Encyclopedia > Thermodynamics

Article Content

Thermodynamics

Thermodynamics is the study of energy, its conversions between various forms such as heat, and the ability of energy to do work. It is closely related to statistical mechanics from which many themodynamic relationships can be derived.

The field delves into a wide range of topics including, but not limited to: efficiency of heat engines and turbines, phase equilibria, PVT relationships. gas laws (both ideal and non ideal[?]), energy balances[?], heats of reactions[?], and combustion reactions. It is governed by 4 basic laws (in brief):

The Laws of Thermodynamics

Zeroth law: A fundamental concept within thermodynamics, however, it was not termed a law until after the first three laws were already widely in use, hence the zero numbering. Stated as:

If A and B are at the same temperature, and B and C are at the same temperature, then A and C are also at the same temperature.

1st Law: 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.

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

All processes cease as temperature approaches zero.

These three laws have been humorously summarised as: (1) you can't win; (2) you can't break even; (3) you can't get out of the game.

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.