ABSTRACT
Thus far, we have considered various forms of energy (including energy in transition as both work and heat) without regard to any limitations on these quantities. It has been assumed that work and heat are mutually interchangeable forms of energy, and it may have appeared to students that the distinction made between these quantities was arbitrary and possibly not necessary. In this chapter, the interconvertibility of these quantities is explored with the object of determining any possible limitations and to express these limitations quantitatively, if they exist. As an example of the point in question, consider the motion of a block sliding along a rough horizontal plane. For motion to proceed along the plane, it is necessary that work be done on the body. All this work subsequently appears as heat at the interface between the block and the plane. There is no question that work has been converted into heat, but can the heat generated in this process be converted into an equivalent amount of work? Let us assume (incorrectly) that this heat can be converted into work without any losses in the process. We know that the energy as heat increases the motion of the individual molecules of the body. By increasing the molecular motion within the body, we have, in a general sense, done work, but it is also possible to distinguish that this form of work is not the same as the external work put into the process. The original transitional energy as work has been converted into heat, and this heat can be expressed as molecular work. However, this form of energy will not be available to return the body to its original state. From this simple example, we note that work can be converted into heat, but that the conversion of heat into useful work may not always be possible. Even though the rst law states that energy is conserved, it does not furnish the necessary information to enable us to determine whether energy has become unavailable.
