moves toward the ground, and heat transfer is from higher to lower temperatures.
Thus, thermodynamics underlies mechanics.
Note that when we introduced thermodynamics, we mentioned that it applies
to macroscopic systems. Macroscopic scale is somewhat arbitrary, but it certainly
is larger than a single molecule and usually includes a large number of molecules
as the domain of heat relationships. As observed in recent nanotechnological
research, physical behavior can be quite different at small scales. For example, the
current distinction between nanoscale and bulk-scale systems is 100 nanometers
(nm). In other words, if a particle has a dimension < 100 nm, it is considered
to be at the nanoscale. Electromagnetic properties can be quite different at the
nanoscale than at the macroscopic scale. In some cases, the emission of energy,
such as light, can be altered significantly (witness that some nanoparticles of gold
are red in color, not gold).
A system has two definitions that apply to green engineering:
1. Generally, a system is a combination of organized elements comprising a
2. From a thermodynamics perspective, a system is a defined physical entity
containing boundaries in space, which can be open (i.e., energy and matter
can be exchanged with the environment) or closed (no energy or matter
The system is what we care about, what we want to study. It may seem obvious,
but in science we must distinguish what we are interested in from everything else.
In physics, we do this by way of the system. What we want to study, explain, or
test is in the system. Everything else is what we call the surroundings (see Fig. 2.2).