Heat transfer is energy in transit due to temperature difference. Whenever there exists a temperature difference in a medium or between media, heat transfer must occur. The basic requirement for heat transfer is the presence of temperature difference. There can be no net heat transfer between two mediums that are at the same temperature. The temperature difference is thedriving force for heat transfer, just as the voltage difference is the driving forcefor electric current flow and pressure difference is the driving force for fluid flow. The rate of heat transfer in a certain direction depends on the magnitude of the temperature gradient (the temperature difference per unit length or the rate of change of temperature) in that direction. The larger the temperature gradient, thehigher the rate of heat transfer.