What condition must be true for heat to flow between two objects?

For heat to flow between two objects, it is essential that the objects have different temperatures. Heat transfer occurs from the object with the higher temperature to the one with the lower temperature. This process is driven by the second law of thermodynamics, which states that heat energy naturally moves toward equilibrium. Therefore, if two objects are at the same temperature, there is no thermal gradient for heat to flow; they are already in thermal equilibrium.

The concept of heat transfer is fundamental in understanding thermodynamics, where temperature differences are required for energy exchange. If both objects were at different temperatures, the heat would flow until they reached the same temperature, at which point the flow of heat would cease. This principle explains many practical scenarios, such as why a hot coffee cup cools down when placed in a colder environment, as heat flows from the cup into the surrounding air until equilibrium is achieved.