ABSTRACT
Physical systems are made up of interacting physical elements; it is these interactions that bind the elements together into a system. Each element is again a system of smaller interacting elements, and so on, until at some low level the elements are electrons and nuclei, and the systems are called atoms. The elements are bound together by a combination of short-range repulsive forces and long-range attractive forces, and the binding of each element can be expressed in terms of a potential energy that equals the work required to remove the element from the system. But, in addition to this potential binding energy, the elements also possess a kinetic energy; we may think of it as the elements rattling around in the “cage” formed by
the potential energy. If this kinetic energy of an element becomes comparable to the potential energy, the binding may be overcome and the system (i.e., the atom) undergoes a change. To restore it to its original condition, we would have to supply a “spare” element (i.e., electron); this would be a maintenance action.
