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 laws have become some of the most important in all of physics and other types of science associated with thermodynamics.

It is wise to distinguish classical thermodynamics, which is focused on systems in thermodynamic equilibrium, from non-equilibrium thermodynamics. The present article is focused on classical or thermodynamic equilibrium thermodynamics.

There are generally considered to be four principles (referred to as "laws"):

0. The zeroth law of thermodynamics, which underlies the definition of temperature.
1. The first law of thermodynamics, which mandates conservation of energy, and states in particular that heat is a form of energy.
2. The second law of thermodynamics, which states that the entropy of an isolated macroscopic system never decreases, or (equivalently) that perpetual motion machines are impossible.
3. The third law of thermodynamics, which concerns the entropy of a perfect crystal at absolute zero temperature, and implies that it is impossible to cool a system all the way to exactly absolute zero.

During the last 80 years or so, occasionally, various writers have suggested additional Laws, but none of them have become well accepted.

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 are at the same temperature. While this is a fundamental concept of thermodynamics, the need to state it explicitly was not perceived until the first third of the 20th century, long after the first three principles were already widely in use, hence the zero numbering. The Zeroth Law asserts that thermal equilibrium, viewed as a binary relation, is a transitive relation (and since any system is always in equilibrium with itself, it is furthermore an equivalence relation).

First law

Main article: First law of thermodynamics

Energy can neither be created nor destroyed. It can only change forms.

In any process in an isolated system, the total energy remains the same.

For a thermodynamic cycle the net heat supplied to the system equals the net work done by the system.

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. 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 assumptions of thermodynamics become meaningless. The First Law, i.e. the law of conservation, has become the most secure of all basic principles of science. At present, it is unquestioned (although it is said to be criticized by people who do not accept the idea that the potential to gain energy is a form of actual energy).

Fundamental Thermodynamic Relation

The first law can be expressed as the Fundamental Thermodynamic Relation:

Heat supplied = internal energy + work done

Internal energy = Heat supplied - work done

$dE=TdS-pdV\,$

Here, E is internal energy, T is temperature, S is entropy, p is pressure, and V is volume. This is a statement of conservation of energy: The net change in internal energy (dE) equals the heat energy that flows in (TdS), minus the energy that flows out via the system performing work (pdV).

Second law

Main article: Second law of thermodynamics

The entropy of an isolated system consisting of two regions of space, isolated from one another, each in thermodynamic equilibrium in itself, but not in equilibrium with each other, will, when the isolation that separates the two regions is broken, so that the two regions become able to exchange matter or energy, tend to increase over time, approaching a maximum value when the jointly communicating system reaches thermodynamic equilibrium.

In a simple manner, the second law states "energy systems have a tendency to increase their entropy rather than decrease it." This can also be stated as "heat can spontaneously flow from a higher-temperature region to a lower-temperature region, but not the other way around." (Heat can flow from cold to hot, but not spontaneously—- for example, when a refrigerator expends electrical power.)

A way of thinking about the second law for non-scientists is to consider entropy as a measure of ignorance of the microscopic details of the system. So, for example, one has less knowledge about the separate fragments of a broken cup than about an intact one, because when the fragments are separated, one does not know exactly whether they will fit together again, or whether perhaps there is a missing shard. Solid crystals, the most regularly structured form of matter, have very low entropy values; and gases, which are very disorganized, have high entropy values. This is because the positions of the crystal atoms are more predictable than are those of the gas atoms.

The entropy of an isolated macroscopic system never decreases. However, a microscopic system may exhibit fluctuations of entropy opposite to that stated by the Second Law (see Maxwell's demon and Fluctuation Theorem).

Third law

Main article: Third law of thermodynamics

As temperature approaches absolute zero, the entropy of a system approaches a constant minimum.

Briefly, this postulates that entropy is temperature dependent and results in the formulation of the idea of absolute zero.

Tentative fourth laws or principles

Over the years, various thermodynamic researchers have come forward to ascribe to or to postulate potential fourth laws of thermodynamics (either suggesting that a widely-accepted principle should be called the fourth law, or proposing entirely new laws); in some cases, even fifth or sixth laws of thermodynamics are proposed. Most fourth law statements, however, are speculative and controversial.

The most commonly proposed Fourth Law is the Onsager reciprocal relations, which give a quantitative relation between the parameters of a system in which heat and matter are simultaneously flowing.

Other tentative fourth law statements are attempts to apply thermodynamics to evolution. During the late 19th century, thermodynamicist Ludwig Boltzmann argued that the fundamental object of contention in the life-struggle in the evolution of the organic world is 'available energy'. Another example is the maximum power principle as put forward initially by biologist Alfred Lotka in his 1922 article Contributions to the Energetics of Evolution. Most variations of hypothetical fourth laws (or principles) have to do with the environmental sciences, biological evolution, or galactic phenomena.

History

The first established principle which eventually became the Second Law was formulated by Sadi Carnot during 1824. By 1860, as formalized 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 were termed "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.

See also

• Conservation law
• Heat death of the universe
• Laws of science
• Philosophy of thermal and statistical physics
• Table of thermodynamic equations

Further reading

• Goldstein, Martin, and Inge F., 1993. The Refrigerator and the Universe. Harvard Univ. Press. A gentle introduction.