Page 19 - Advanced Thermodynamics for Engineers, Second Edition

1.3 THERMAL EQUILIBRIUM AND THE ZEROTH LAW 3

1.2.4.2 Density (r)
Density is defined as mass per unit volume. Density is also a continuum concept. Typical units of
3
density are kilograms per cubic metre denoted by kg/m .
1.2.4.3 Specific volume (v)
Specific volume is the reciprocal of density and is defined as the volume per unit mass; this is also a
3
continuum concept. Typical units of specific volume are cubic metres per kilogram denoted by m /kg.
1.2.4.4 Temperature (t or T)
Temperature is a property introduced by thermodynamics: thermodynamic processes are driven by
temperature or rather temperature differences. Temperature is measured by a device in thermal
(temperature) equilibrium with the body.

1.3 THERMAL EQUILIBRIUM AND THE ZEROTH LAW
If a system changes state slowly (compared to the reaction time of the system) it passes through a series
of equilibrium states called a path.

1.3.1 PROCESS
The path through a succession of equilibrium points is called a process:itis defined by the two end
states and the path.

1.3.2 CYCLE
A cycle is a process whose end states are identical. The concept of a cycle is an extremely important
one in thermodynamics, and many of the proofs in thermodynamics are derived by considering cycles.

1.3.3 RELATIONSHIP BETWEEN PROPERTIES
The state of simple systems of constant mass and composition can be defined by two independent
properties: this is referred to as the two-property rule, or the Gibbs rule. Any other property can be
evaluated from these two properties if the characteristics of the substance inside the system boundaries
are known. Hence, two independent properties are sufficient to define the state of simple systems
of constant mass.

1.3.4 THERMAL EQUILIBRIUM
Consider two systems, A and B, which are not at the same state, connected through a wall, which can
be either a perfect insulator or a perfect conductor. If the systems do not change state with time, then
the wall is an adiabatic one. If the systems spontaneously change state then the wall is a diathermal
one, and the systems will ultimately reach a steady state: this steady state is called thermal equilibrium.

1.3.5 ZEROTH LAW OF THERMODYNAMICS

Two systems are in thermal equilibrium with each other if they are both in thermal equilibrium with a
third system.
Systems in thermal equilibrium with each other are said to be at the same temperature.