5. PROPERTIES OF A SYSTEM
Any characteristic of a system is called a property.
Intensive properties are those that are independent
of the mass of a system.
Extensive properties are those whose values depend
on the size or extent of the system such as
proportional to mass of system.
Extensive properties per unit mass are called specific
properties.
A continuous, homogeneous matter with no holes,
that is, a continuum.
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7. The mass of the whole is the sum of the masses of the
parts, and the overall volume is the sum of the
volumes of the parts. However, the temperature of
the whole is not the sum of the temperatures of the
parts; it is the same for each part. Mass and volume
are extensive, but temperature is intensive.
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8. Density and specific gravity
Density is defined as mass per unit volume.
The reciprocal of density is the specific volume v,
which is defined as volume per unit mass.
Specific gravity or relative density is ratio of the
density of a substance to the density of some standard
substance at a specified temperature.
The weight of a unit volume of a substance is called
specific weight.
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9. State and Equilibrium
Each unique condition of system is called state.
In an equilibrium state there are no unbalanced
potentials (or driving forces) within the system.
The system will be in thermal equilibrium if its
temperature is same throughout the system.
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10. Figure1: A closed system reaching thermal
equilibrium
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11. State and Equilibrium
Mechanical equilibrium is related to pressure, and
a system is in mechanical equilibrium if there is no
change in pressure at any point of the system with
time.
If a system involves two phases, it is in phase
equilibrium when the mass of each phase reaches an
equilibrium level and stays there.
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12. State and Equilibrium
A system is in chemical equilibrium if its chemical
composition does not change with time,
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14. State Postulate
The number of properties required to fix the state of a
system is given by the state postulate.
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15. Process and Cycle
Any change that a system undergoes from one
equilibrium state to another is called a process, and
the series of states through which a system passes
during a process is called the path of the process.
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16. Process and Cycle
When a process proceeds in such a manner that the
system remains infinitesimally close to an equilibrium
state at all times, it is called a quasi- static, or quasi-
equilibrium, process. A quasi-equilibrium process
can be viewed as a sufficiently slow process that
allows the system to adjust itself internally so that
properties in one part of the system do not change
any faster than those at other parts.
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19. Process and Cycle
The prefix iso- is often used to designate a process
for which a particular property remains constant.
Isothermal process
During an isothermal process there are no temperature
changes, i.e., dT = 0.
Isobaric process
For an isobaric process the pressure is constant (dP = 0).
Isometric or Isochoric process
In isometric process the specific volume remains
unchanged (dv = 0).
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20. Process and Cycle
Adiabatic process
In adiabatic process there is no heat transfer, i.e., when
the system is perfectly insulated.
The terms steady and uniform are used frequently in
engineering
The term steady implies no change with time. The
opposite of steady is unsteady, or transient. The term
uniform, however, implies no change with location
over a specified region.
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22. TEMPERATURE AND THE ZEROTH LAW OF
THERMODYNAMICS
Although we are familiar with temperature as a
measure of “hotness” or “coldness,” it is not easy to
give an exact definition for it. Based on our
physiological sensations, we express the level of
temperature qualitatively with words like freezing
cold, cold, warm, hot, and red-hot.
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24. TEMPERATURE AND THE ZEROTH LAW OF
THERMODYNAMICS
The zeroth law of thermodynamics states that if
two bodies are in thermal equilibrium with a third
body, they are also in thermal equilibrium with each
other.
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25. Temperature Scales
Temperature scales enable us to use a common basis
for temperature measurements.
The Kelvin scale is related to the Celsius scale by
The Rankine scale is related to the Fahrenheit scale
by
The temperature scales in the two unit systems are
related by
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the state of a system is described by its properties. The state postulate requires that the two properties specified be indepen- dent to fix the state. Two properties are independent if one property can be varied while the other one is held constant. Temperature and specific vol- ume, for example, are always independent properties, and together they can
FIGURE 1–25
Nitrogen
T = 25°C
v = 0.9 m3/kg
fix the state of a simple compressible system (Fig. 1–25). Temperature and pressure, however, are independent properties for single-phase systems, but are dependent properties for multiphase systems. Higher altitudes Example
To describe a process completely, one should specify the initial and final states of the process, as well as the path it follows, and the interactions with the surroundings.
Turbines, pumps, boilers, con- densers, and heat exchangers or power plants or refrigeration systems are examples of steady flow condition. reciprocating engines or compressors.
It is a common experience that a cup of hot coffee left on the table even-tually cools off and a cold drink eventually warms up. That is, when a body is brought into contact with another body that is at a different temperature, heat is transferred from the body at higher temperature to the one at lower temperature until both bodies attain the same temperature (Fig. 1–31). At that point, the heat transfer stops, and the two bodies are said to have reached thermal equilibrium. The equality of temperature is the only requirement for thermal equilibrium.
