.
Phasor circuit analysis and impedance
Using
phasorsIn physics and engineering, a phase vector, or phasor, is a representation of a sine wave whose amplitude and angular frequency are timeinvariant. It is a subset of a more general concept called analytic representation. Phasors decompose the behavior of a sinusoid into three independent...
, the equivalent
impedanceElectrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
of an inductance is given by:
where
 j is the imaginary unit
In mathematics, the imaginary unit allows the real number system ℝ to be extended to the complex number system ℂ, which in turn provides at least one root for every polynomial . The imaginary unit is denoted by , , or the Greek...
,
 L is the inductance,
 ω = 2πf is the angular frequency
In physics, angular frequency ω is a scalar measure of rotation rate. Angular frequency is the magnitude of the vector quantity angular velocity...
,
 f is the frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
and
 Lω = X_{L} is the inductive reactance.
Nonlinear inductance
Many inductors make use of magnetic materials. These materials over a large enough range exhibit a nonlinear permeability with such effects as
saturationSeen in some magnetic materials, saturation is the state reached when an increase in applied external magnetizing field H cannot increase the magnetization of the material further, so the total magnetic field B levels off...
. This inturn makes the resulting inductance a function of the applied current. Faraday's Law still holds but inductance is ambiguous and is different whether you are calculating circuit parameters or magnetic fluxes.
The secant or largesignal inductance is used in flux calculations. It is defined as:
The differential or smallsignal inductance, on the other hand, is used in calculating voltage. It is defined as:
The circuit voltage for a nonlinear inductor is obtained via the differential inductance as shown by Faraday's Law and the
chain ruleIn calculus, the chain rule is a formula for computing the derivative of the composition of two or more functions. That is, if f is a function and g is a function, then the chain rule expresses the derivative of the composite function in terms of the derivatives of f and g.In integration, the...
of calculus.
There are similar definitions for nonlinear mutual inductances.
General references
 Küpfmüller K.
Karl Küpfmüller was a German electrical engineer, who was prolific in the areas of communications technology, measurement and control engineering, acoustics, communication theory and theoretical electrotechnology....
, Einführung in die theoretische Elektrotechnik, SpringerVerlag, 1959.
 Heaviside O., Electrical Papers. Vol.1. – L.; N.Y.: Macmillan, 1892, p. 429560.
 F. LangfordSmith, editor, 1953, Radiotron Designer's Handbook, 4th Edition, Wireless Press for Amalgamated Wireless Valve Company PTY, LTD, Sydney, Australia together with Eectron Tube Division of the Radio Corporation of America [RCA], Harrison, N. J. No Library of Congress Card Catalog Number or ISBN. Chapter 10 pp. 429448 Calculation of Inductance includes a wealth of approximate formulas and nomographs for singlelayer solenoids of various coil diameters and pitch of windings and lengths, the effects of screens, formulas and nomographs for multilayer coils (long and short), for toroidal coils, for flat spirals, and a nomograph for the mutual inductance between coaxial solenoids. With 56 references.
External links