The main driving force for heat transfer is:

The main driving force for heat transfer is the temperature difference. Heat naturally flows from areas of higher temperature to areas of lower temperature, a principle embodied in the second law of thermodynamics. This phenomenon occurs because energy in the form of thermal energy is transferred in an attempt to reach thermal equilibrium, where the temperature across the different bodies or systems becomes uniform.

When there is a temperature gradient, the molecular activity is higher at the hotter region, leading to the transfer of kinetic energy to the cooler molecules through conduction, convection, or radiation. As such, without a temperature difference, there would be no net flow of heat energy.

While pressure differences, volume differences, and density differences can influence various thermodynamic processes, they are not the primary drivers of heat transfer. In heat transfer processes, the presence of a temperature differential is crucial to initiate and sustain the flow of heat.