When we think about the force of gravity, and the spontaneous
tendency for a stream to flow or a ball to roll downhill,
it is convenient to regard it as a tendency for the water
or the ball to go from a region of high to low potential energy.
Only when such a drop in potential is possible can useful
work be obtained from the process. The attractive force that
the Earth exerts on a ball of mass m
is F = mg, in which g
is the gravitational constant. If we lift the ball to a height
h above some starting level, we give
it an extra potential energy of Ep
= mgh. The ball can convert this potential energy
to kinetic energy of motion by rolling downhill to the original
level, as represented on the next page. Spontaneous motion
occurs from a region of high potential to one of low potential.
The same language is useful in studying chemical reactions.
We know that a spontaneous chemical reaction at constant overall
temperature and pressure is one that leads to a decrease in
free energy, G. The combination of hydrogen with oxygen to
form water is highly spontaneous, and can be explosively fast: