Heat is a change in the amount of energy a body contains. It is the change in energy that results when a hot body is placed in contact with a cold one, for example. In this case, heat is transferred, or flows, from the hot body to the cold one. The molecular motions within the hot body slow down (the hot body loses energy) and the ones in the cold body speed up (the cold body gains energy) until their temperatures are the same. The symbol for heat is "q" and for energy to be conserved, the amount of heat lost by the hot body must exactly equal the amount of heat gained by the cold one. We write this symbolically -q(hot) = q(cold).

Heat explained. Heat transfer explained.