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The third law of thermodynamics
Thermodynamics

In physics, thermodynamics is the study of the conversion of heat energy into different forms of energy ; different energy conversions into heat energy; and its relation to macroscopic variables such as temperature, pressure, and volume....
 is a statistical law of nature regarding entropy
Entropy

In many branches of science, entropy is a measure of the disorder of a system. The concept of entropy is particularly notable as it is applied across physics, information theory and mathematics....
 and the impossibility of reaching absolute zero
Absolute zero

Absolute zero is a temperature marked by a 0 entropy configuration. It is the coldest temperature theoretically possible, and cannot be reached, by artificial or natural means....
 of temperature
Temperature

In physics, temperature is a physical property of a Physical system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature....
. The most common enunciation of third law of thermodynamics is:

Note that the minimum value is not necessarily zero, although it is almost always zero in a perfect, pure crystal
Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions....
; see the article Residual entropy
Residual entropy

Residual entropy is physically significant entropy, which is present even after a substance is cooled arbitrarily close to absolute zero. That is, if a material is reduced to its ground state, residual entropy occurs if the material can exist in multiple different ground states that have the same zero-point energy....
 for more information.

The essence of the postulate is that the entropy of the given system near absolute zero depends only on the temperature (i.e.






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The third law of thermodynamics
Thermodynamics

In physics, thermodynamics is the study of the conversion of heat energy into different forms of energy ; different energy conversions into heat energy; and its relation to macroscopic variables such as temperature, pressure, and volume....
 is a statistical law of nature regarding entropy
Entropy

In many branches of science, entropy is a measure of the disorder of a system. The concept of entropy is particularly notable as it is applied across physics, information theory and mathematics....
 and the impossibility of reaching absolute zero
Absolute zero

Absolute zero is a temperature marked by a 0 entropy configuration. It is the coldest temperature theoretically possible, and cannot be reached, by artificial or natural means....
 of temperature
Temperature

In physics, temperature is a physical property of a Physical system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature....
. The most common enunciation of third law of thermodynamics is:

Note that the minimum value is not necessarily zero, although it is almost always zero in a perfect, pure crystal
Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions....
; see the article Residual entropy
Residual entropy

Residual entropy is physically significant entropy, which is present even after a substance is cooled arbitrarily close to absolute zero. That is, if a material is reduced to its ground state, residual entropy occurs if the material can exist in multiple different ground states that have the same zero-point energy....
 for more information.

The essence of the postulate is that the entropy of the given system near absolute zero depends only on the temperature (i.e. tends to a constant independently of the other parameters).

History

The third law was developed by Walther Nernst
Walther Nernst

Walther Hermann Nernst was a Germany physical chemist who is known for his theories behind the calculation of chemical affinity as embodied in the third law of thermodynamics, for which he won the 1920 Nobel Prize in chemistry....
, during the years 1906-1912, and is thus sometimes referred to as Nernst's theorem or Nernst's postulate. The third law of thermodynamics states that the entropy of a system at zero is a well-defined constant. This is because a system at zero temperature exists in its ground state, so that its entropy is determined only by the degeneracy
Degeneracy

Degeneracy , from the Latin de-generare "to depart from its kind or genus, to fall from its proper or ancestral quality" can refer to:*In science and mathematics:...
 of the ground state; or, it states that "it is impossible by any procedure, no matter how idealised, to reduce any system to the absolute zero of temperature in a finite number of operations".

An alternative version of the third law of thermodynamics as stated by Gilbert N. Lewis
Gilbert N. Lewis

Gilbert Newton Lewis was a famous American physical chemistry known for the discovery of the covalent bond , his purification of heavy water, his reformulation of chemical thermodynamics in a mathematically rigorous manner accessible to ordinary chemists, his theory of Lewis acids and bases, and his photochemical experiments....
 and Merle Randall
Merle Randall

Merle Randall was an American physical chemist famous for his work, over the period of 25 years, in measuring Thermodynamic free energy calculations of compounds with Gilbert N....
 in 1923:

This version states not only ?S will reach zero at 0 Kelvin, but S itself will also reach zero, at least for perfect crystalline substances. (This statement is now known to have some rare exceptions.)

Overview

In simple terms, the Third Law states that the entropy of most pure substances approaches zero as the absolute temperature approaches zero. This law provides an absolute reference point for the determination of entropy. The entropy determined relative to this point is the absolute entropy.

A special case of this is systems with a unique ground state, such as most crystal lattices. The entropy of a perfect crystal lattice as defined by Nernst's theorem is zero (if its ground state is singular and unique, whereby log(1) = 0). An example of a system which does not have a unique ground state is one containing half-integer spin
Spin (physics)

In quantum mechanics, spin is a fundamental property of atomic nucleus, hadrons, and elementary particles. For particles with non-zero spin, spin direction is an important intrinsic degrees of freedom ....
s, for which time-reversal symmetry gives two degenerate ground states. Of course, this entropy is generally considered to be negligible on a macroscopic scale. Additionally, other exotic systems are known that exhibit geometrical frustration
Geometrical frustration

frustration is a phenomenon in condensed matter physics in which the geometrical properties of the crystal lattice or the presence of conflicting atomic forces forbid simultaneous minimization of the interaction energies acting at a given site....
, where the structure of the crystal lattice prevents the emergence of a unique ground state.

Real crystals with frozen defects obey this same law, so long as one considers a particular defect configuration to be fixed. The defects would not be present in thermal equilibrium, so if one considers a collection of different possible defects, the collection would have some entropy, but not actually have a temperature. Such considerations become more interesting and problematic in considering various forms of glass
Glass

Glass generally refers to a Hardness, brittle, transparency amorphous solid, such as that used for windows, many Glass Bottles, or eyewear, including, but not limited to, soda-lime glass, borosilicate glass, acrylic glass, sugar glass, Muscovite , or aluminium oxynitride....
, since glasses have large collections of nearly degenerate states, in which they become trapped out of equilibrium.

Another application of the third law is with respect to the magnetic moments of a material. Paramagnetic materials (moments random) will order as T approaches 0 K. They may order in a ferromagnetic sense, with all moments parallel to each other, or they may order in an antiferromagnetic sense, with all moments antiparallel to each other.

Yet another application of the third law is the fact that at 0 K no solid solutions should exist. Phases in equilibrium at 0 K should either be pure elements or atomically ordered phases.

See also

  • Adiabatic process
    Adiabatic process

    In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which no heat is transferred to or from the working fluid....
  • Ground state
  • Laws of thermodynamics
    Laws of thermodynamics

    The laws of thermodynamics, in principle, describe the specifics for the transport of heat and Work in thermodynamic processes. Since their inception, however, these Physical laws have become some of the most important in all of physics and other branches of science connected to thermodynamics....
  • Residual entropy
    Residual entropy

    Residual entropy is physically significant entropy, which is present even after a substance is cooled arbitrarily close to absolute zero. That is, if a material is reduced to its ground state, residual entropy occurs if the material can exist in multiple different ground states that have the same zero-point energy....
  • Thermodynamic entropy
  • Timeline of thermodynamics, statistical mechanics, and random processes
    Timeline of thermodynamics, statistical mechanics, and random processes

    A Chronology of events related to thermodynamics, statistical mechanics, and random processes....


Further reading

  • Goldstein, Martin & Inge F. (1993) The Refrigerator and the Universe. Cambridge MA: Harvard University Press. ISBN 0674753240. Chpt. 14 is a nontechnical discussion of the Third Law, one including the requisite elementary quantum mechanics
    Introduction to quantum mechanics

    Quantum mechanics is a branch of physics dealing with the behavior of matter and energy on the minute scale of atoms and subatomic particles. Quantum mechanics is fundamental to our understanding of all of the fundamental forces of nature except gravity....
    .