Basic Concepts of Thermodynamics Objective Definition of Thermodynamic System Concepts of State and...
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Transcript of Basic Concepts of Thermodynamics Objective Definition of Thermodynamic System Concepts of State and...
Basic Concepts of Thermodynamics
ObjectiveObjective
• Definition of Thermodynamic System
• Concepts of State and its properties
• Pressure, Temperature and Specific Volume
• Quasi-static or Quasi-equilibrium Process
and Reversible Process
• Thermodynamic Cycle
Thermodynamic System
System—A quantity of matter or
a region in space bounded by an
arbitrary surfaces for study.
Surrounding—The mass or regi
on outside the system.
Boundary—The surfaces separ
ate the system from its surroundi
ngs.
The boundary can be real or imaginary, fixed or variable.
–It is critically important to define your system before
attempting to solve a thermodynamic problem.
Thermodynamic System
Closed system—No mass can cross its boundary. It consists of a fixed amount of mass. (also known as control mass)
Open system —A selected region in space. Both mass and energy can cross its boundary. (also known as control volume)
Thermodynamic System
–Simple Thermodynamic System
Adiabatic System —No heat cross its boundary.
SurroundingsHeat
One Type Work
Isolated System— Nether mass nor energy can cross its boundary.
Thermodynamic System
Single Composition System —Consists of only one kind
of substance.
Multi-Composition System —Consists of tow or more
kinds of substances.
Single Phase System —Consists of only one phase.
Multi-phases System —Consists of tow or more
phases.Homogeneous System —The composition and property are identical all over the system.
Thermodynamic State
Thermodynamic state of a system –the condition of the system as characterized by the values of its properties.
Property –any characteristic of a system
• Basic Property
–can be directly measured
• Derived Property
–indirectly calculated from basic ones
Pressure
ForcePressure=
Area
[ ]2
lbfpsi
in=
[ ]2
NPa
m=
[ ]torr mmHg=
[ ] 5bar 10 Pa=
Common Use Units:
English
SI
Other
Pressure Measurement
Gage and Absolute Pressure Scales
We commonly use three different kinds pressure scales (not
units).
Absolute — is relative to full vacuum (absolute zero).
Gage (Gauge) — is relative to the ambient pressure.
Used for pressure above the ambient pressure.
Vacuum — is relative to the ambient pressure.
Used for pressure below the ambient pressure.
g a b
v b a
p p p
p p p
= -
= -
Gage and Absolute Pressure Scales
0ap =
bp
vp
ap
bp
Absolute
Vacuum
bp
ap
gp
Absolute
Vacuum
0.1MPa 14.5psiap = =
Temperature
• Two objects in thermal equilibrium are at the same
temperature.
•Zeroth law — if two bodies are in thermal equilibrium
with a third body, they are also in thermal
equilibrium with each other. Thermometers — measure the temperature
Celsius (Centigrade) scale and Fahrenheit scale —Two points scale — Dependent of physical properties of substances
Temperature Scale
ice point
steam point
0℃ F32O
100℃ F212O
Celsius Fahrenheit
o o( F) 1.8 ( C) 32T T= +
Temperature Scale•Thermodynamic temperature scale— based on the principle that at low pressure, the temperature of gas is proportional to its pressure at constant volume.— One point scale— independent of property of substance
HydrogenHelium
Temperature Scale
so that: T = a + b P
For gases at low pressures (ideal gasses)
For Celcius scale, a=-273.15
For Fahrenheit scale,a=-459.67
Temperature Scale
Four temperature scales are
in common use:
–Rankine
–Fahrenheit
–Kelvin
–Celsius
UnitAbsolute Relative
SI Kelvin Celsius
English Rankine Fahrenheitdiffer in:- dependent / independent of property of substance- location of the zero points- size of the degree unit
Temperature Scale Comparisons
o
o
o o
o
o
o
(K) ( C) 273.15
(R) ( F) 459.67
( F) 1.8 ( C) 32
(R) 1.8 (K)
(K) ( C)
(R) ( F)
(R) 1.8 ( C)
T t
T t
t t
T T
T t
T t
T t
= +
= +
= +
=
D =D
D =D
D = D
Specific Volume
Specific Volume 3/ (m )v V m=
31/ (kg/m )vr =
Density
vv
Characteristic of State Property
Mathematical
1
2A
C
B
integral2
2 11
0
f df f f
df
D = = -
=
ò
ò
differential
2 2
( ) ( )y x
f fdf dx dy
x y
f f
x y y x
¶ ¶= +
¶ ¶
¶ ¶=
¶ ¶ ¶ ¶
intensive and extensive
Characteristic of State Property
–Intensive property: a property that is independent of the mass of the system.
• Example –temperature, pressure, density, …
–Extensive property: a property whose value is proportional to the mass of the system.
• Example – volume, total enthalpy, energy ,…
–Specific property: An extensive property per unit mass. Specific properties are intensive.
• Example –specific volume (= volume/mass)
Equilibrium
•Sufficient and Necessary Conditions for Equilibrium: –Thermal: the temperature is the same throughout the system. –Mechanical: the pressure is the same. –Phase: there is no driving force for the total mass in each phase to change. –Chemical: there is no driving force for chemical composition to change.
Stable equilibrium state —a state in which the system is not capable of spontaneous change to another state without a finite change in the surroundings.
—There are no driving forces to carry out a change.
State Equation
We need only specify a certain number of properties to
fix the state of system. The exact number depends on compositions and phases
of system?
State postulate(相律)
2f a b= - +f
a
b
-
-
-
number of independent properties
number of different compositions of system
number of different phases of system
State equation ( , , ) 0f p v T =
Property Diagram
1p
2p
1 Initial state
2 Final state • Any point illustrates the state of system.• The state must be Equilibrium.
Thermodynamic Process
Thermodynamic Processes—transformations from one equilibrium state to another. Path—The series of states through which the system passes.
1P
2P
1 Initial state
2 Final state
path
Quasi-Equilibrium Process
1P
2P
1 Initial state
2 Final state
P
V1V2V
System remains infinitesimally close to an equilibrium state at all times.
Process occurs slow enough to keep the properties identical throughout the system.The interval of process change takes much longer time than system spontaneously adjusts to a new state after the previous equilibrium state was destroyed.
Quasi-equilibrium Process
To connect states with line must be QE process.
Reversible Process
Reversible process: the reverse process could be
performed so that the system and surroundings can be
restored to their initial states with no change in the
system or surroundings.
Sufficient and necessary conditions:•Quasi-equilibrium process• no dissipation in process
-friction, non-elastic deformation, resistance…
Thermodynamic Cycle
Cycle –a collection of processes that ultimately lead to the system being returned to the original state.
P
V
•clockwise
heat work
•Anti-clockwise
work heat