According to the principle of heat flow, what is the direction of heat transfer between substances?

Heat transfer between substances follows the fundamental principle that energy naturally moves from a region of higher temperature to a region of lower temperature. This process occurs until thermal equilibrium is achieved, meaning that the temperatures of the two substances become equal.

In practical terms, this can be observed in everyday situations, such as touching a cold metal object; the warmth from your hand is transferred to the metal, which is at a lower temperature compared to your hand. This continuous flow of heat from hot to cold is crucial in understanding how fires behave and spread, as well as in developing strategies for effective firefighting. It's essential for those in firefighting to recognize this principle to manage and control fire behavior effectively.