However, although it may seem clear that heat flows from a hot object to a cold one, how does this actually happen? When heat flows, what is happening at the molecular level?
Recall that the temperature of an object is proportional to the average kinetic energy of the object’s particles. Therefore, the particles of an object at high temperature move with high kinetic energy on average, while the particles of an object at a low temperature have low kinetic energy on average. But what will happen if the hot object is brought into contact with the cold object?
By bringing two objects at different temperatures together, their particles will begin to collide with one another. As these collisions occur, the particles with high kinetic energy transfer some of their energy to the particles with low kinetic energy. As most of the particles in the hot object have higher kinetic energy than most of the particles in the cold object, this results in a net transfer of kinetic energy from the particles of the hot object to the particles of the cold object until the particles of each object have the same average kinetic energy. When this happens, each object is at the same temperature. This energy transferred from a hot object to a cold object through collisions between particles is called heat. Once the objects have reached the same temperature, kinetic energy is still transferred between individual particles, but the average kinetic energy of the particles does not change so that heat no longer flows between the objects.
Worked Example
Several of the expected consequences of climate change involve the transfer of heat from regions of high temperature to regions of low temperature. Describe how the transfer of heat between a hot object and a cold object occurs at the molecular level.
Heat is transferred between objects at different temperatures through collisions between the particles of the objects. When these collisions occur, kinetic energy is transferred from particles with high kinetic energy to particles with low kinetic energy. As many collisions occur, the average kinetic energy of the hot object decreases, while the average kinetic energy of the cold object increases. In this way, energy is transferred from the hot object to the cold object, due to a difference in temperature; this transfer of energy is called heat. Eventually, the average kinetic energies and, therefore, the temperatures of the objects are equal. When this occurs, heat is no longer transferred in one direction, because there is no longer a temperature difference between the objects. Although individual collisions between particles still occur, there is no longer a net flow of heat.