CHAP. 3: FUNDAMENTALS OF THERMODYNAMICS [CONTENTS] 63
Chapter 3
Fundamentals of thermodynamics
3.1 Basic postulates.
Thermodynamics is a branch of science based on six postulates which ensued from the gener-
alization of observed and experimental facts. The first postulate relates to the transition of a
system to the state of equilibrium [see1.4.2]. The second postulate states that the internal
energy of a system is an extensive quantity. The remaining four postulates are, for historical
reasons, called the laws of thermodynamics.
3.1.1 The zeroth law of thermodynamics
When two systems are in thermal equilibrium, i.e. no heat flows from one system to the other
during their thermal contact, then both systems have the same temperature. If system A has
the same temperature as system B and system B has the same temperature as system C, then
system A also has the same temperature as system C.
Note:The zeroth law of thermodynamics is sometimes called the postulate on the existence
of temperature as a thermodynamic quantity. It allows us to find out whether two systems
that are not in thermal contact have the same temperature.