energy - the capacity to do work. There are many forms of energy, any of which can be converted into any other form of energy. To produce electrical power in a steam turbine-generator system, the chemical energy in coal is converted in heat energy, which (through steam) is converted to the mechanical energy of the turbine, and in turn, converted into electrical energy. Electrical energy may then be converted into the mechanical energy of a vacuum cleaner, the radiant and heat energy of a light bulb, the chemical energy of a charged battery, etc. Conversion from one form of energy to another results in some energy being lost in the process (usually as heat). There are two kinds of mechanical energy: kinetic energy, imparted by virtue of a body's motion, and potential energy, impacted by virtue of a body's position (e.g., a coiled spring, or a stone on the edge of a cliff). Solar (radiant) energy is the basis of all life through the process of photosynthesis, by which green plants convert solar energy into chemical energy. Nuclear energy is the result of the conversion of a small amount of the mass of an unstable (radioactive) atom into energy. The fundamental unit of energy in the Systeme International is the joule. It can be expressed in other energy units, such as the calorie, British thermal unit (Btu), kilowatt-hour, etc., by use of appropriate conversion factors.