Heat transfer is the transport of thermal energy from one point to another. Heat is transferred only under the influence of a temperature gradient or temperature difference. The direction of heat transfer is always from the point at the higher temperature to the point at the lower temperature@ in accordance with the second law of thermodynamics. The fundamental modes of heat transfer are conduction@ convection@ and radiation. Conduction is the net transfer of energy within a fluid or solid occurring by the collisions of molecules@ atoms@ or electrons. Convection is the transfer of energy resulting from fluid motion. Convection involves the processes of conduction@ fluid motion@ and mass transfer. Radiation is the transfer of energy from one point to another in the absence of a transporting medium. In practical applications several modes of heat transfer occur simultaneously. For example@ aerodynamic heating of a vehicle surface includes convection to the surface@ radiation away from the surface@ and conduction through the surface structure. Since the three modes of heat transfer obey different laws@ a real insight into such a problem can be gained only after they have been studied separately.