The direction of net heat flow between two reservoirs depends primarily on what factor?

The direction of net heat flow between two reservoirs is primarily determined by the temperature difference between the two systems. Heat naturally flows from a warmer object to a cooler one, following the second law of thermodynamics. This principle states that energy spontaneously disperses or spreads out if it is not hindered from doing so. Therefore, when there is a temperature differential, heat flows from the hotter reservoir to the cooler one until thermal equilibrium is reached.

Although pressure, volume, and humidity can influence specific heat capacities and the state of a substance, they do not directly dictate the fundamental direction of heat transfer like temperature does. The concept of heat flow relies on the tendency for energy to move towards a state of equilibrium, which is primarily governed by the relative temperatures of the two reservoirs.