Chapter 5 – Systems, processes, and cycles:
The language of thermodynamics
In thermodynamics, the totality of the world is usually termed the
, and is
divided into two parts.
is the part of the universe in which we are particularly interested.
The rest of the universe is
Separating the system from the surroundings is the system boundary (see Fig. 5.1).
The system, the surroundings, and the system boundary.
Systems can be of three types, depending on what can pass through the system boundary.
If matter, heat, and work can pass through the system boundary, then the
an open system
If heat and work can pass through the boundary, but matter cannot, then the
a closed system
If neither matter, not heat, nor work can pass through the boundary, then the
an isolated system
Properties of systems are sometimes divided into two types as follows.
these depend on the mass of substance present, for
example volume, the greater the mass of air, the greater will be the volume.
these do not depend on the mass of substance present,
for example pressure and temperature.
Specific volume is defined as the
volume per unit mass, and so must be an intensive property.
Extensive properties are usually given an upper case symbol, for example
Intensive properties are usually given a lower case symbol, such as
for pressure, and
for specific volume.
However, the use of T for the intensive property temperature is an
exception to this rule.