One very common form of energy is heat energy. Strictly speaking,
this is not an additional type of energy, since heat energy is
the kinetic energy of the individual molecules in a system. The
faster the average motion of the molecules, the higher the temperature
of the system. Heat can do work. When heat is applied to a liquid,
the liquid may eventually boil, changing to a gas which takes
up more space than does the liquid. And the gas from a boiling
liquid can exert great force. It drives the turbines that generate
the electricity of large cities.
The great importance of heat energy arises from the fact that
most of the times that energy is used to do work, part of the
energy is wasted as heat. For example, when a hammer is used to
pound a nail into a board, much of the energy of the hammer goes
to heating up the nail, the head of the hammer, the parts
of the board that touch the nail, and sound. Only a small part of the
total
energy actually moves the nail into the board.
The same is true of an automobile engine. Such engines would
be much more efficient if all of the chemical energy generated
by the explosion of gasoline and air changed to the kinetic energy
that moves the pistons. Instead, much of the chemical energy changes
to heat energy, which is of no help in running the car.
