Thermodynamic Equations - Laws of Thermodynamics

Laws of Thermodynamics

The behavior of a thermodynamic system is summarized in the laws of thermodynamics, which concisely are:

• Zeroth law of thermodynamics

  If A, B, C are thermodynamic systems such that A is in thermal equilibrium with B and B is in thermal equilibrium with C, then A is in thermal equilibrium with C.

The zeroth law is of importance in thermometry, because it implies the existence of temperature scales. In practice, C is a thermometer, and the zeroth law says that systems that are in thermodynamic equilibrium with each other have the same temperature. The law was actually the last of the laws to be formulated.

• First law of thermodynamics

  where is the infinitesimal increase in internal energy of the system, is the infinitesimal heat flow into the system, and is the infinitesimal work done by the system.

The first law is the law of conservation of energy. The symbol instead of the plain d, originated in the work of German mathematician Carl Gottfried Neumann and is used to denote an inexact differential and to indicate that Q and W are path-dependent (i.e., they are not state functions). In some fields such as physical chemistry, positive work is conventionally considered work done on the system rather than by the system, and the law is expressed as .

• Second law of thermodynamics

  The entropy of an isolated system never decreases: for an isolated system.

A concept related to the second law which is important in thermodynamics is that of reversibility. A process within a given isolated system is said to be reversible if throughout the process the entropy never increases (i.e. the entropy remains unchanged).

• Third law of thermodynamics

  when

The third law of thermodynamics states that at the absolute zero of temperature, the entropy is zero for a perfect crystalline structure.

• Onsager reciprocal relations – sometimes called the Fourth law of thermodynamics

  ;

  .

The fourth law of thermodynamics is not yet an agreed upon law (many supposed variations exist); historically, however, the Onsager reciprocal relations have been frequently referred to as the fourth law.