Start with a simple example. Think about a situation in which there is a temperature gradient across a body of uniform composition. Heat will flow from the high temperature region to the low temperature region until the temperature distribution is the same throughtout the body. Or, there may be a temperature gradient across an object. Heat will flow so as to equalize the temperature throughout the object.

There are three processes that can aid in transferring heat:

1. radiation
2. conduction
3. convection

A key concept is that heat is molecular or atomic motion. Read the section Temperature and Absolute Zero. Spend a little time with the Box Temperature Animation. Blue atoms are cold and red atoms are hot. Conduction of heat occurs when thermally agitated atoms and molecules jostle one another mechanically transferring the vibrational motion from a hot region to a cold region. There is no physical movement of the material itself. Some materials are good conductors and the rate of heat flow through them is high. Others are insulators and the rate of heat flow is much lower.

Read the brief article on Convection and watch the Short Movie. "It functions because heated fluids, due to their lower density, rise and cooled fluids fall. A heated fluid will rise to the top of a column, radiate heat away and then fall to be re-heated, rise and so on. Gasses, like our atmosphere, are fluids, too. A packet of fluid can become trapped in this cycle. When it does, it becomes part of a convection cell."

There is a critical temperature at which convection begins. Within the Earth materials may be capable of flow if suffient time is allowed for the process to operate. For example, here is an animation of Convection in the Earth's Mantle

Now, take the Heat Flow Quiz