Page 232 - The Mechatronics Handbook

referred to as a control mass or closed system. When there is flow of mass through the control surface, the
system is called a control volume or open system. An isolated system is a closed system that does not
interact in any way with its surroundings.
State, Property
The condition of a system at any instant of time is called its state. The state at a given instant of time is
described by the properties of the system. A property is any quantity whose numerical value depends on
the state, but not the history of the system. The value of a property is determined in principle by some
type of physical operation or test.
Extensive properties depend on the size or extent of the system. Volume, mass, energy, entropy, and
exergy are examples of extensive properties. An extensive property is additive in the sense that its value
for the whole system equals the sum of the values for its parts. Intensive properties are independent of
the size or extent of the system. Pressure and temperature are examples of intensive properties.

Process, Cycle
Two states are identical if, and only if, the properties of the two states are identical. When any property
of a system changes in value there is a change in state, and the system is said to undergo a process. When
a system in a given initial state goes through a sequence of processes and finally returns to its initial state,
it is said to have undergone a thermodynamic cycle.

Phase and Pure Substance
The term phase refers to a quantity of matter that is homogeneous throughout in both chemical com-
position and physical structure. Homogeneity in physical structure means that the matter is all solid, or
all liquid, or all vapor (or equivalently all gas). A system can contain one or more phases. For example,
a system of liquid water and water vapor (steam) contains two phases. A pure substance is one that is
uniform and invariable in chemical composition. A pure substance can exist in more than one phase,
but its chemical composition must be the same in each phase. For example, if liquid water and water
vapor form a system with two phases, the system can be regarded as a pure substance because each phase
has the same composition. The nature of phases that coexist in equilibrium is addressed by the phase
rule (for discussion see Moran and Shapiro, 2000).
Equilibrium
Equilibrium means a condition of balance. In thermodynamics the concept includes not only a balance
of forces, but also a balance of other influences. Each kind of influence refers to a particular aspect of
thermodynamic (complete) equilibrium. Thermal equilibrium refers to an equality of temperature,
mechanical equilibrium to an equality of pressure, and phase equilibrium to an equality of chemical
potentials (for discussion see Moran and Shapiro, 2000). Chemical equilibrium is also established in
terms of chemical potentials. For complete equilibrium the several types of equilibrium must exist
individually.

Temperature
A scale of temperature independent of the thermometric substance is called a thermodynamic temperature
scale. The Kelvin scale, a thermodynamic scale, can be elicited from the second law of thermodynamics.
The definition of temperature following from the second law is valid over all temperature ranges and
provides an essential connection between the several empirical measures of temperature. In particular,
temperatures evaluated using a constant-volume gas thermometer are identical to those of the Kelvin scale
over the range of temperatures where gas thermometry can be used. On the Kelvin scale the unit is the
kelvin (K).
The Celsius temperature scale (also called the centigrade scale) uses the degree Celsius (∞C), which
has the same magnitude as the kelvin. Thus, temperature differences are identical on both scales.
However, the zero point on the Celsius scale is shifted to 273.15 K, the triple point of water (Fig. 12.1b),

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