Thermodynamics

Thermodynamics (from the Greek thermos meaning heat and dynamis meaning power) is a branch of physics that studies the effects of temperature, pressure, and volume changes on physical systems at the macroscopic scale. In simpler terms, heat means ‘energy in transit’ and dynamics relates to ‘movement’. Thus, in essence thermodynamics studies how energy instills movement.

The starting point for most thermodynamic considerations are the laws of thermodynamics. These laws postulate that energy can be exchanged between physical systems in the form of heat and work, as well as the existence of a quantity named entropy, which can be associated with every system.

From its inception, thermodynamics developed out of the need to increase the efficiency of early steam engines. The first engine constructed was the 1698 Savery engine as shown below:

Thermodynamics in most regards is held to be a difficult subject. In chemical engineering for example, which teaches one of the more rigorous variations of such, thermodynamics is considered a weeder course. One of the better ways to learn thermodynamics is to follow a ground up development of its concepts and principles, beginning with units as SI and English, parameters as pressure, temperature, and volume, etc., properties of substances as gas, vapor, liquid, and solid, etc., phase diagrams, the laws of thermodynamics, equations of state, continuing onward through such advanced subjects as multi-phase reaction thermodynamics, high-speed gas flow thermodynamics, or molecular thermodynamics, etc.

A difficult concept in thermodynamics is that of "entropy". In particular, the entropy of a system exchanging no heat with the outside can never decrease with time. As such, entropy allows predictions on the transformations and energy exchanges that are accessible to a given system. Related to entropy, Statistical mechanics or statistical thermodynamics is one of the underlying theories that sustain thermodynamics; it provides a way to predict the entropy of a thermodynamic system, based on the statistical analysis of the fluctuations the system experiences over a set of microstates

A short history of thermodynamics begins with the British physicist and chemist Robert Boyle who in 1656, in coordination with English scientist Robert Hooke, invented the air pump. Using this pump, Boyle and Hooke noticed the pressure-temperature-volume correlation. In time, the ideal gas law was formulated. Soon thereafter, in 1679, based on these concepts, an associate of Boyle's named Denis Papin built a bone digester, which is a closed vessel with a tightly fitting lid that confines steam until a high pressure is generated.

Later designs implemented a steam release valve to keep the machine from exploding. By watching the valve rhythmically move up and down, Papin conceived of the idea of a piston and cylinder engine. He did not however follow through with his design. Nevertheless, in 1698, based on Papin’s designs, engineer Thomas Savery built the first engine. These early engines being crude and inefficient attracted the attention of the leading scientists of the time. One such scientist was Sadi Carnot, the “father of thermodynamics”, who in 1824 published “Reflections on the Motive Power of Fire”, a discourse on heat, power, and engine efficiency. This marks the start of thermodynamics as a modern science.

Of most importance in thermodynamics is the concept of the “system”. A system is the region of the universe under study. A system is separated from the remainder of the universe by a boundary which may be imaginary or not, but which, by convention delimits a finite volume. The possible exchanges of work, heat, or matter between the system and the surroundings take place across this boundary. There are four dominate classes of systems:

1. Isolated Systems – matter and energy may not cross the boundary.
2. Adiabatic Systems – heat and matter may not cross the boundary.
3. Closed Systems – matter may not cross the boundary.
4. Open Systems – heat, work, and matter may cross the boundary.

For closed systems, as time goes by, internal differences in the system tend to even out; pressures and temperatures tend to equalize, as do density differences. A system in which all equalizing processes have gone practically to completion, is considered to be in a state of thermodynamic equilibrium.

In thermodynamic equilibrium, a system's properties are, by definition, unchanging in time. Systems in equilibrium are much simpler and easier to understand than systems which are not in equilibrium. Often, when analyzing a thermodynamic process, it can be assumed that each intermediate state in the process is at equilibrium. This will also considerably simplify the situation. Thermodynamic processes which develop so slowly as to allow each intermediate step to be an equilibrium state are said to be reversible processes.

The central concept of thermodynamics is that of energy, the ability to do work. As stipulated by the first law, the total energy of the system and its surroundings is conserved. It may be transferred into a body by heating, compression, or addition of matter, and extracted from a body either by expansion, cooling, or extraction of matter. Just as in mechanics, energy transfer is effected by a force causing a displacement, with the product of the two being the amount of energy transferred. In a similar way, thermodynamic systems can be thought of as transferring energy as the result of a generalized force causing a generalized displacement, with the product of the two being the amount of energy transferred. These thermodynamic force-displacement pairs are known as conjugate variables. The most common conjugate thermodynamic variables are pressure-volume (mechanical parameters), temperature-entropy (thermal parameters), and chemical potential-particle number (material parameters).

There are two types of thermodynamic instruments, the meter and the reservoir. A thermodynamic meter is any device which measures any parameter of a thermodynamic systems. In some cases, the thermodynamic parameter is actually defined in terms of an idealized measuring instrument. For example, the zeroth law states that if two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. This principle, as noted by James Maxwell in 1872, asserts that it is possible to measure temperature. An idealized thermometer is a sample of an ideal gas at constant pressure. From the ideal gas law PV=NRT, the volume of such a sample can be used as an indicator of temperature; in this manner it defines temperature. Although pressure is defined mechanically, a pressure-measuring device, called a barometer may also be constructed from a sample of an ideal gas held at a constant temperature. A calorimeter is a device which is used to measure and define the internal energy of a system.

A thermodynamic reservoir is a system which is so large that it does not appreciably alter its state parameters when brought into contact with the test system. It is used to impose a particular value of a state parameter upon the system. For example, a pressure reservoir is a system at a particular pressure, which imposes that pressure upon any test system that it is mechanically connected to. The earths atmosphere is often used as a pressure reservoir.

When a system is at equilibrium under a given set of conditions, it is said to be in a definite state. The state of the system can be described by a number of intensive variables and extensive variables. The properties of the system can be described by an equation of state which specifies the relationship between these variables. State may be thought of as the instantaneous quantitative description of a system with a set number of variables held constant.

A thermodynamic process may be defined as the energetic evolution of a thermodynamic system proceeding from an initial state to a final state. Typically, each thermodynamic process is distinguished from other processes, in energetic character, according to what parameters, as temperature, pressure, or volume, etc., are held fixed. Furthermore, it is useful to group these processes into pairs, in which each variable held constant is one member of a conjugate pair. The five most common thermodynamic processes are shown below:

1. An isobaric process occurs at constant pressure.
2. An isochoric process occurs at constant volume.
3. An isothermal process occurs at a constant temperature.
4. An isentropic process occurs at a constant entropy.
5. An adiabatic process occurs without loss or gain of heat.

In thermodynamics, there are four laws of very general validity, and as such they do not depend on the details of the interactions or the systems being studied. Hence, they can be applied to systems about which one knows nothing other than the balance of energy and matter transfer. Examples of this include Einstein's prediction of spontaneous emission around the turn of the 20th century and current research into the thermodynamics of black holes.

The four laws are:

• Zeroth law of thermodynamics, about the transitivity of thermodynamic equilibrium
  • If systems A and B are in thermal equilibrium, and systems B and C are in thermal equilibrium, then A and C are also in thermal equilibrium.
  • Two systems in thermal equilibrium with a third system, all must be in equilibrium with each other.
• First law of thermodynamics, or a statement about the conservation of energy
  • The work exchanged in an adiabatic process depends only on the initial and the final state and not on the details of the process.
  • The heat energy flowing into a system is equal to the sum of the increase in the internal energy of the system and the work done by the system.
  • The change in internal energy of a system is ${\displaystyle \Delta }$U = q + w, where q is heat flow and w is work.
• Second law of thermodynamics, about entropy
  • The entropy of an isolated system never decreases (see Maxwell's demon)
  • A system operating in contact with a thermal reservoir cannot produce positive work in its surroundings (Kelvin)
  • A system operating in a cycle cannot produce a positive heat flow from a colder body to a hotter body (Clausius)
• Third law of thermodynamics, about absolute zero temperature
  • All processes cease as temperature approaches zero.
  • As temperature goes to 0, the entropy of a system approaches a constant

As derived from the energy balance equation on a thermodynamic system there exist energetic quantities called thermodynamic potentials, being the quantitative measure of the stored energy in the system. The four most well known potentials are:

Internal energy	${\displaystyle U\,}$
Helmholtz free energy	${\displaystyle A=U-TS\,}$
Enthalpy	${\displaystyle H=U+PV\,}$
Gibbs free energy	${\displaystyle G=U+PV-TS\,}$

Potentials are used to measure energy changes in systems as they evolve from an initial state to a final state. The potential used depends on the constraints of the system, such as constant temperature or pressure. Internal energy is the internal energy of the system, enthalpy is the internal energy of the system plus the energy related to pressure-volume work, and Helmholtz and Gibbs free energy are the energies available in a system to do useful work when the temperature and volume or the pressure and temperature are fixed, respectively.

In 1875 Austrian physicist Ludwig Boltzmann declared: "the general struggle for existence of animate beings is a struggle for entropy". Ever since, there has been a continuous search to elucidate the thermodynamic mechanism behind evolution. As it is generally agreed that life evolved from non-life, a process called abiogenesis, by some form of chemical evolution, and as it is understood that both life and non-life abide by the laws of thermodynamics, then, in theory, it is reasoned that there should exist a functionable model of thermodynamic evolution. This line of research defines the field of thermodynamic evolution.

• A common scientific joke expresses the three laws simply and surprisingly accurately as:

Zeroth: "You must play the game."

First: "You can't win."

Second: "You can't break even."

Third: "You can't quit the game."

• History of thermodynamics
• Legendre transformation
• Onsager reciprocal relations - sometimes called the Fourth Law of Thermodynamics
• Philosophy of thermal and statistical physics
• Statistical Mechanics
• Thermodynamic equations
• Thermodynamic properties

• List of important publications in thermodynamics
• List of notable textbooks in statistical mechanics
• Timeline of thermodynamics, statistical mechanics, and random processes

• Calorimetry
• Fluid dynamics
• Phase equilibrium
• Thermal analysis
• Thermochemistry (also known as chemical thermodynamics)
• Biological thermodynamics
• Black hole thermodynamics
• Philosophy of thermal and statistical physics
• Thermochemistry

• Engineering Thermodynamics

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• History of Thermodynamics
• Shakespeare & Thermodynamics
• Biochemistry Thermodynamics

• 10+ Variations of the 0th Law
• 30+ Variations of the 1st Law
• 110+ Variations of the 2nd Law
• 20+ Variations of the 3rd Law
• 10+ Variations of the 4th Law