The factor that is necessary for the flow of heat from one object to another is that the two objects must have different temperatures. Heat always "flows," or moves, from the hotter object to the cooler one, unless there is some sort of external agent doing work.
As the heat flows, the warmer object cools down and the cooler object warms up until they reach the same temperature, or "thermal equilibrium." Once that happens, the heat transfer process stops.
The exchange of heat energy involves a change of either an intensive factor or an extensive (also known as capacity) factor, regardless of whether the heat energy is exchanged through the performance of work or through heat flow. In heat exchange, the intensive factor is the temperature, and the extensive factor is entropy.
There are three different ways for heat to flow from one object to another: conduction, convection and radiation. Conduction involves heat transfer through atoms bumping into each other. Convection, which happens in air and fluids, takes it a step further. The initial heat transfer is through conduction, but then a loop forms with warm areas rising and cool areas falling, creating a circular effect called a "convection cell." In heat radiation, thermal equilibrium means that particles are absorbing the same amount of radiation from their surroundings that they are radiating out themselves, so nothing changes. Heat transfer occurs when particles are emitting more radiation than they are absorbing from their environment.