Which condition is necessary for heat transfer to occur?

For heat transfer to occur, there must be a temperature difference between two objects. This principle is grounded in the second law of thermodynamics, which states that heat naturally flows from a region of higher temperature to one of lower temperature. If both objects are at the same temperature, no heat transfer occurs, as there is no driving force behind the thermal exchange. The presence of a temperature gradient is essential, as it creates a condition whereby energy can be transferred until thermal equilibrium is approached.

In practical scenarios like a hot cup of coffee placed on a cool table, heat will flow from the coffee, which is at a higher temperature, to the table, which is cooler, until both reach the same temperature. This fundamental understanding of heat transfer is critical in fields such as engineering, where effective thermal management is important for the operation of systems and equipment.