Perfection - Physics and Chemistry

Physics and Chemistry

A variety of physical and chemical concepts include, in their names, the word "perfect."

The physicist designates as a perfectly rigid body, one that "is not deformed by forces applied to it." He uses the concept in the full awareness that this is a fictitious body, that no such body exists in nature. The concept is an ideal construct.

A perfectly plastic body is one that is deformed infinitely at a constant load corresponding to the body's limit of plasticity: this is a physical model, not a body observed in nature.

A perfectly black body would be one that absorbed completely, radiation falling upon it — that is, a body with a coefficient of absorption equal to unity.

A crystal is perfect when its physically equivalent walls are equally developed; it has a perfect structure when it answers the requirements of spatial symmetry and is free of structural defects, dislocation, lacunae and other flaws.

A perfect fluid is one that is incompressible and non-viscous — this, again, is an ideal fluid that does not exist in nature.

A perfect gas is one whose molecules do not interact with each other and which have no volume of their own. Such a gas is fictitious, just as are perfectly solid, perfectly rigid, perfectly plastic and perfectly black bodies. They are termed "perfect" in the strict (non-metaphorical) sense of the word. These are all concepts that are necessary in physics, insofar as they are limiting, ideal, fictitious — insofar as they set the extreme which nature may at the most approach.

In a looser sense, real things are called "perfect" if they approximate perfection more or less closely, though they be not, strictly speaking, perfect.

The relation of these perfect bodies to real bodies may be illustrated by the relation of a perfect gas to a real one. The equation of state of a perfect gas is a first approximation to a quantum equation of state that results from statistical physics. Thus, the equation of state of a real gas within classical limits assumes the form of the equation of state of a perfect gas. That is, the equation of state of a perfect gas describes an ideal gas (comprising points, that is, dimensionless molecules that do not act upon one another).

The perfect gas equation arose from the work of Robert Boyle, Edme Mariotte and Joseph Louis Gay-Lussac, who, in studying the properties of real gases, found formulas applicable not to these but to an ideal, perfect gas.