Heat may be defined as the transfer of thermal energy across a well‎-defined boundary around a thermodynamic system‎.‎ The thermodynamic free energy is the amount of work that a thermodynamic system can perform‎.‎ Enthalpy is a thermodynamic potential that is the sum of the internal energy of the system plus the product of pressure and volume‎.‎ Heat transfer is the exchange of thermal energy between physical systems‎,‎ depending on the temperature and pressure‎,‎ by dissipating heat‎.‎ The fundamental modes of heat transfer are conduction or diffusion‎,‎ convection and radiation‎.‎ Heat transfer always occurs from a region of high temperature to another region of lower temperature‎.‎ In the simplest of terms‎,‎ the discipline of heat transfer is concerned with only two things‎:‎ temperature‎,‎ and the flow of heat‎.‎ Temperature represents the amount of thermal energy available‎,‎ whereas heat flow represents the movement of thermal energy from place to place‎.‎Heat transfer is a process function‎,‎ as opposed to functions of state‎;‎ therefore‎,‎ the amount of heat transferred in a thermodynamic process that changes the state of a system depends on how that process occurs‎,‎ not only the net difference between the initial and final states of the process‎.‎ Several material properties serve to modulate the heat transferred between two regions at differing temperatures‎.‎ Examples include thermal conductivities‎,‎ specific heats‎,‎ material densities‎,‎ fluid velocities‎,‎ fluid viscosities‎,‎ surface emissivities‎,‎ and more‎.‎ Taken together‎,‎ these properties serve to make the solution of many heat transfer problems an involved process‎.‎ Foundations of Heat Transfer focuses on the basic modes of heat transfer‎.‎