Thermal Properties

Abstract

All solids can conduct heat — some better, others worse. In an isotropic solid, the spread of heat obeys Fourier’s law, discovered in 1882

$$ Q = - k grad T = - k\left( {\frac{{\partial T}}{{\partial n}}} \right),$$

((3.1))

where Q is the surface density of the heat stream; it is a vector whose module is equal to the heat flow across the cross section perpendicular to Q; T is the temperature; $\frac{{\partial T}}{{\partial n}}$ is the gradient of temperature along a normal n to the isothermic surface; and k is heat conductivity. The minus sign to the right of the expression (3.1) is connected to the fact that heat flows in a direction opposite to the temperature gradient, from the hot to the cold side. In anisotropic solids, the tensor of second rank and its form depend on crystal symmetry.