In electromagnetism and electronics, **inductance** is the property of a conductor by which a change in current flowing through it “induces” (creates) a voltage (electromotive force) in both the conductor itself (self-inductance)^{[1]}^{[2]}^{[3]} and in any nearby conductors (mutual inductance).^{[1]}^{[3]}

## Explanation

These effects are derived from two fundamental observations of physics: First, that a steady current creates a steady magnetic field (Oersted’s law),^{[4]} and second, that a time-varying magnetic field induces voltage in nearby conductors (Faraday’s law of induction).^{[5]} According to Lenz’s law,^{[6]} a changing electric current through a circuit that contains inductance, induces a proportional voltage, which opposes the change in current (self-inductance). The varying field in this circuit may also induce an e.m.f. in neighbouring circuits (mutual inductance).

## Origin of term[edit]

The term ‘inductance’ was coined by Oliver Heaviside in February 1886.^{[7]} It is customary to use the symbol *L* for inductance, in honour of the physicist Heinrich Lenz.^{[8]}^{[9]} In the SI system the measurement unit for inductance is the henry, H, named in honor of the scientist who discovered inductance, Joseph Henry.

## Circuit analysis

To add inductance to a circuit, electrical or electronic components called inductors are used. Inductors are typically manufactured out of coils of wire, with this design delivering two circumstances, one, a concentration of the magnetic field, and two, a linking of the magnetic field into the circuit more than once.

The relationship between the self-inductance *L* of an electrical circuit (in henries), voltage, and current is

Where *v(t)* denotes the voltage in volts across the circuit, and *i(t)* the current in amperes through the circuit. The formula implicitly states that a voltage is induced across an inductor, equal to the product of the inductor’s inductance, and current’s rate of change through the inductor.

All practical circuits have some inductance, which may provide beneficial or detrimental effects. For a tuned circuit, inductance is used to provide a frequency selective circuit. Practical inductors may be used to provide filtering, or energy storage, in a given network. The inductance of a transmission line is one of the properties that determines its characteristic impedance; balancing the inductance and capacitance of cables is important for distortion-free telegraphy and telephony. The inductance of lengthy power transmission lines effectively results in a lessened delivery of AC power, due to the combination of inductance, coupled with transmission lines being spread across great distances. Sensitive circuits, such as microphone and computer network cables, may utilize special cabling construction, limiting the mutual inductance between signal circuits.

The generalization to the case of *K* electrical circuits with currents *i _{m}* and voltages

*v*reads

_{m}Here, inductance *L* is a symmetric matrix. The diagonal coefficients *L _{m,m}* are called coefficients of self-inductance, the off-diagonal elements are called coefficients of mutual inductance. The coefficients of inductance are constant, as long as no magnetizable material with nonlinear characteristics are involved. This is a direct consequence of the linearity of 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 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 linkage of loop *m* is given by

Here *N _{m}* denotes the number of turns in loop

*m*, Φ

*the magnetic flux through this loop, and*

_{m}*L*are some constants. This equation follows from Ampere’s law – magnetic fields and fluxes are linear functions of the currents. By Faraday’s law of induction we have

_{m,n}where *v _{m}* denotes the voltage induced in circuit

*m*. This agrees with the definition of inductance above if the coefficients

*L*are identified with the coefficients of inductance. Because the total currents

_{m,n}*N*contribute to Φ

_{n}i_{n}*it also follows that*

_{m}*L*is proportional to the product of turns

_{m,n}*N*.

_{m}N_{n}## Inductance and magnetic field energy

Multiplying the equation for *v _{m}* above with

*i*and summing over

_{m}dt*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.^{[10]} The integrability condition

requires *L _{m,n}=L_{n,m}*. The inductance matrix

*L*thus is symmetric. The integral of the energy transfer is the magnetic field energy as a function of the currents,

_{m,n}This equation also is a direct consequence of the linearity of Maxwell’s equations. It is helpful to associate changing electric currents with a build-up 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 and mutual inductance

**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 transformerswork, 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 *Neumannformula*, 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 inductors 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,- d
*i*_{1}/d*t*is the derivative, with respect to time, of the current through the inductor of interest, - d
*i*_{2}/d*t*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.^{[11]}

When one inductor is closely coupled to another inductor through mutual inductance, such as in a transformer, 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 inductors 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 over-coupling. 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 effect, 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 of two wire loops

The mutual inductance by a filamentary circuit *m* on a filamentary circuit *n* is given by the double integral *Neumann formula*^{[12]}

The symbol μ_{0} denotes the magnetic constant (4π × 10^{−7} H/m), *C _{m}* and

*C*are the curves spanned by the wires. See a derivation of this equation.

_{n}### Self-inductance of a wire loop

Formally the self-inductance of a wire loop would be given by the above equation with *m* = *n*. The problem, however, is that *1/| x–x’|* 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

*|*>

**x**–**x’**|*a*/2 and a correction term,

^{[13]}

- 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 effect),*Y*= 1/2 when the current is homogeneous across the wire. This approximation is accurate when the wires are long compared to their cross-sectional dimensions.### 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 images). 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 capacitance 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,

^{[14]}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 effect, 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 non-conductive medium enveloping the conductors.

## Self-inductance of simple electrical circuits in air

The self-inductance 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^{[15]}

for w << 1

for w >> 1*N*: Number of turns

*r*: Radius

*l*: Length

*w = r/l*

*m = 4w*^{2}

*E,K*: Elliptic integralsCoaxial cable,

high frequencya _{1}: Outer radius

a: Inner radius

*l*: LengthCircular loop ^{[16]}r: Loop radius

a: Wire radiusRectangle ^{[17]}b, d: Border length

d >> a, b >> a

a: Wire radiusPair of parallel

wiresa: Wire radius

d: Distance, d ≥ 2a

*l*: Length of pairPair of parallel

wires, high

frequencya: Wire radius

d: Distance, d ≥ 2a

*l*: Length of pairWire parallel to

perfectly

conducting walla: Wire radius

d: Distance, d ≥ a

*l*: LengthWire parallel to

conducting wall,

high frequencya: Wire radius

d: Distance, d ≥ a

*l*: LengthThe symbol μ

_{0}denotes the magnetic constant (4π×10^{−7}H/m). For high frequencies the electric current flows in the conductor surface (skin effect), 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*= 1/2 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.## Inductance with physical symmetry

### Inductance of a solenoid

A solenoid is a long, thin coil, i.e. a coil whose length is much greater than the diameter. Under these conditions, and without any magnetic material used, the magnetic flux density within the coil is practically constant and is given by

where is the magnetic constant, the number of turns, the current and the length of the coil. Ignoring end effects, the total magnetic flux through the coil is obtained by multiplying the flux density by the cross-section area :

When this is combined with the definition of inductance,

it follows that the inductance of a solenoid is given by:

A table of inductance for short solenoids of various diameter to length ratios has been calculated by Dellinger, Whittmore, and Ould

^{[18]}This, and the inductance of more complicated shapes, can be derived from Maxwell’s equations. For rigid air-core coils, inductance is a function of coil geometry and number of turns, and is independent of current.

Similar analysis applies to a solenoid with a magnetic core, but only if the length of the coil is much greater than the product of the relative permeability of the magnetic core and the diameter. That limits the simple analysis to low-permeability cores, or extremely long thin solenoids. Although rarely useful, the equations are,

where the relative permeability of the material within the solenoid,

from which it follows that the inductance of a solenoid is given by:

where N is squared because of the definition of inductance.

Note that since the permeability of ferromagnetic materials changes with applied magnetic flux, the inductance of a coil with a ferromagnetic core will generally vary with current.

### Inductance of a coaxial line

Let the inner conductor have radius and permeability , let the dielectric between the inner and outer conductor have permeability , and let the outer conductor have inner radius , outer radius , and permeability . Assume that a DC current flows in opposite directions in the two conductors, with uniform current density. The magnetic field generated by these currents points in the azimuthal direction and is a function of radius ; it can be computed using Ampère’s law:

The flux per length in the region between the conductors can be computed by drawing a surface containing the axis:

Inside the conductors, L can be computed by equating the energy stored in an inductor, , with the energy stored in the magnetic field:

For a cylindrical geometry with no dependence, the energy per unit length is

where is the inductance per unit length. For the inner conductor, the integral on the right-hand-side is ; for the outer conductor it is

Solving for and summing the terms for each region together gives a total inductance per unit length of:

However, for a typical coaxial line application we are interested in passing (non-DC) signals at frequencies for which the resistive skin effect cannot be neglected. In most cases, the inner and outer conductor terms are negligible, in which case one may approximate

## Phasor circuit analysis and impedance

If signals of current and voltage are sine, using phasors, the equivalent impedance of an inductance is given by:

where

*j*is the imaginary unit,*L*is the inductance,*ω = 2πf*is the angular frequency,*f*is the frequency and*ωL = X*is the inductive reactance._{L}

## Nonlinear inductance

Many inductors make use of magnetic materials. These materials over a large enough range exhibit a nonlinear permeability with such effects as saturation. This in-turn 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 large-signal inductance is used in flux calculations. It is defined as:

The differential or small-signal 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 rule of calculus.

There are similar definitions for nonlinear mutual inductances.

- A wire at distance