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Erschienen in:

2019 | OriginalPaper | Buchkapitel

3. System and State

verfasst von : Achim Schmidt

Erschienen in: Technical Thermodynamics for Engineers

Verlag: Springer International Publishing

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Abstract

In this chapter it is clarified what a thermodynamic system is and how its state can be described. Any system is separated from an environment by a system boundary as it is done in other technical disciplines, e.g. technical mechanics, as well. First, the permeability of the system boundary is categorised. However, after classifying the system and the system boundary, it is the next step to identify the internal state. This leads to so-called state values, e.g. pressure and temperature, that fix the state of a system. Our everyday experience shows, that the state can be varied by external impacts across the boundary.

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Vorheriges Kapitel Energy and Work

Nächstes Kapitel Thermodynamic Equilibrium

The content of a bottle can be heated up, e.g. by a lighter. By doing so, its internal state, given by the temperature for instance, changes. Thus, the lighter is the external impact acting at the boundary.

The expression extensive state value will follow in the next chapters. However, extensive state values can be counted.

The density of a system is a state value like pressure, temperature and many more, see Sect. 3.2 Assigning state values to a heterogenous system can cause difficulties as they might be equivocal.

Though nobody has ever visited the boundary of space...

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.

Just think of your fridge at home: A bottle of beer, that needs to be cooled down, is placed inside the fridge. In this compartment the temperature is low. However, at this low temperature, a heat flux needs to leave the bottle of beer in order to decrease its internal energy, i.e. cooling it down. This heat is supplied to the refrigerant cycle of the fridge.

You might have already realised that the backside of your fridge has a larger temperature than the environment and thus releases heat.

Also known as second law of thermodynamics. Placing your hand on a hotplate proves the second law of thermodynamics impressively.

This is the reason why the fridge needs to be connected with a plug.

In case of a fridge the working fluid releases heat to the surrounding air, that can be regarded as second mass flow.

At least as long as we talk about sensible heat, i.e. the fluids do not underlie a phase change. Part II covers fluids, that also can change their aggregate states. If this is done isobarically for instance, supplied heat is not utilized to vary the temperature but to perform the phase change. Heat then is called latent heat and the phase change runs isothermally.

Just keep in mind, that Pa is the SI-unit. Nevertheless, for many applications bar is the more common unit. The conversion follows \(1 \times 10^{5} \mathrm{Pa}\equiv 1\mathrm{bar}\).

The thermodynamic explanation of the triple point will be given in part II!

Enthalpy and entropy are introduced at that point though the physical explanation is going to follow at a later point of time. Currently we just assume, they exist.

E.g. entropy generation.

E.g. loss of exergy.

Just think of your bathtub: The amount of water as a function of time in the tub is influenced by the flux of incoming water as well as by the flux at the sink. This example visualises Eq. 3.9 though this example does not include sinks or sources. Another example was given in Fig. 3.2.

Going back to the example with the cup of coffee: When the coffee is decanted in two cups, the mass m as well as the volume V sure vary, but its density \(\rho =\frac{1}{v}=\frac{m}{V}\) remains constant.

However, the equation is given at this time, though the explanation follows later on.

Isotopes are variants of a chemical element which differ in the neutron number but having the same number of protons in its core.

However, this definition is arbitrary.

Which equals \(\left( 6.022045 \pm 0.000031\right) \times 10^{23}\) atoms.

This can be compared with buying fruits in a super-market: ten apples have a larger mass than ten grapes!

Titel: System and State
verfasst von: Achim Schmidt
Verlag: Springer International Publishing
Buch: Technical Thermodynamics for Engineers
Print ISBN: 978-3-030-20396-2

Electronic ISBN: 978-3-030-20397-9

Copyright-Jahr: 2019
DOI: https://doi.org/10.1007/978-3-030-20397-9_3

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