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2019 | OriginalPaper | Buchkapitel

7. Changes of State

verfasst von : Achim Schmidt

Erschienen in: Technical Thermodynamics for Engineers

Verlag: Springer International Publishing

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Abstract

So far thermodynamic systems have been discussed and state values to quantify their internal state have been categorised. In this chapter the focus will be on changes of state, i.e. bringing a system from an initial state to a new state.

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Vorheriges Kapitel Thermal Equation of State

Nächstes Kapitel Thermodynamic Processes

Key to solve problems in thermodynamics is to understand the path a change of state takes!

The gas as part of the investigated system may also be at equilibrium though it is not in thermal equilibrium with the outer environment. In this case, the gas is adiabatically insulated and is not affected by the environment. In this case the temperature inside the cylinder is constant with respect to time and uniform!

The experiments performed by Boyle-Mariotte as well as by Gay-Lussac, see Chap. 6, have been conducted under the premise of thermodynamic equilibrium.

The system would drive itself into thermal, mechanical as well as chemical equilibrium!

Think of taking pictures of a car that is accelerating. If your camera has a very short exposure time, each of the plenty of photos looks like static, all images are sharp. In this very short period of time the car movement is neglectable. If the shutter speed is too large, the resulting pictures do not look like static at all, since the movement results in blurred images.

In the following sections, it is investigated what too high means!

Mass fractions \(\xi _{i}\) will be introduced in part II though.

And thus a homogeneous phase!

Later on, internal friction will be called dissipation!

To cope with this example a knowledge of first and second law of thermodynamics is required.

The system will be further discussed later in Problem 11.8!

Benefit of a differential notation is, that it shows what is ongoing inbetween initial and final equilibrium state.

The term friction \(\Psi _{\text {fric.}}\) is explained in Sect. 9.2.5.

If the gas inside the cylinder expands, i.e. \(\text {d}V>0\), work is released by the system to the environment. Thus, \(\delta W_{\text {12,env}}\), as assumed in the sketch, is negative! The related work can be calculated easily since the ambient pressure is constant.

For a technical application mathematical functions for \(\psi _{\text {12,G}}\) and \(\psi _{\text {fric.}}\) are required. These functions need to correlate the specific dissipation with the distance a system moves and its velocity. A detailed example is given with Problem 11.9.

With the mass of the gas still ignored.

To cope with this problem a knowledge of first and second law of thermodynamics is required.

Titel: Changes of 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_7

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