rhein-main-verzeichnis.info: Thermodynamics & Heat: Entropy
Our starting point for this discussion is the definition of a measurable entropy change: This definition may be used to calculate the entropy of a system at a. Scientists use the formula (delta)S = (delta)Q /(delta)T. "S" is the entropy value, "Q " is the measure of heat, and "T" is the temperature of the system measured in. The central concepts of thermodynamics are: entropy S, temperature T, and .. the relation between thermodynamic temperature T and Celsius temperature ϑ.
History of entropy The French mathematician Lazare Carnot proposed in his paper Fundamental Principles of Equilibrium and Movement that in any machine the accelerations and shocks of the moving parts represent losses of moment of activity.
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In other words, in any natural process there exists an inherent tendency towards the dissipation of useful energy. Building on this work, in Lazare's son Sadi Carnot published Reflections on the Motive Power of Fire which posited that in all heat-engines, whenever " caloric " what is now known as heat falls through a temperature difference, work or motive power can be produced from the actions of its fall from a hot to cold body.
He made the analogy with that of how water falls in a water wheel. This was an early insight into the second law of thermodynamics.
The first law of thermodynamicsdeduced from the heat-friction experiments of James Joule inexpresses the concept of energy, and its conservation in all processes; the first law, however, is unable to quantify the effects of friction and dissipation.
In the s and s, German physicist Rudolf Clausius objected to the supposition that no change occurs in the working body, and gave this "change" a mathematical interpretation by questioning the nature of the inherent loss of usable heat when work is done, e.
In Boltzmann visualized a probabilistic way to measure the entropy of an ensemble of ideal gas particles, in which he defined entropy to be proportional to the natural logarithm of the number of microstates such a gas could occupy. Henceforth, the essential problem in statistical thermodynamicsi.
Definitions and descriptions[ edit ] Any method involving the notion of entropy, the very existence of which depends on the second law of thermodynamics, will doubtless seem to many far-fetched, and may repel beginners as obscure and difficult of comprehension. Historically, the classical thermodynamics definition developed first. In the classical thermodynamics viewpoint, the system is composed of very large numbers of constituents atoms, molecules and the state of the system is described by the average thermodynamic properties of those constituents; the details of the system's constituents are not directly considered, but their behavior is described by macroscopically averaged properties, e.
The early classical definition of the properties of the system assumed equilibrium.
The classical thermodynamic definition of entropy has more recently been extended into the area of non-equilibrium thermodynamics. Later, the thermodynamic properties, including entropy, were given an alternative definition in terms of the statistics of the motions of the microscopic constituents of a system — modeled at first classically, e.
homework and exercises - A relationship between entropy and temperature - Physics Stack Exchange
These statements have been a source of unending confusion for students of thermodynamics for over a hundred years. What has been sorely needed is a precise mathematical definition of the Second Law that avoids all the complicated rhetoric. The sad part about all this is that such a precise definition has existed all along.
The definition was formulated by Clausius back in the 's.
Clausius wondered what would happen if he evaluated the following integral over each of the possible process paths between the initial and final equilibrium states of a closed system: He carried out extensive calculations on many systems undergoing a variety of both reversible and irreversible paths and discovered something astonishing.
He found that, for any closed system, the values calculated for the integral over all the possible reversible and irreversible paths between the initial and final equilibrium states was not arbitrary; instead, there was a unique upper bound maximum to the value of the integral.
Clausius also found that this result was consistent with all the "word definitions" of the Second Law. Clearly, if there was an upper bound for this integral, this upper bound had to depend only on the two equilibrium states, and not on the path between them.
It must therefore be regarded as a point function of state. Clausius named this point function Entropy.
The Relationship between Entropy and Temperature
But how could the value of this point function be determined without evaluating the integral over every possible process path between the initial and final equilibrium states to find the maximum? Clausius made another discovery. He determined that, out of the infinite number of possible process paths, there existed a well-defined subset, each member of which gave the same maximum value for the integral.
This subset consisted of what we call today the reversible process paths. So, to determine the change in entropy between two equilibrium states, one must first conceive of a reversible path between the states and then evaluate the integral. Any other process path will give a value for the integral lower than the entropy change.Thermodynamics 27 : Entropy and Temperature