In structural analysis, constitutive relations connect applied stresses or forces to strains or deformations. The constitutive relations for linear materials are linear, and termed Hooke's law.

More generally, in physics, a constitutive equation is a relation between two physical quantities (often tensors) that is specific to a material or substance, and does not follow directly from physical law. It is combined with other equations that do represent physical laws to solve some physical problem, like the flow of a fluid in a pipe, or the response of a crystal to an electric field.

Some constitutive equations are simply phenomenological; others are derived from first principles. A constitutive equation frequently has a parameter taken to be a constant of proportionality in ideal systems.

Examples

• Friction

F_f = F_pμ_f

• Drag equation

• Electric susceptibility (Permittivity)

P_j = ε₀χ_ijE_i

D_j = ε_ijE_i

• Magnetic susceptibility (Permeability (electromagnetism))

M_j = μ₀χ_m,ijH_i

B_j = μ_ijH_i

• Linear elasticity (Hooke's law)

F = - kx

or

and in tensor form,

or, equivalently,

• Ohm's law

or

J_j = σ_ijE_i

• Newtonian fluid mechanics:

• Heat capacity

q = c_pT

• Thermal conductivity

• Diffusion (Fick's law)