Chapter 4 - UCSB HEP

SEC. 4.12 THE GENERAL LAW OF CONSERVATION OF ENERGY 185
By a series of meticulous experiments on the heating of water by
a paddle wheel driven by a falling weight, he showed that the loss
of mechanicaf energy by friction is accompanied by the appearance
of an equivalent amount of heat. Joule concluded that heat must
be a form of energy and that the sum of the mechanical energy
and the heat energy of a system is conserved.
We now have a more detailed picture of heat energy than was
available to Joule. We know that solids are composed of atoms
held together by strong interatomic forces. Each atom can oscillate
about its equilibrium position and has mechanical energy in
the form of kinetic and potential energies. As the solid is heated,
the ampfitude of oscillation increases and the average energy of
each atom grows larger. The heat energy of a solid is the mechanical
energy of the random vibrations of the atoms.
There is a fundamental difference between mechanical energy
on the atomic level and that on the level of everyday events. The
atomic vibrations in a solid are random; at any instant there are
atoms moving in all possible directions, and the center of mass of
the block has no tendency to move on the average. Kinetic energy
of the block represents a collective motion; when the block moves
with velocity v, each atom has, on the average, the same velocity v.
Mechanical energy is turned into heat energy by friction, but
the reverse process is never observed. No one has ever seen a
hot block at rest on a table suddenly cool off and start moving,
although this would not violate consewation of energy. The
reason is that collective motion can easily become randomized.
For instance, when a block hits an obstacle, the collective translational
motion ceases and, under the impact, the atoms start to
jitter more violently. Kinetic energy has been transf~rmed to
heat energy. The reverse process where the random motion of
the atoms suddenly turns to collective motion is so improbable
that for all practical purposes it never occurs. It is for this reason
that we can distinguish between the heat energy and the rnechanical
energy of a chunk of matter even though on the atomic scale
the distinction vanishes.
We now recognize that in addition to mechanical energy and
heat there are many other forms of energy. These include the
radiant energy of light, the energy of nuclear forces, and, as we
shall discuss in Chap. 13, the energy associated with mass. It is
apparent that the concept of energy is much wider than the simple
idea of kinetic and potential energy of a mechanical system. We
believe that the total energy of a system is conserved if all forms
of energy are taken into account.