Environmental Engineering Reference
In-Depth Information
3.4
ENERGY CONSERVATION IN COMPLEX SYSTEMS
Consider what happens when you drop a heavy stone on a sandy beach. Initially
the stone has gravitational potential energy, which converts into kinetic energy as
the stone falls. When the stone hits the beach, the grains of sand are given kinetic
energy in the collision and they jostle against each other briefly, but in a very short
time everything comes to rest with the stone embedded in the sand. Where does
the energy go? We could say that friction acts between the stone and the sand,
and since friction is a non-conservative force the total mechanical energy of the
stone is not a constant of the motion once the stone hits the sand. But that is not
to answer the question.
You might be familiar with the heating that occurs when an electric drill is used
on a resilient surface, such as brick or ceramic tiles: the drill bit can become too hot
to handle. This type of experience demonstrates clearly that heat can result from
motion. In a series of careful experiments, Joule showed that heat could be regarded
as a form of energy and that a loss of mechanical energy could be associated with
a predictable temperature rise. In effect Joule demonstrated that the conservation
of energy can be rescued provided we are prepared to count thermal energy when
we are doing the book-keeping. So, when considering the stone that falls into the
sand we can now say that the total energy is conserved but that mechanical energy
is transformed into thermal energy as a result of the collision, and that we expect a
rise in temperature of the sand and stone that depends on the amount of mechanical
energy dissipated in the collision.
But is thermal energy really a fundamentally new type of energy? To gain a fuller
understanding requires us to view the system in terms of its microscopic constituent
particles. On this level, the details of the collision are very complicated. Many
atoms, interacting by way of electrostatic forces, jostle each other and the original
mechanical energy of the stone is dissipated into kinetic energy of the atoms within
the sand and the stone itself. In this way thermal energy can be viewed as nothing
more than atomic kinetic energy. Nevertheless, from a macroscopic perspective
it makes much more sense to talk about heat energy, since keeping track of the
kinetic energy of the individual atoms is not practicable. Figure 3.11 illustrates
another way to think about the distinction between kinetic and thermal energy: in
the former case there tends to be a collective, ordered, motion whilst in the latter
the motion tends to be disordered.
Disordered Motion
Ordered Motion
Figure 3.11
Disordered and ordered motion of the molecules in a body.
