HEAT transfer is energy in transit because of a temperature difference. The thermal energy is transferred from one region to another by three modes of heat transfer: conduction, radiation, and convection. Heat transfer is among a group of energy transport phenomena that includes mass transfer (see Chapter 5), momentum transfer (see Chapter 2), and electrical conduction. Transport phenomena have similar rate equations, in which flux is proportional to a potential difference. In heat transfer by conduction and convection, the potential difference is the temperature difference. Heat, mass, and momentum transfer are often considered together because of their similarities and interrelationship in many common physical processes.