Thermodynamics is concerned with the amount of heat transfer as a system undergoes a process from one equilibrium state to another, and it gives no indication about how long the process will take. A thermodynamic analysis simply tells us how much heat must be transferred to realize a specified change of state to satisfy the conservation of energy principle.

In practice, we are concerned with the rate of heat transfer (heat transfer per unit time) than we are with the amount of heat transfer. For example, we can determine the amount of heat transferred from a thermos flask as the hot milk inside cools from 95oC to 85oC by a thermodynamic analysis alone. But, a designer of the thermos flask is primarily interested in how long it will be before the hot milk inside cools to 85oC, and a thermodynamic analysis cannot answer this question. Determining the rates of heat transfer to or from a system and thus the time of cooling or heating, as well as the variation of temperature, is the subject of heat transfer.

Thermodynamics deals with equilibrium states and changes from one equilibrium state to another. Heat transfer, on the other hand, deals with systems that lack thermal equilibrium, and thus it is a non-equilibrium phenomenon. Therefore, the study of heat transfer cannot be based on the principles of thermodynamics alone. However, the laws of thermodynamics lay the framework for the science of heat transfer. The first law requires that the rate of energy transfer into a system be equal to the rate of increase of the energy of that system. The second law requires that heat be transferred in the direction of decreasing temperature. It is analogous to the electric current flowing in the direction of decreasing voltage or the fluid flowing in the direction of decreasing pressure.