In physics and engineering, a constitutive equation or constitutive relation is a relation between two physical quantities (especially kinetic quantities as related to kinematic quantities) that is specific to a material or substance, and approximates the response of that material to external stimuli, usually as applied fields or forces. They are combined with other equations governing physical laws to solve physical problems; for example in fluid mechanics the flow of a fluid in a pipe, in solid state physics the response of a crystal to an electric field, or in structural analysis, the connection between applied stresses or forces to strains or deformations.
Some constitutive equations are simply phenomenological; others are derived from first principles. A common approximate constitutive equation frequently is expressed as a simple proportionality using a parameter taken to be a property of the material, such as electrical conductivity or a spring constant. However, it is often necessary to account for the directional dependence of the material, and the scalar parameter is generalized to a tensor. Constitutive relations are also modified to account for the rate of response of materials and their non-linear behavior. See the article Linear response function.
Mechanical properties of matter 
The first constitutive equation (constitutive law) was developed by Robert Hooke and is known as Hooke's law. It deals with the case of linear elastic materials. Following this discovery, this type of equation, often called a "stress-strain relation" in this example, but also called a "constitutive assumption" or an "equation of state" was commonly used. Walter Noll advanced the use of constitutive equations, clarifying their classification and the role of invariance requirements, constraints, and definitions of terms like "material", "isotropic", "aeolotropic", etc. The class of "constitutive relations" of the form stress rate = f (velocity gradient, stress, density) was the subject of Walter Noll's dissertation in 1954 under Clifford Truesdell.
Quantity (common name/s) (Common) symbol/s Defining equation SI units Dimension General stress, P, σ
F may be any force applied to area A
Pa = N m−2 [M] [T]−2[L]−1 General strain ε
- D = dimension (length, area, volume)
- ΔD = change in dimension
dimensionless dimensionless General elastic modulus Emod Pa = N m−2 [M] [T]−2 [L]−1 Young's modulus E, Y Pa = N m−2 [M] [T] −2[L]−1 Shear modulus G Pa = N m−2 [M] [T]−2[L]−1 Bulk modulus B Pa = N m−2 [M] [T]−2[L]−1 Compressibility C Pa−1 = m2 N−1 [L] [T]2[M]−1
Deformation of solids 
Friction is a complicated phenomenon, macroscopically the friction force F between the interface of two materials can be modelled as proportional to the reaction force R at a point of contact between two interfaces, through a dimensionless coefficient of friction μf which depends on the pair of materials:
This can be applied to static friction (friction preventing two stationary objects from slipping on their own), kinetic friction (friction between two objects scraping/sliding past each other), or rolling (frictional force which prevents slipping but causes a torque to exert on a round object). Surprisingly, the friction force does not depend on the surface area of common contact.
Stress and strain 
The stress-strain constitutive relation for linear materials is commonly known as Hooke's law. In its simplest form, the law defines the spring constant constant (or elasticity constant) k in a scalar equation, stating the tensile/compressive force is proportional to the extended (or contracted) displacement x:
In general, forces which deform solids can be normal to a surface of the material (normal forces), or tangential (shear forces), this can be described mathematically using the stress tensor:
Solid-state deformations 
Using Hooke's law, several classes of deformations in elastic materials are the following:
- Elastic: if the material satisfies Hooke's law.
- Anelastic: if the material almost satisfies Hooke's law, in which the applied force induces additional time-dependent resistive forces (i.e. depend on rate of change of extension/compression, in addition to the extension/compression). Metals and ceramics have this characteristic, but are usually negligible, although not so much when heating due to friction occurs (such as vibrations or shear stresses in machines).
- Viscoelastic: If the time-dependent resistive contributions are large, and cannot be neglected. Rubbers and plastics have this property, and certainly do not satisfy Hooke's law. In fact, elastic hysteresis occurs.
- Plastic: The applied force induces non-linear displacements in the material, i.e. force is not proportional to displacement, but now a non-linear function.
The relative speed of separation vapproach of an object A after a collision with another object B is related to the relative speed of approach vseparation by the coefficient of restitution, defined by Newton's experimental impact law:
which depends the materials A and B are made from, since the collision involves interactions at the surfaces of A and B. Usually 0 ≤ e ≤ 1, in which e = 1 for completely elastic collisions, and e = 0 for completely inelastic collisions. It's possible for e ≥ 1 to occur - for superelastic (or explosive) collisions.
Deformation of fluids 
where the drag coefficient (dimensionless) cd depends on the geometry of the object and the drag forces at the interface between the fluid and object.
with u(y) the variation of the flow velocity u in the cross-flow (transverse) direction y. In general, for a Newtonian fluid, the relationship between the elements τij of the shear stress tensor and the deformation of the fluid is given by
- with and
where vi are the components of the flow velocity vector in the corresponding xi coordinate directions, eij are the components of the strain rate tensor, Δ is the volumetric strain rate (or dilatation rate) and δij is the Kronecker delta.
The ideal gas law is a constitutive relation in the sense the pressure p and volume V are related to the temperature T, via the number of moles n of gas:
where R is the gas constant (J K−1 mol−1).
In both classical and quantum physics, the precise dynamics of a system form a set of coupled differential equations, which are almost always too complicated to be solved exactly, even at the level of statistical mechanics. In the context of electromagnetism, this remark applies to not only the dynamics of free charges and currents (which enter Maxwell's equations directly), but also the dynamics of bound charges and currents (which enter Maxwell's equations through the constitutive relations). As a result, various approximation schemes are typically used.
For example, in real materials, complex transport equations must be solved to determine the time and spatial response of charges, for example, the Boltzmann equation or the Fokker–Planck equation or the Navier-Stokes equations. For example, see magnetohydrodynamics, fluid dynamics, electrohydrodynamics, superconductivity, plasma modeling. An entire physical apparatus for dealing with these matters has developed. See for example, linear response theory, Green–Kubo relations and Green's function (many-body theory).
These complex theories provide detailed formulas for the constitutive relations describing the electrical response of various materials, such as permittivities, permeabilities, conductivities and so forth.
It is necessary to specify the relations between displacement field D and E, and the magnetic H-field H and B, before doing calculations in electromagnetism (i.e. applying Maxwell's macroscopic equations). These equations specify the response of bound charge and current to the applied fields and are called constitutive relations.
Determining the constitutive relationship between the auxiliary fields D and H and the E and B fields starts with the definition of the auxiliary fields themselves:
where P is the polarization field and M is the magnetization field which are defined in terms of microscopic bound charges and bound current respectively. Before getting to how to calculate M and P it is useful to examine the following special cases.
Without magnetic or dielectric materials 
In the absence of magnetic or dielectric materials, the constitutive relations are simple:
Isotropic linear materials 
In an (isotropic) linear material, where P is proportional to E, and M is proportional to B, the constitutive relations are also straightforward. In terms of the polarization P and the magnetization M they are:
General case 
For real-world materials, the constitutive relations are not linear, except approximately. Calculating the constitutive relations from first principles involves determining how P and M are created from a given E and B.[note 1] These relations may be empirical (based directly upon measurements), or theoretical (based upon statistical mechanics, transport theory or other tools of condensed matter physics). The detail employed may be macroscopic or microscopic, depending upon the level necessary to the problem under scrutiny.
In general, the constitutive relations can usually still be written:
but ε and μ are not, in general, simple constants, but rather functions of E, B, position and time, and tensorial in nature. Examples are:
- Dispersion and absorption where ε and μ are functions of frequency. (Causality does not permit materials to be nondispersive; see, for example, Kramers–Kronig relations). Neither do the fields need to be in phase which leads to ε and μ being complex. This also leads to absorption.
- Nonlinearity where ε and μ are functions of E and B.
- Anisotropy (such as birefringence or dichroism) which occurs when ε and μ are second-rank tensors,
- Dependence of P and M on E and B at other locations and times. This could be due to spatial inhomogeneity; for example in a domained structure, heterostructure or a liquid crystal, or most commonly in the situation where there are simply multiple materials occupying different regions of space). Or it could be due to a time varying medium or due to hysteresis. In such cases P and M can be calculated as:
- in which the permittivity and permeability functions are replaced by integrals over the more general electric and magnetic susceptibilities.
In practice, some materials properties have a negligible impact in particular circumstances, permitting neglect of small effects. For example: optical nonlinearities can be neglected for low field strengths; material dispersion is unimportant when frequency is limited to a narrow bandwidth; material absorption can be neglected for wavelengths for which a material is transparent; and metals with finite conductivity often are approximated at microwave or longer wavelengths as perfect metals with infinite conductivity (forming hard barriers with zero skin depth of field penetration).
Calculation of constitutive relations 
The theoretical calculation of a material's constitutive equations is a common, important, and sometimes difficult task in theoretical condensed-matter physics and materials science. In general, the constitutive equations are theoretically determined by calculating how a molecule responds to the local fields through the Lorentz force. Other forces may need to be modeled as well such as lattice vibrations in crystals or bond forces. Including all of the forces leads to changes in the molecule which are used to calculate P and M as a function of the local fields.
The local fields differ from the applied fields due to the fields produced by the polarization and magnetization of nearby material; an effect which also needs to be modeled. Further, real materials are not continuous media; the local fields of real materials vary wildly on the atomic scale. The fields need to be averaged over a suitable volume to form a continuum approximation.
These continuum approximations often require some type of quantum mechanical analysis such as quantum field theory as applied to condensed matter physics. See, for example, density functional theory, Green–Kubo relations and Green's function.
A different set of homogenization methods (evolving from a tradition in treating materials such as conglomerates and laminates) are based upon approximation of an inhomogeneous material by a homogeneous effective medium (valid for excitations with wavelengths much larger than the scale of the inhomogeneity).
The theoretical modeling of the continuum-approximation properties of many real materials often rely upon experimental measurement as well. For example, ε of an insulator at low frequencies can be measured by making it into a parallel-plate capacitor, and ε at optical-light frequencies is often measured by ellipsometry.
Thermoelectric and electromagnetic properties of matter 
Electromagnetic properties of solids Property/effect Stimuli/response parameters of system Constitutive tensor of system Equation Hall effect ρ = electrical resistivity (Ω m) Direct Piezoelectric Effect
- σ = Stress (Pa)
- P = (dielectric) polarization (C m−2)
d = direct piezoelectric coefficient (K−1) Converse Piezoelectric Effect
- ε = Strain (dimensionless)
- E = electric field strength (N C−1)
d = direct piezoelectric coefficient (K−1) Piezomagnetic effect
- σ = Stress (Pa)
- M = magnetization (A m−1)
q = piezomagnetic coefficient (K−1)
Thermoelectric properties of solids Property/effect Stimuli/response parameters of system Constitutive tensor of system Equation Pyroelectricity
- P = (dielectric) polarization (C m−2)
- T = temperature (K)
p = pyroelectric coefficient (C m−2 K−1) Electrocaloric effect
- S = entropy (J K−1)
- E = electric field strength (N C−1)
p = pyroelectric coefficient (C m−2 K−1) Seebeck effect
- E = electric field strength (N C−1 = V m−1)
- T = temperature (K)
- x = displacement (m)
β = thermopower (V K−1) Peltier effect
- E = electric field strength (N C−1)
- J = electric current density (A m−2)
- q = heat flux (W m−2)
Π = Peltier coefficient (W A−1)
The (absolute) refractive index of a medium n (dimensionless) is an inherently important property of geometric and physical optics defined as the ratio of the luminal speed in vacuum c0 to that in the medium c:
where ε is the permittivity and εr the relative permittivity of the medium, likewise μ is the permeability and μr are the relative permmeability of the medium. The vacuum permittivity is ε0 and vacuum permeability is μ0. In general, n (also εr) are complex numbers.
The relative refractive index is defined as the ratio of the two refractive indices. Absolute is for on material, relative applies to every possible pair of interfaces;
- Speed of light in matter
As a consequence of the definition, the speed of light in matter is
for special case of vacuum; ε = ε0 and μ = μ0,
The piezooptic effect relates the stresses in solids σ to the dielectric impermeability a, which are coupled by a fourth-rank tensor called the piezooptic coefficient Π (units K−1):
Transport phenomena 
Definitions (thermal properties of matter) Quantity (Common Name/s) (Common) Symbol/s Defining Equation SI Units Dimension General heat capacity C = heat capacity of substance J K−1 [M][L]2[T]−2[Θ]−1 Linear thermal expansion
- L = length of material (m)
- α = coefficient linear thermal expansion (dimensionless)
- ε = strain tensor (dimensionless)
K−1 [Θ]−1 Volumetric thermal expansion β, γ
- V = volume of object (m3)
- p = constant pressure of surroundings
K−1 [Θ]−1 Thermal conductivity κ, K, λ, W m−1 K−1 [M][L][T]−3[Θ]−1 Thermal conductance U W m−2 K−1 [M][T]−3[Θ]−1 Thermal resistance R
Δx = displacement of heat transfer (m)
m2 K W−1 [M]−1[L][T]3[Θ]
Definitions (Electrical/magnetic properties of matter) Quantity (Common Name/s) (Common) Symbol/s Defining Equation SI Units Dimension Electrical resistance R Ω = V A−1 = J s C−2 [M] [L]2 [T]−3 [I]−2 Resistivity ρ Ω m [M]2 [L]2 [T]−3 [I]−2 Resistivity temperature coefficient, linear temperature dependence α K−1 [Θ]−1 Electrical conductance G S = Ω−1 [T]3 [I]2 [M]−1 [L]−2 Electrical conductivity σ Ω−1 m−1 [I]2 [T]3 [M]−2 [L]−2 Magnetic reluctance R, Rm, A Wb−1 = H−1 [M]−1[L]−2[T]2 Magnetic permeance P, Pm, Λ, Wb A−1 = H [M][L]2[T]−2
Definitive laws 
There are several laws which describe the transport of matter, or properties of it, in an almost identical way. In every case, in words they read:
- Flux (density) is proportional to a gradient, the constant of proportionality is the characteristic of the material.
In general the constant must be replaced by a 2nd rank tensor, to account for directional dependences of the material.
Property/effect Nomenclature Equation Fick's law of diffusion, defines diffusion coefficient D Darcy's law for porous flow in matter, defines permeability κ Ohm's law of electric conduction, defines electric conductivity (and hence resistivity and resistance)
- V = potential difference in material (V)
- I = electrical current through material (A)
- R = resistance of material (Ω)
- ∂V/∂x = potential gradient (electric field) through material (V m−1)
- J = electric current density through material (A m−2)
- σ = electric conductivity of material (Ω−1 m−1)
- ρ = electrical resistivity of material (Ω m)
- Simplist form is:
- More general forms are:
Fourier's law of thermal conduction, defines thermal conductivity λ Stefan–Boltzmann law of black-body radiation, defines emmisivity ε
- For a single radiator:
- 0 ≤ ε ≤ 1
- ε = 0 for perfect reflector
- ε = 1 for perfect absorber (true black body)
See also 
- Clifford Truesdell & Walter Noll; Stuart S. Antman, editor (2004). The Non-linear Field Theories of Mechanics. Springer. p. 4. ISBN 3-540-02779-3.
- See Truesdell's account in Truesdell The naturalization and apotheosis of Walter Noll. See also Noll's account and the classic treatise by both authors: Clifford Truesdell & Walter Noll - Stuart S. Antman (editor) (2004). "Preface" (Originally published as Volume III/3 of the famous Encyclopedia of Physics in 1965). The Non-linear Field Theories of Mechanics (3rd ed.). Springer. p. xiii. ISBN 3-540-02779-3.
- Jørgen Rammer (2007). Quantum Field Theory of Nonequilibrium States. Cambridge University Press. ISBN 978-0-521-87499-1.
- Encyclopaedia of Physics (2nd Edition), R.G. Lerner, G.L. Trigg, VHC publishers, 1991, ISBN (Verlagsgesellschaft) 3-527-26954-1, ISBN (VHC Inc.) 0-89573-752-3
- Essential Principles of Physics, P.M. Whelan, M.J. Hodgeson, 2nd Edition, 1978, John Murray, ISBN 0 7195 3382 1
- Kay, J.M. (1985). Fluid Mechanics and Transfer Processes. Cambridge University Press. pp. 10 & 122–124. ISBN 9780521316248.
- The generalization to non-isotropic materials is straight forward; simply replace the constants with tensor quantities.
- Halevi, Peter (1992). Spatial dispersion in solids and plasmas. Amsterdam: North-Holland. ISBN 978-0-444-87405-4.
- Jackson, John David (1999). Classical Electrodynamics (3rd ed.). New York: Wiley. ISBN 0-471-30932-X.
- Note that the 'magnetic susceptibility' term used here is in terms of B and is different from the standard definition in terms of H.
- TG Mackay and A Lakhtakia (2010). Electromagnetic Anisotropy and Bianisotropy: A Field Guide. World Scientific.
- Aspnes, D.E., "Local-field effects and effective-medium theory: A microscopic perspective", Am. J. Phys. 50, p. 704-709 (1982).
- Habib Ammari & Hyeonbae Kang (2006). Inverse problems, multi-scale analysis and effective medium theory : workshop in Seoul, Inverse problems, multi-scale analysis, and homogenization, June 22–24, 2005, Seoul National University, Seoul, Korea. Providence RI: American Mathematical Society. p. 282. ISBN 0-8218-3968-3.
- O. C. Zienkiewicz, Robert Leroy Taylor, J. Z. Zhu, Perumal Nithiarasu (2005). The Finite Element Method (Sixth ed.). Oxford UK: Butterworth-Heinemann. p. 550 ff. ISBN 0-7506-6321-9.
- N. Bakhvalov and G. Panasenko, Homogenization: Averaging Processes in Periodic Media (Kluwer: Dordrecht, 1989); V. V. Jikov, S. M. Kozlov and O. A. Oleinik, Homogenization of Differential Operators and Integral Functionals (Springer: Berlin, 1994).
- Vitaliy Lomakin, Steinberg BZ, Heyman E, & Felsen LB (2003). "Multiresolution Homogenization of Field and Network Formulations for Multiscale Laminate Dielectric Slabs". IEEE Transactions on Antennas and Propagation 51 (10): 2761 ff. Bibcode:2003ITAP...51.2761L. doi:10.1109/TAP.2003.816356.
- AC Gilbert (Ronald R Coifman, Editor) (2000-05). Topics in Analysis and Its Applications: Selected Theses. Singapore: World Scientific Publishing Company. p. 155. ISBN 981-02-4094-5.
- Edward D. Palik & Ghosh G (1998). Handbook of Optical Constants of Solids. London UK: Academic Press. p. 1114. ISBN 0-12-544422-2.
- The free charges and currents respond to the fields through the Lorentz force law and this response is calculated at a fundamental level using mechanics. The response of bound charges and currents is dealt with using grosser methods subsumed under the notions of magnetization and polarization. Depending upon the problem, one may choose to have no free charges whatsoever.