2 The Laws of Thermodynamics

Alternative statements that are mathematically equivalent can be given for each law.

• Zeroth lawThe zeroth law of thermodynamics is the law which states that thermal equilibrium is transitive''. Two systems are said to be in thermal equilibrium when 1) both of the systems are in a state of equilibrium, and 2) they remain so when they are brought int: Thermodynamic equilibrium. When two systems are put in contact with each other, energy and/or matter will be exchanged between them unless they are in thermodynamic equilibrium. Two systems are in thermodynamic equilibrium with each other if they stay the same after being put in contact. The zeroth law is stated as

If A and B are in thermodynamic equilibrium, and B and C are in thermodynamic equilibrium, then A and C are also in thermodynamic equilibrium.

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. There is still some discussion about its status.

Thermodynamic equilibrium includes thermal equilibrium (associated to heat exchange and parameterized by temperature), mechanical equilibrium (associated to work exchange and parameterized generalized forces such as pressure), and chemical equilibrium (associated to matter exchange and parameterized by chemical potential).

• 1st LawConservation of energy the first law of thermodynamics is one of several conservation laws. It states that the total inflow of energy into a system must equal the total outflow of energy from the system, plus the change in the energy contained within the: Conservation of energy. This is a fundamental principle of mechanics, and more generally of physics. In thermodynamics, it is used to give a precise definition of heat. It is stated as follows:

The work exchanged in an adiabatic processThis article covers adiabatic processes in thermodynamics. For adiabatic processes in quantum mechanics, see adiabatic process (quantum mechanics). In thermodynamics, an adiabatic process is a process in which no heat is gained or lost in the working flui depends only on the initial and the final state and not on the details of the process.

or

The heat flowing into a system equals the increase in internal energyThe internal energy of a system (abbreviated E or U is the total kinetic energy due to the motion of molecules, and the total potential energy associated with the rotational, vibrational, and electric energy of atoms within molecules. Internal energy is a of the system minus the work done by the system.

or

Energy cannot be created, or destroyed, only modified in form.

• 2nd LawIn physics, the second law of thermodynamics in its many forms, is a statement about the quality and direction of energy flow, and it is closely related to the concept of entropy. This law and its derivatives, such as the law of friction, define the arrow: A far reaching and powerful law, it can be stated many ways, the most popular of which is:

It is impossible to obtain a process such that the unique effect is the subtraction of a positive heat from a reservoir and the production of a positive work.

Specifically,

A system operating in contact with a thermal reservoir cannot produce positive work in its surroundings ( Lord KelvinWilliam Thomson, Archbishop of York, has the same name as this man. William Thomson, 1st Baron Kelvin ( June 26, 1824 December 17, 1907) was a mathematical physicist who did important work in thermodynamics. In recognition of his achievements, he was in 1)

or

A system operating in a cycle cannot produce a positive heat flow from a colder body to a hotter body ( ClausiusRudolf Julius Emanuel Clausius ( January 2, 1822 August 24, 1888), was a German physicist and mathematician. Clausius was one of the founders of thermodynamics. His most important paper, on the mechanical theory of heat, published in 1850, first stated th)

The entropy of a thermally isolated macroscopic system never decreases (see Maxwell's demon), however a microscopic system may exhibit fluctuations of entropy opposite to that dictated by the second law (see Fluctuation Theorem).

• 3rd Law: This law explains why it is so hard to cool something to absolute zero:

All processes cease as temperature approaches zero.

As temperature goes to 0, the entropy of a system approaches a constant

These laws have been humorously summarised as Ginsberg's theorem: (1) you can't win, (2) you can't break even, and (3) you can't get out of the game.

Or, alternatively: (1) you can't get anything without working for it, (2) the most you can accomplish by working is to break even, and (3) you can only break even at absolute zero.

Or, (1) you can't get out more than you put in (2) even the best-designed machine eventually loses energy and stops (3) you can't get to absolute zero.

Or, (1) you cannot win or quit the game, (2)you cannot have a tie in the game unless it is very cold, (3) the weather doesn't get that cold.

2.1 More about the 2nd Law

The Second Law is exhibited (coarsely) by a box of electrical cables. Cables added from time to time tangle, inside the 'closed system' (cables in a box) by adding and then removing cables. The best way to untangle is to start by taking the cables out of the box and placing them stretched out. The cables in a closed system (the box) will never untangle, but giving them some extra space starts the process of untangling (by going outside the closed system).

C.P._Snow said the following in a Rede Lecture in 1959 entitled "The Two Cultures and the Scientific Revolution."

"A good many times I have been present at gatherings of people who, by the standards of the traditional culture, are thought highly educated and who have with considerable gusto been expressing their incredulity at the illiteracy of scientists. Once or twice I have been provoked and have asked the company how many of them could describe the Second Law of Thermodynamics. The response was cold: it was also negative."

3 Basics

The following is a list of the major concepts in thermodynamics, together with the algebraic symbols used to represent them.

Internal energy	U
Temperature	T
Entropy	S
Pressure	P
Volume	V
Density	ρ
Helmholtz free energy	F
Gibbs free energy	G
Enthalpy	H
Chemical potential	μ
Particle number	N
Phase (matter)
Intensive variable
Extensive variable
State function

The rest of this discussion is about reversible transformation of systems in equilibrium. For irreversible processes or systems out of equilibrium, see non-equilibrium thermodynamics.

4 Examples

4.1 Substances describable by temperature alone

Blackbody radiation is an example, since photon number is not conserved. Such a state is completely described by its temperature, although if phase transitions or spontaneous symmetry breaking occur other variables may be needed to discriminate among the phases. (This problem does not arise for blackbody radiation.) Given the internal energy as a function of temperature, we can define F = U - TS.

4.2 Substances describable by temperature and pressure alone

Most "pure" nonmagnetic substances fall into this category. This state is completely described by its temperature and pressure, except at phase transitions and perhaps spontaneous symmetry breaking in the ordered phase. Given U and V (or the density ρ) as a function of T and P, we can define the Helmholtz energy as before and the Gibbs energy as G = U - TS + PV and the enthalpy as H = U + PV.

4.3 Substances describable by temperature, pressure and chemical potential

If there are more than one kind of atom/molecule, a substance would fall into this category. This state is completely described by its temperature, pressure and chemical potentials, except at phase transitions and perhaps spontaneous symmetry breaking in the ordered phase.

4.4 Substances describable by temperature and magnetic field

If a substance is a ferromagnet or a superconductor, for example, it would fall into this category. It is completely described by its temperature and magnetic field, except at phase transitions and perhaps spontaneous symmetry breaking in the ordered phase.

5 See also

• Thermodynamic properties
• Thermodynamics also touches upon the fields of:
  • Phase equilibrium
  • Fluid dynamics
  • Calorimetry
  • Thermal analysis
  • Thermochemistry also known as chemical thermodynamics

5.1 Quotes

“[Thermodynamics] is the only physical theory of universal content which, within the framework of the applicability of its basic concepts, I am convinced will never be overthrown.” — Albert Einstein

“In this house, we OBEY the LAWS of THERMODYNAMICS!” — Homer Simpson

6 Units

• Joule
• Calorie
• Coulomb
• Faraday
• Kelvin

7 Etymology

Thermodynamics comes from the greek thermos=heat and dynamos=power.

8 See also

• Partition function (statistical mechanics)
• Legendre transformation
• List of thermodynamics topics
• important publications in thermodynamics
• Onsager reciprocal relations - sometimes called the Fourth Law of Thermodynamics

9 Wikibooks

• Engineering Thermodynamics

General subfields within physics

Classical mechanics | Condensed matter physics | Continuum mechanics | Electromagnetism | General relativity | Particle physics | Quantum field theory | Quantum mechanics | Solid state physics | Special relativity | Statistical mechanics | Thermodynamics

Thermodynamics

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