2. Syst Flashcards
A ?(1)? is a definable part of the world that is interesting to us.
It is seperated from its ?(2)? by a system boundary.
(1) system
(2) environment
(3) boundary
Thermodynamic processes can be understood as a mutual influence of system and environment or of two systems among themselves. The influence takes place by ??
transfer of energy
Name 3 types of energy transfer!
Mechanical
Thermal
Through material flow
Types of energy transfer: Mechanical
Energy in form of ?(1)? crosses the system boundary
work(W)
Types of energy transfer: Thermal
Energy in the form of ?? is transferred across the system boundaries.
heat (Q)
Types of energy transfer: through material flow
Different types of energy (e.g. ??) cross the system boundary with the material flow.
kinetic energy
General Energy balancing Formula?
Change in the total energy of system
(=)
Total Energy entering the system
(-)
Total Energy leaving the system
Elements of Thermodynamics: ?? (3)
System and system boundaries
State/status and variables describing the status
Process and process variables
State variables
Name 3 thermal and 3 caloric state variables!
thermal: Temp. (T), Pressure (p), Volume (V)
caloric: Internal energy (U), Enthalpy (H), Entropy (S)
-> specific jeweils mit /m
State variables
?(1)? -> depend on the prevailing temperature
?(2)? -> describe the energy content of a system
(1) thermal
(2) caloric
Name 4 Process variables!
Work: W
Power: P = dW / dt
Heat: Q
Heat flow: Qdot = dQ / dt
The (electrical) work done on an adiabatic system is equal to?
the increase in the energy of the system
The energy change of a system during a process is equal to?
the net work and heat transfer between the system and its surroundings
Ex1: A rigid tank contains a hot fluid that is cooled while being stirred by a paddle wheel. Initially, the internal energy of the fluid loses 500kJ of heat, and the paddle wheel does 100kJ of work on the fluid.
Determine the final internal energy of the fluid (U2). Neglect the energy stored in the paddle wheel!
Wsh,in - Qout = deltaU = U2 - U1
100kJ - 500kJ = U2 - 800kJ
–> U2 = 400kJ
(
For a closed system delta U = delta Q + delta W
-> if we add work or heat to the system, the internal energy will increase
-> this will result in increased state variables like temp., pressure or volume
-> A transfer of heat will result in an increase of internal energy and is proportional to the increase in temperature by a factor c
-> Attention: This does not apply to phase transition and c is not constant for the same substance across all temperature levels!
-> c is the thermal capacity of the system and it depends on the substance as well as the conditions under which the heat transfer takes place (constant p or V)
)
What does the Law of Conservation of Energy say?
The sum of all forms of energy always remains the same.
-> but: Energy = Exergy + Anergy
Energie = Exergy + Anergy
Welche Energie ist useful?
Exergy -> Anergy is not useful
??: The part of the energy that can be converted into any other form of energy under given thermodynamic conditions of the environment.
Exergy (Availability)
The exergy share ?? in all conversion processes.
decreases
??: The part of the energy that cannot be converted into other forms of energy, e.g. thermal energy at the temperature level of the environment
Anergy
What does the 1st law of thermodynamics say?
Energy consumption in the thermodynamic sense does not exist, as the sum of exergy and anergy is constant.
What does the 2nd law of thermodynamics say?
Energy can be degraded from exergy to anergy and it is never possible to convert anergy into exergy.
(This principle could be interpreted as energy consumption)
?(1)? = fully usuable, unlimitedly convertible part of energy (e.g. work); entropy-free
?(2)? = non-usuable part of the energy in the considered environment; afflicted with entropy
(1) Exergy
(2) Anergy
(A cup of hot coffee does not get hotter in a cooler room)
Ex. 2: A piston–cylinder device contains a liquid-vapor mixture of water at 300 K. During a constant pressure process, 750 kJ of heat is transferred to the water. As a result, part of the liquid in the cylinder vaporizes.
Determine the entropy change of the water during this process.
delta S system = delta Q / Tsystem
delta S system = 750 kJ / 300K = 2,5 kJ/K
Formel gilt bei konstanter Temp. und reversiblen Prozess
-> liegt hier vor, denn:
1) Der Prozess wird bei konstanter Temp. durchgeführt. delta Q wird ausschließlich dazu verwendet, die Phasenumwandlung bei konstanter Temp. und Druck zu ermöglichen.
2) Bei Phasenwechselprozessen (wie Verdampfen) unter konstantem Druck und konstanter Temp handelt es sich um quasi-statische (nahezu reversible) Prozesse.
T-v Diagram and P-v Diagram
-> see slide 15
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