Laws of Thermodynamics

- When thermodynamics is studied as a nonstatistical subject (phenomenology) --> four postulates

- These postulates are theoretically based in our statistical formulation.

Zeroth law: If two systems are in thermal equilibrium with a third system, they must be in thermal equilibrium with each other. Immediate consequence of the condition for thermal equilibrium

First law: Heat is a form of energy and together with work can contribute to the change of the internal energy of the system --> practically the principle of energy conservation.

Second law: the law of increase of entropy. If a closed system is in a configuration that is not the equilibrium configuration , the most probable consequence will be that the entropy of the system will increase monotonically in successive instants of time. Many other formulations are known….

Third law: The entropy of a system approaches a constant value (must come in flat with T) as the temperature approaches zero. This constant value might not necessarily be: 0

Entropy as a logarithm:two important consequences of the fact that entropy is defined as the logarithm of the number of accessible states, instead of as the number of accessible states itself:

1. The entropy becomes an extensive parameter

2. The entropy is insensitive to the precision with which the energy of a closed system is defined or fixed. Let D(U) be the number of accessible states per unit energy range. From here:

Typically , as for the system of N spins, g(U) is of order 2N, D(U)2N/N, where is the average energy per particle.