is 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,
imparted 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 Système 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.