Topics: Thermodynamics, Entropy, Laws of thermodynamics Pages: 6 (1204 words) Published: August 11, 2008

The first established principle of thermodynamics (which eventually became the Second Law) was formulated by Sadi Carnot in 1824. By 1860, as found in the works of those such as Rudolf Clausius and William Thomson, there were two established "principles" of thermodynamics, the first principle and the second principle. As the years passed, these principles turned into "laws." By 1873, for example, thermodynamicist Josiah Willard Gibbs, in his “Graphical Methods in the Thermodynamics of Fluids”, clearly stated that there were two absolute laws of thermodynamics, a first law and a second law.

Over the last 80 years or so, occasionally, various writers have suggested adding Laws, but none of them have been widely accepted.

[edit] Overview

* Zeroth law of thermodynamics

A \sim B \wedge B \sim C \Rightarrow A \sim C

* First law of thermodynamics

\mathrm{d}U=\delta Q-\delta W\,

* Second law of thermodynamics

\oint \frac{\delta Q}{T} \ge 0

* Third law of thermodynamics

T \rightarrow 0, S \rightarrow C

* Onsager reciprocal relations - sometimes called the Fourth Law of Thermodynamics

\mathbf{J}_{u} = L_{uu}\, \nabla(1/T) - L_{ur}\, \nabla(m/T) \!; \mathbf{J}_{r} = L_{ru}\, \nabla(1/T) - L_{rr}\, \nabla(m/T) \!.

[edit] Zeroth law

Main article: Zeroth law of thermodynamics

If two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other.

When two systems are put in contact with each other, there will be a net exchange of energy between them unless or until they are in thermal equilibrium, that is, they contain the same amount of thermal energy for a given volume (say, 1 cubic centimeter, or 1 cubic inch.) While this is a fundamental concept of thermodynamics, the need to state it explicitly as a law was not perceived until the first third of the 20th century, long after the first three laws were already widely in use, hence the zero numbering. The Zeroth Law asserts that thermal equilibrium, viewed as a binary relation, is an equivalence relation.

[edit] First law

Main article: First law of thermodynamics

In any process, the total energy of the universe remains the same.

It can also be defined as:

for a thermodynamic cycle the sum of net heat supplied to the system and the net work done by the system is equal to zero.

More simply, the First Law states that energy cannot be created or destroyed; rather, the amount of energy lost in a steady state process cannot be greater than the amount of energy gained.

A pithy summation would be "You Can't Win".

This is the statement of conservation of energy for a thermodynamic system. It refers to the two ways that a closed system transfers energy to and from its surroundings - by the process of heating (or cooling) and the process of mechanical work. The rate of gain or loss in the stored energy of a system is determined by the rates of these two processes. In open systems, the flow of matter is another energy transfer mechanism, and extra terms must be included in the expression of the first law.

The First Law clarifies the nature of energy. It is a stored quantity which is independent of any particular process path, i.e., it is independent of the system history. If a system undergoes a thermodynamic cycle, whether it becomes warmer, cooler, larger, or smaller, then it will have the same amount of energy each time it returns to a particular state. Mathematically speaking, energy is a state function and infinitesimal changes in the energy are exact differentials.

All laws of thermodynamics but the First are statistical and simply describe the tendencies of macroscopic systems. For microscopic systems with few particles, the variations in the parameters become larger than the parameters themselves, and the...
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