Page 21 - Advanced thermodynamics for engineers

4 CHAPTER 1 INTRODUCTION AND REVISION

1.4 TEMPERATURE SCALES
There are two different scales of temperature:
• The continuous scale of temperature is based on the Second Law of thermodynamics and is
independent of the thermometric substance.
• The discontinuous scales of temperature are defined as the International Temperature Scale
(ITS-90). The Celsius (or Centigrade) and Fahrenheit scales are the original ITS scales.

1.5 INTERACTIONS BETWEEN SYSTEMS AND SURROUNDINGS
A system can interact with its surroundings through transfers across the system boundaries. For a
closed system there can be no mass flow across the system boundaries, and the only possible
interactions are by two mechanisms: work and heat – both of these are transfers of energy across the
boundary.

1.5.1 WORK – THERMODYNAMIC DEFINITION
Work is done by a system when the sole effect on everything external to the system (the
surroundings) would be the raising of a weight.
For engineers the main interest is in using systems to produce work, and hence the following
definition is adopted for the sign convention of work:
• positive work is done by a system on the surroundings;
• negative work is done by the surroundings on a system.
This sign convention is completely arbitrary and some engineering textbooks use the opposite
convention, which is also used by physicists and chemists. While the convention is arbitrary it does
have a significant effect on the equations of thermodynamics. It is important to use the sign convention
consistently and correctly.
1.5.1.1 Negative work
Energy cannot be created or destroyed, so when a system does positive work then the surroundings
must do an equal amount of negative work.

1.5.1.2 Representation of work
Positive work will be represented by an arrow pointing out of the system, and negative work will be
represented by an arrow pointing into the system.

1.5.1.3 Displacement work
Displacement work is an important feature of engines operating on thermodynamic principles, and this
is often generated by a piston and a cylinder.
The incremental piston, or displacement work, is
dW ¼ pdV; (1.1)