What is defined as heat flow?

Heat flow refers to the transfer of thermal energy from one object or substance to another due to a temperature difference. The fundamental principle of heat transfer dictates that energy naturally moves from areas of higher temperature to areas of lower temperature. Therefore, when a hot substance comes into contact with a cold substance, the heat energy will flow from the hot substance to the cold one until thermal equilibrium is reached, meaning both substances reach the same temperature.

This principle is crucial in many practical applications, such as in firefighting, where understanding how heat travels can help in strategies for controlling fires and protecting people and property. The incorrect options suggest alternative scenarios that do not align with the established understanding of thermodynamics. For instance, heat does not flow from cold to hot or between substances of equal temperature, as there would be no temperature gradient to drive the heat transfer. Additionally, stating that heat flow is a constant process lacks the context that it specifically depends on the presence of a temperature difference to initiate the flow.