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Classical Thermodynamics

Thermodynamics is a branch of physics which deals with the heat energy and its transfer during the various physical and chemical processes. The processes which are subject to thermodynamics consideration include not only the natural phenomena that occur about us every day but also controlled chemical reactions, the performance of engines and even hypothetical processes such as chemical reactions that do not occur but can be imagined. Thermodynamics is exceedingly general in its applicability, and this makes it a powerful tool for solving many kinds of important problem. The thermodynamics methods do not make any assumptions as to the atoms and molecules. The only quantities and concepts which enter thermodynamics are the experimental properties of matter such as pressure, volume, temperature and composition. Such properties are the properties of matter in bulk rather than of individual isolated molecules. Thermodynamics provides a convenient and powerful method of relating, systematizing and discussing such properties.

Thermodynamics provides the most general and efficient methods for studying and understanding complex physical and chemical phenomena. Thermodynamics is the science of heat and temperature and, in particular, of the laws governing the conversion of heat in to mechanical, electrical or other macroscopic forms of energy. An important characteristic of thermodynamics is that it permits the derivation of relationship between different laws of nature, even though the laws themselves are not a consequence of thermodynamics.

Classical thermodynamics is based on the four laws of thermodynamics. In thermodynamics we are concerned with the behavior of vase quantities of particles in the substances that we study. The laws of thermodynamics are the laws of the generalized behavior of the particles.


Temperature and the Zeroth Law

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Temperature is a more subtle property than pressure. Its origin is the so-called zeroth law of thermodynamics. The Zeroth law is based on experiments (as are all physical laws) and is concerned with properties of systems in thermal equilibrium, that is, system is equilibrium connected by a diathermal wall.

The law states:

If two systems are separately in thermal equilibrium with a third system, they are in equilibrium with each other.

Let systems A, B and C each consist of a mass of fluid in an insulated container. We shall use C as a reference. We choose to describe the state of each system in terms of the state variables P and V. We place A in contact with C through a diathermal wall.

The First Law

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The first law of thermodynamics is essentially the statement of the principle of the conservation of energy for thermodynamical systems. As such, it may be expressed by stating that the variation in energy of a system during any transformation is equal to the amount of energy that the system receives from it environment. In order to give a precise meaning to this statement, it is necessary to define phrases “energy of the system” and “energy that the system receives from its environment during a transformation.”

The Second Law

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The Kelvin-Planck statement of the second law of thermodynamics is based on this limitation on the thermal efficiency of heat engines. The Kelvin-Planck statement of the second law is:

It is impossible for any device that operates on a cycle to exchange heat with just a single reservoir and produce a net amount of work.

That is, to keep operating, a heat engine must exchange heat with a low-temperature sink as well as a high-temperature source.

The Kelvin-Planck statement can also be expressed as:

For a power plant to operate, the working fluid must exchange heat with the environment as well as the furnace.


No heat engine can have a thermal efficiency of 100 per cent.

It is essential to realize that the impossibility of having a 100 per cent efficient heat engine is not due to friction or other dissipative effects, but is a limitation that applies to both the idealized and the actual heat engines.

The Third Law

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Experiment shows that the fundamental feature of all cooling processes is that, the lower the temperature attained, the more difficult it is to cool further. For example, the colder a liquid is, the lower the vapour pressure, and the harder it is to produce further cooling by pumping away the vapour. The same is true for magneto caloric effect. If one demagnetization produces a temperature T1, say one-fifth of the original temperature T, then second demagnetization from the same original field will produce a temperature T2 which is also approximately one-fifth of T1. Under these circumstances, an infinite number of adiabatic demagnetization would be required to attain absolute zero. We may generalize this experience by saying: ‘By no finite series of processes is the absolute zero attainable’.

This is known as the principle of the unattainability of absolute zero, or the unattainability statement of the third law of thermodynamics. As in the case of the second law of thermodynamics, the third law has a number of alternatives of equivalent statements. Another statement of this law is the outcome of experiments leading to the calculations of the way that the entropy change of a condensed system during a reversible isothermal process behaves as the temperature approaches zero. This is known as the Nernst-Simon statement of the third law, which state: ‘The entropy change associated with any isothermal reversible process of a condensed system approaches zero as the temperature approaches zero’.

Thermodynamic Potentials: Free Energies and Enthalpy

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A thermodynamic potential is a scalar potential function that describes the thermodynamic state of a system. For example, internal energy, U, is a thermodynamic potential. Along with U, three other common thermodynamic potentials are the Helmholtz free energy, F[J], the enthalpy, H [J], and the Gibbs free energy, G[J]. Neither F, H, nor G have an insightful physical interpretation as U does, but they are useful in determining how properties of a system change when it changes state, as we shall see below. In particular, thermodynamic potentials are used for determining the parameters that describe equilibrium states of a system, which are states that correspond to energy minima (stable states). The equations of four thermodynamic potentials are;

Internal energy:
U = TS - PV

dU = TdS - PdV

Helmholtz free energy:
F = U - TS

dF = - SdT - PdV

H = U + PV

dH = TdS + VdP

Gibbs free energy:
G = U - TS + PV

dG = - SdT + VdP
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