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.