An energy form inherent in every system is the internal energy, which arises from the molecular state of motion of matter. The symbol U is used for the internal energy and the unit of measurement is the joules (J).
Internal energy increases with rising temperature and with changes of state or phase from solid to liquid and liquid to gas. Planetary bodies can be thought of as combinations of heat reservoirs and heat engines. The heat reservoirs store internal energy E, and the heat engines convert some of this thermal energy into various types of mechanical, electrical and chemical energies.