# Stationary process

A stationary process in physics and chemistry is a process associated with the movement of substance or energy, in which the state variables of the system under consideration do not change over time as long as the process lasts. For example, in a heat exchanger, the continuous heat transport is stationary if the temperature is constant at a given, arbitrary point on the heat exchanger.

An external stimulus to the system - such as switching on, load change, disruption in the process - is followed by a compensation process that subsides over time. If the stationary process is disturbed by an intervention, the old one changes into a new stationary process, but not abruptly at the time of change, but steadily .

Two characteristics can be seen:

1. An aperiodic process occurs which appears to be stationary despite a throughput. In a flow process, a steady state is established in which all the state variables that characterize the process - with a finite reaction speed - assume values ​​that are constant over time. For example, an electrical conductor is heated over time from the time a current is switched on until an increased, but then constant temperature is established through heat dissipation, see Current Heat Act .
2. A periodic process occurs, which is called stationary oscillation , if its characteristic values ​​- peak value and frequency - are independent of time. Periodic processes are stationary from the start. This includes, for example, steady processes in alternating current technology .

Any kind of damping of a stable oscillating system leads to the decay of the oscillation to the rest position. (The same applies to the rotation, which is made up of two oscillations.) However, if the process is continuously supplied with energy that compensates for the damping losses, the process can run indefinitely. examples are

• a clock in a mechanical or electronic clockwork under constant mechanical or electrical voltage
or generally an oscillator ;
• an unloaded machine adjusting to idle speed as long as it is connected to the network;
• a non-fading vibration of a violin as long as it is played.

A chemical reaction usually proceeds up to a chemical equilibrium with the opposite reaction. The overall reaction then appears dormant, although the individual reactions continue to run. There are also oscillating reactions . For example, they act as a clock for periodic biological processes.

## Individual evidence

1. Jürgen U. Keller: Technical Thermodynamics in Examples, Part 1: Basics. de Gruyter, 1979, p. 258
2. Walter J. Moore: Fundamentals of physical chemistry. de Gruyter, 1990, p. 338
3. Frank Ahnert: Introduction to Geomorphology. Eugen Ulmer, 5th edition 2015, p. 24 ff
4. Ralf Bürgel: Handbook high temperature materials technology: Fundamentals .... Vieweg, 3rd ed., 2006, p. 27
5. Wilfried Weißgerber: Electrical engineering for engineers 3: compensation processes, Fourier analysis, four-pole theory. Springer Vieweg, 8th edition, 2013, p. 1
6. Günter Ludyk: Theoretical control engineering 1: Basics, synthesis of linear control systems. Springer, 1995, p. 179
7. a b Tilo Peifer, Paul Profos (Ed.): Handbook of industrial measurement technology. Oldenbourg, 6th edition 1994, p. 53
8. Erwin Haibach: Durable components: Determination and proof of durability, structural and entrepreneurial aspects. Springer, 1992, p. 42
9. Hans Peter Latscha, Helmut Alfons Klein: Inorganische Chemie: Chemie-Basiswissen I. Springer, 8th edition 2002, p. 252
10. ^ Fritz Kurt Kneubühl: Repetitorium der Physik. Teubner, 4th ed. 1990, p. 248
11. Wolfgang Demtröder: Experimentalphysik 1: Mechanics and heat. Springer, 7th edition 2015, p. 384
12. Thomas Frey, Martin Bossert: Signal and System Theory. Teubner, 2004, p. 133