where the constant of proportionality, formula_34, is the thermal conductivity. This is called Fourier's law of heat conduction. In actuality, it is not a law but a "definition" of thermal conductivity in terms of the independent physical quantities formula_28 and formula_32. As such, its usefulness depends on the ability to determine formula_1 for a given material under given conditions. The constant formula_1 itself usually depends on formula_32 and thereby implicitly on space and time. An explicit space and time dependence could also occur if the material is inhomogeneous or changing with time.