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Dissipation (Latin for "dispersion") describes in physics the process in a dynamic system . B. by friction, the energy of a macroscopically directed movement, which can be converted into other forms of energy, is converted into thermal energy , d. H. into energy of a disordered movement of the molecules, which can then only be partially converted. Such a system is called dissipative . This term occurs in the physical fields of thermodynamics and acoustics or in general in wave theory . An example of a dissipative system is the damped oscillation .

In thermodynamics, the work that is converted into thermal energy ( internal energy ) due to friction, throttling or impact processes is referred to as dissipation work. These are irreversible processes in which the entropy increases, in other words: exergy is converted into anergy (cf. second law of thermodynamics ). These works are process variables , i. H. path-dependent.

The dissipation constant for a thermistor ( Negative Temperature Coefficient , NTC) is the thermal conductivity value , which is usually specified for stagnant air. The dissipation constant changes on contact with water.

In materials technology , in connection with the material cycle, dissipation means the consumption of raw materials - i.e. losses that cannot be compensated for, e.g. B. Corrosion , abrasion and other loss in widespread distribution over the entire surface of the earth, so that the raw material cannot be recovered.

Dissipation in Thermodynamics

Dissipative processes in a closed system

The picture illustrates various dissipative processes in a closed adiabatic system:

  • Due to the rigid system limit, the work of a fan does not perform any volume change work , is therefore completely dissipated by friction,
  • likewise the electrical work transferred across the system boundary.
  • With a temperature equalization process within the system, reversible work could be generated via a Carnot process and released to the outside. However, since the heat in the picture flows unused from “warm” to “cold”, its exergetic component is dissipated.
  • Instead of the throttling process shown, the pressure equalization could be reversible via a movable piston (with power transmission to the outside), or a wind turbine could do work behind the nozzle. These possibilities also remain unused, the work is instead dissipated.

In an internal combustion engine , the friction between the piston rings and the cylinder walls converts part of the volume work that has already been generated from thermal energy back into internal energy, so that the work transferred to the outside is reduced.