# Thermodynamics

Thermodynamics is the study of energy, it's conversions between various forms, and the ability of energy to do work. The field delves into a wide range of topics including, but not limited to: efficiency of engines, phase equilibria, PVT relationships (both ideal and non ideal, energy balances, heats of reactions, and combustion reactions. It is governed by 4 basic laws (in brief):

• 0th law: A fundamental concept within thermodynamics, however, it was not termed a law until after the first 3 laws were already widely in use, hence the 0 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.
In a closed system (see below) the total inflow of energy must equal the total outflow of energy.
• 2nd Law: A far reaching and powerful law, it can be stated many ways, the most popular of which is:
The entropy of the universe is always increasing.
• 3rd Law: This often neglected, under utilized, but still important law is stated:
At absolute zero the entropy of a perfect crystal is zero.

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.
• 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 closed gas cylinder 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. On open gas cylinder 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.

Related topics:

Thermodynamics also touches upon the fields of: