Question

In a steady state of heat flow through conduction, the thermal current H through a bar of cross section area A is given by H = kA(ΔT/Δl), where ΔT/Δl is temperature gradient along the bar and k is the coefficient of thermal conductivity of the material of the bar. In the above expression the term Δl/(kA) is called thermal resistance offered by a section of bar having length Δl.

A cup of hot coffee is left on table after some time coffee cools down. What is the mode of transfer of heat for this cooling?