Conduction and convection follow a linear process, while radiation follows a non-linear process. That is, the flow of heat through conduction and convection can be modeled as a current through a resistor. The value of the
resistor is the calculated thermal resis- tance of the heat transfer process being modeled. There is a final piece to this puzzle, and that is capacitance. All materials have some thermal capacity; i.e., they stay hot for a while after heat is removed.