Page 19 - Advanced thermodynamics for engineers

2 CHAPTER 1 INTRODUCTION AND REVISION

Second Law of Thermodynamics, which defines the ‘quality’ of energy and the direction of
processes – and, even, time.
Third Law of Thermodynamics, which defines conditions at a temperature of absolute zero.

1.2 DEFINITIONS
Thermodynamics introduces new concepts and requires a new vocabulary based on a set of definitions.

1.2.1 SYSTEM
A system is a collection of matter enclosed within prescribed boundaries.

1.2.1.1 Closed system
A closed system is a prescribed and identifiable collection of matter enclosed within a prescribed
boundary. There is no flow of matter across the boundary but energy exchanges can occur.

1.2.1.2 Surroundings
The surroundings are anything not contained within the system boundaries.

1.2.1.3 Open system
An open system is defined by its boundaries, and there can be mass transfer across the boundaries:
the matter contained within an open system will vary with time. The space occupied by an open
system is called a control volume and the boundaries of an open system are control surfaces.

1.2.2 PROPERTIES
A property is any quantity whose change is fixed by the end states, and is independent of the path
between the end states.

Intensive properties are independent of the size/mass of the system.
Extensive properties are dependent on the size/mass (or extent) of the system.
Specific properties are defined as the extensive property per unit mass.

1.2.3 STATE
The state of a system is a complete description of all the properties of the system, i.e. both the
mechanical and thermodynamic properties. e.g. a simple system is defined by its volume, pressure,
velocity and position.

1.2.3.1 Change of state
When the state of a system alters, the change of state is defined by the two end states.

1.2.4 SOME IMPORTANT PROPERTIES
1.2.4.1 Pressure (p)
Pressure is a continuum concept and is the force per unit area exerted by a system on its surface in the
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direction normal to that surface. Typical units of pressure are N/m , bar and lb f /in .