The basic requirement for heat transfer is the presence of a temperature difference. There can be no net heat transfer between two bodies that are at the same temperature. The temperature difference is the driving force for heat transfer, just as the voltage difference is the driving force for electric cur- rent 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 tem- perature gradient (the temperature difference per unit length or the rate of change of temperature) in that direction. The larger the temperature gradient, the higher the rate of heat transfer.