All substances are composed of molecules. The energy associated with molecules is called thermal energy. Thermal energy, like mechanical energy, exists in two stored forms and in one transitional form. The two stored forms of thermal energy are called internal potential energy, and internal kinetic energy. Thermal energy in transition is called heat.Although molecules are too small to be seen, they behave in some ways pretty much like the larger objects we considered in the discussion of mechanical energy. Molecules have energy of position (internal potential energy) because of the forces which attract molecules to each other. In this way, they are somewhat like the rock and the earth we considered before. Molecules have energy of motion (internal kinetic energy) because they are constantly in motion. Thus, the two stored forms of thermal energy-internal potential energy and internal kinetic energy-are in some ways similar to mechanical potential energy and mechanical kinetic energy, except that everything is on a smaller scale.
