13.11 Conservation of energy  (Page 2/7)

Hence, “thermal” energy is actually a macroscopic reflection of kinetic energy of the atoms and molecules. On the other hand, “internal” energy is potential and kinetic energy of the particles constituting the system. Similarly, various forms of electromagnetic energy (electrical and magnetic energy) are actually potential and kinetic energy. Besides, field energy like radiation energy does not involve even matter.

Nevertheless, analysis of energy at macroscopic level requires that we treat these forms of energy as a different energy with respect to mechanical energy, which we associate with the motion of the system

Process

In mechanics, we are concerned with motion – a change in position. On the other hand, energy is a very general concept that extends beyond change in position i.e. motion. It may involve thermal, chemical, electric and such other changes called processes. For example, a body may not involve motion as a whole, but atoms/molecules constituting the body may be undergoing motion all the time. For example, work for gas compression does not involve locomotion of the gas mass. It brings about change in internal and heat energy of the system.

Clearly, we need to change our terminology to suit the context of energy.

From the energy point of view, a motion, besides involving a change in position, also involves heat due to friction. For example, heat is produced, when a block slides down a rough incline. Thus, we see that even a process involving motion (mechanical process) can involve energy other than mechanical energy (potential and kinetic energy). The important point to underline is that though “mechanical energy” excludes thermal energy, but “mechanical process” does not.

System

A system comprises of many particles, which are interacted by different kinds of force. It is characterized by a boundary. We are at liberty to define our system to suit analysis of a motion or process. Everything else other than system is “surrounding”.

The boundary of the system, in turn, is characterized either to be “open”, “closed” or “isolated”. Accordingly, a system is open, closed or isolated. We shall define each of these systems.

In an “open” system, the exchange of both “matter” and “energy” are permitted between the system and its surrounding. In other words, nothing is barred from or to the system.

A “closed” system, however, permits exchange of energy, but no exchange of matter. The exchange of energy can take place in two ways. It depends on the type of process. The energy can be exchanged in the form of “energy” itself. Such may be the case in thermal process in which heat energy may flow “in” or “out” of the system. Alternatively, energy can be transferred by “work” on the system or by the system. In the nutshell, transfer of energy can take place either as “energy” or as “work”.

An “isolated” system neither permits exchange of energy nor that of matter. In other words, everything is barred “to” and “from” the system.

Interestingly, there is no exchange of mass in both “closed” and “isolated” system. Barring system involving nuclear reaction, the conservation of mass in these systems means that total numbers of atoms remain a constant.