Thermodynamic
Thermodynamics is the study of energy, its conversions between various forms, the ability of energy to do work, and the spontaneity of processes. It is closely related to statistical mechanics from which many thermodynamic relationships can be derived. It is not concerned with the concept of time, or that of rate of change (derivative in time). As a result, it has been suggested that this science should rather have been called thermostatics. Time-dependent thermodynamic processes are studied by non-equilibrium thermodynamics.
Thermodynamic laws are of very general validity, and they do not depend on the details of the interactions or the systems being studied. This means they can be applied to systems about which one knows nothing other than the balance of energy and matter transfer between them and the environment.
The basic concepts of Thermodynamics
The basic abstraction of thermodynamics is the division of the world into systems delimited by real or ideal boundaries. The systems not directly under consideration are lumped into the environment. It is possible to subdivide a system into subsystems, or to group several systems together into a larger system.
There are three kinds of systems depending on the kinds of exchanges taking place between a system and its environment:
- isolated systems: not exchanging heat, matter or work with their environment.
- closed systems: exchanging energy (heat and work) but not matter with their environment. Whether a system exchanges heat, work or both is usually thought of as a property of its boundary, which can be
- adiabatic boundary: not allowing heat exchange;
- rigid boundary: not allowing exchange of work.
- open systems: exchanging energy (heat and work) and matter with their environment. A boundary allowing matter exchange is called permeable.
The Laws of Thermodynamics
Alternative statements can be given for each law which are mathematically equivalent.
- Zeroth law: Thermodynamic equilibrium. When two systems are put in contact with each other, energy 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 thermal equilibrium, and B and C are in thermal equilibrium, then A and C are also in thermal 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 by pressure and other generalized forces), and chemical equilibrium (associated to matter exchange and parameterized by chemical potential).
- 1st Law: 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 process depends only on the initial and the final state and not on the details of the process.
- 2nd Law: 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.
- All processes cease as temperature approaches zero.
These laws have been humorously stated as: (0) temperature is a good thing; (1) you can't win, you can only break even; (2) you can only break even at absolute zero; (3) you can never reach absolute zero. In sum, you can't even break even.
Basics
The following is a list of the major concepts in thermodynamics, together with the algebraic symbols used to represent them.
The rest of this discussion is about reversible transformation of systems in equilibrium. For irreversible processes or systems out of equilibrium, see nonequilibrium thermodynamics.
Substances describable by temperature alone
Blackbody radiation is an example. The reason why this is the case is because photon number isn't conserved. The state is completely described by its temperature except at phase transitions and perhaps spontaneous symmetry breaking in the ordered phase. given the internal energy as a function of temperature, we can define F=U-TS.
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.
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.
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.
Thermodynamic Systems
A thermodynamic system is that part of the universe that is under consideration. A real or imaginary boundary separates the system from the rest of the universe, which is referred to as the surroundings. Often thermodynamic systems are characterized by the nature of this boundary as follows:
- Isolated systems are completely isolated from their surroundings. Neither heat nor matter can be exchanged between the system and the surroundings. An example of an isolated system would be an insulated container, such as an insulated gas cylinder. (In reality, a system can never be absolutely isolated from its environment, because there is always at least some slight coupling, even if only via minimal gravitational attraction).
- Closed systems are separated from the surroundings by an impermeable barrier. Heat can be exchanged between the system and the surroundings, but matter cannot. A greenhouse is an example of a closed system.
- Open systems can exchange both heat and matter with their surroundings. Portions of the boundary between the open system and its surroundings may be impermeable and/or adiabatic, however at least part of this boundary is subject to heat and mass exchange with the surroundings. The ocean would be an example of an open system.
Thermodynamic State
A key concept in thermodynamics is the state of a system. When a system is at equilibrium under a given set of conditions, it is said to be in a definite state. For a given thermodynamic state, many of the system's properties have a specific value corresponding to that state. The values of these properties are a function of the state of the system and are independent of the path by which the system arrived at that state. The number of properties that must be specified to describe the state of a given system is given by Gibbs phase rule. Since the state can be described by specifying a small number of properties, while the values of many properties are determined by the state of the system, it is possible to develop relationships between the various state properties. One of the main goals of Thermodynamics is to understand these relationships between the various state properties of a system. Equations of state are examples of some of these relationships.
See also: thermodynamic properties
Thermodynamics also touches upon the fields of:
See also:
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