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Chapter 4: Definitions
Having gained some insight into the underlying concepts of thermodynamics, we need now to define our terms more precisely before we set out to analyse objects of interest. In thermodynamics, we have a number of terms which occur frequently, and you need to be familiar with them.
System
Fundamental to our ability to analyse processes occurring in equipment is the need to know just what we are including in our analysis of the object or “system”. A system is defined in two ways because there are two distinct types. It is either “A Region in space” or “A particular collection of matter”, whichever is more convenient for the particular application. A system boundary is arbitrarily defined to make analysis as simple as possible. However, once it is defined, it must be treated consistently.
Closed System
Some “systems” have a sealed container preventing mass from entering or leaving, but whose dimensions may (or may not) be able to change. A simple definition of a closed system is: “A system in which the mass remains constant”.
Open System
An open system is one which allows mass to flow in and out (e.g. a jet engine). The total quantity of mass inside the system may or may not change. The simple definition is: “A system in which the mass changes”.
Pure Substance
In thermodynamics we are more concerned with the consistent composition of working fluids rather than their chemical purity. We therefore define a “pure” substance as: “A single substance or mixture of substances which has a consistent composition throughout” (i.e. homogeneous, and the molecular structure does not change).
Working Substance
Thermodynamics deals with heat and work, so working substances are usually fluids which can be deformed (i.e. expanded or compressed, e.g. air, steam, etc.).
State
The situation a thermodynamic system is in at any particular instant is called its “state”. The State of a system is defined by the values of its “Properties” (e.g. Pressure, Volume, etc.) at that instant. (Note: we are not simply referring to whether it is solid, liquid, or gas, although that is also relevant.)
Properties
Properties are characteristics of a System which can be used to describe its “State” (e.g. Pressure, Volume, Temperature). Properties can be measured and given a numerical value unique to the “State” (i.e. independent of how the state was reached, e.g. volume).
CLOSED SYSTEM
“a particular collection of matter"
OPEN SYSTEM
"a region in space"
Two Property Rule (empirical)
If TWO independent properties of a closed system containing a Pure Substance are known, then ALL other Properties (and the State) are known. Independent properties can be plotted against each other on graphs (charts), and since two co-ordinates define a point on a chart, we have established the state of the system. At boiling (and other phase changes), Pressure and Temperature are not independent, and so cannot alone define a single point at that condition.
Process
When the State (condition) of a system is changed by some operation, the System is said to have undergone a Process (e.g. a piston compressing gas in a cylinder).
Cycle
A cycle is a series of Processes where the final state of the System is the same as the first (i.e. it is ready to start the same set of processes again). Being able to return to its initial condition is obviously essential for any activity to continue indefinitely, so we often refer to any machine running indefinitely as operating in a cycle.
Work
If the Boundary of a System is moved against a force, Work is done on or by the System. (Note: when movement ceases, Work ceases. i.e. it is transient , therefore not a Property)
Heat
Heat is the energy transferred from one body to another due to temperature difference. (Note: heat disperses into other energy forms such as “Internal Energy”, Work, etc. i.e. it is transient , therefore not a property)
Sign Convention
When we analyse thermodynamic processes, we need a consistent mathematical approach. For convenience, a new arbitrary sign convention has been adopted for heat and work “flows”:
Heat and Work: “in” is +ve “out” is -ve
Temperature (and Zeroth Law of Thermodynamics)
Changes in temperature can be measured by some variation of a thermometric Property (e.g. length of a column of liquid, pressure of a gas at constant volume, resistance of a wire, EMF of a thermocouple). The concept of an “absolute zero” of temperature is derived from the Second Law of Thermodynamics, and will be dealt with later.
The (unofficial) Zeroth Law of Thermodynamics identifies what we intuitively know about heat flowing from one body to another (i.e. only if there is a temperature difference), and states that “If two bodies are separately in thermal equilibrium with a third body, then they must be in thermal equilibrium with each other.” We say, they are at the same temperature.
Perfect Gas
The reasoning behind the notion of a Perfect Gas will be dealt with later, but it is useful to note that a Perfect Gas obeys the common relationship:
p. V = m. R. T
Dryness Fraction
When we have a system containing a mixture of liquid and vapour (i.e. both phases present, often called a “Wet vapour”), we almost always need to know how much of each phase is present (we call the liquid “wet” and the vapour part “dry”). The most common way to express this is the “Dryness Fraction”, defined as follows:
Dryness Fraction = x =
Mass of Vapour
Total Mass
