The constant of proportionality k is the thermal conductivity; it is a physical property of the material. In the present scenario, since T 2 > T 1 heat flows in the minus x-direction and q is negative, which in turn means that k > 0 . In general, k is always defined to be positive. The same definition of k can also be extended to gases and liquids, provided other modes of energy transport, such as convection and radiation, are eliminated or accounted for.