In which direction does heat flow?

Heat flows primarily from hot to cold. This fundamental principle of thermodynamics is often referred to as the second law of thermodynamics, which states that energy will naturally move towards a state of lower energy. In practical terms, when two objects at different temperatures come into contact, heat energy will transfer from the object with higher temperature (hot) to the one with lower temperature (cold) until they reach thermal equilibrium.

This is easily observed in everyday situations, such as when a warm cup of coffee cools down in a cooler room. The heat from the coffee (the hotter object) dissipates into the surrounding air (the cooler object), causing the coffee to lose temperature. Understanding this concept is crucial for activities like soldering or brazing, where controlling heat flow is essential for ensuring proper joint formation without damaging components.

While heat can radiate outwards in all directions from a source, the overall flow of heat energy is directed from regions of higher temperature to regions of lower temperature. Therefore, saying that heat flows from hot to cold accurately reflects the underlying physics.