Measuring the Energy of a System: Energetics and the First Law of Thermodynamics

Abstract

Originally thermodynamics was concerned with the study of heat and its transformation into mechanical work and vice versa. Historically heat was thought to represent a unique physical phenomenon akin to electric charge. Heat was regarded as a fluid, the caloric, which penetrated matter and would be expected to flow from one body to another according to a set of rules. Classical thermodynamics was built on the belief that a new physical model of heat flow should be approached in much the same manner that electrical studies had been undertaken. Thus careful observations of the natural state were to be made, leading to experimental results that would empirically validate and thus establish axiomatic principles, or laws of thermodynamics This approach was practically successful, and today classical thermodynamics remains valid and important, useful both for the practical applicability of the laws and elegant because it is in itself a formal system of significant grace and beauty. However, the search for a new physical model was futile because it turned out that heat was simply the disordered motion of ordinary matter and obeys the laws of mechanics (generally quantum mechanics). Thus the classical empirical laws of thermodynamics are descended from mechanics in terms of theoretical modeling and in principle can be derived from the laws of mechanics.