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2022 | OriginalPaper | Chapter

3. System and State

Author : Achim Schmidt

Published in: Technical Thermodynamics for Engineers

Publisher: Springer International Publishing

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Abstract

This chapter clarifies what a thermodynamic system is and how its state can be described. Every system is separated from an environment by a system boundary.

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next chapter Thermodynamic EquilibriumEquilibriumthermodynamic
Footnotes
1
The fluid inside a bottle can be heated, e.g. by a lighter. This changes the fluid’s inner state, which is given by the temperature, for example. The lighter is therefore an external influence that acts across the system boundary.
 
2
Even though they can be generated, e.g. entropy, or they can be destroyed, e.g. exergy.
 
3
The expression extensive state value is specified in the next chapters. Extensive state values have in common that they can be counted.
 
4
The density of a system is a state value like pressure, temperature and many others, see Sect. 3.2. The assignment of state values to a heterogeneous system can lead to difficulties because they can be ambiguous.
 
5
Though nobody has ever visited the boundary of space...
 
6
Every student knows, that a perfectly insulated thermos does not exist. After a three-hour lecture the coffee inside a thermos is inedible. However, in thermodynamics ideal systems are assumed to exist.
 
7
Just think of your refrigerator at home: A bottle of beer that needs to be chilled is placed in the fridge. There is a low temperature in this compartment. At this low temperature, however, a flow of heat must leave the beer bottle to reduce its internal energy, i.e. to cool it down. This heat is supplied to the refrigerant cycle.
 
8
You may have noticed that the back of your refrigerator has a higher temperature than the surroundings and therefore releases heat.
 
9
Also known as second law of thermodynamics. Placing your hand on a hotplate demonstrates the second law of thermodynamics impressively.
 
10
This is the reason why the fridge needs to be connected with a plug.
 
11
Thus, entropy obviously has something to do with heat and temperature!
 
12
In case of a fridge the working fluid releases heat to the ambient air, that can be regarded as second mass flow. This ambient air rises due to buoyancy effects.
 
13
At least as long as it is sensible heat, i.e. the fluids are not subject to a phase change. Part II deals with fluids that can also change their state of aggregation. If this happens e.g. isobaric, the heat supplied is not utilised to modify the temperature, but to carry out the phase change. The heat is then called latent heat and the phase change is isothermal.
 
14
Just keep in mind, that \({\text {Pa}}\) is the SI-unit. Nevertheless, for many applications \({\text {bar}}\) is the more common unit. The conversion follows \(1 \times 10^{5}\,{\text {Pa}}\equiv {1}\,{\text {bar}}\).
 
15
The thermodynamic explanation of the triple point is given in Part II!
 
16
Enthalpy and entropy are introduced at this point, although the physical explanation will follow at a later stage. At present, it is simply assumed that they exist.
 
17
A sink can be treated by \(\dot{Z}_{\text {Source}}<0\).
 
18
Just think of your bathtub: the amount of water as a function of time in the tub is influenced by both the flux of incoming water and the mass flux at the outflow. This example is described by Eq. 3.9, although this example does not contain any sources or sinks. Another example has already been given in Fig. 3.2.
 
19
To come back to the example with the cup of coffee: If the coffee is decanted into two cups, the mass m and the volume V change, but the density \(\rho =\frac{1}{v}=\frac{m}{V}\) remains constant.
 
20
However, the equation is given at this time, though the explanation follows later on.
 
21
Isotopes are variants of a chemical element which differ in the neutron number but having the same number of protons in its core.
 
22
However, this definition is arbitrary.
 
23
Which equals \(6.022045 \pm (0.000031) \times 10^{23}\) atoms.
 
24
This is comparable to buying fruit in a supermarket: ten apples have a greater mass than ten grapes!
 
Metadata
Title
System and State
Author
Achim Schmidt
Copyright Year
2022
Book
Technical Thermodynamics for Engineers

Print ISBN: 978-3-030-97149-6

Electronic ISBN: 978-3-030-97150-2

Copyright Year: 2022

DOI
https://doi.org/10.1007/978-3-030-97150-2_3

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