Inthermodynamics,dissipationis the result of anirreversible processthat affects athermodynamic system.In a dissipative process,energy(internal,bulk flowkinetic,or systempotential)transformsfrom an initial form to a final form, where the capacity of the final form to dothermodynamic workis less than that of the initial form. For example,transfer of energy as heatis dissipative because it is a transfer of energy other than by thermodynamic work or by transfer of matter, andspreadspreviously concentrated energy. Following thesecond law of thermodynamics,in conduction and radiation from one body to another, theentropyvaries withtemperature(reduces the capacity of the combination of the two bodies to do work), but never decreases in an isolated system.
Inmechanical engineering,dissipationis the irreversible conversion ofmechanical energyintothermal energywith an associated increase in entropy.[1]
Processes with defined local temperatureproduce entropyat a certain rate. The entropy production rate times local temperature gives the dissipatedpower.Important examples of irreversible processes are:heat flowthrough athermal resistance,fluid flowthrough a flow resistance, diffusion (mi xing ),chemical reactions,andelectric currentflow through anelectrical resistance(Joule heating).
Definition
editDissipative thermodynamic processes are essentially irreversible because theyproduce entropy.Planckregarded friction as the prime example of an irreversible thermodynamic process.[2]In a process in which the temperature is locally continuously defined, the local density of rate of entropy production times local temperature gives the local density of dissipated power.[definition needed]
A particular occurrence of a dissipative process cannot be described by a single individualHamiltonianformalism. A dissipative process requires a collection of admissible individual Hamiltonian descriptions, exactly which one describes the actual particular occurrence of the process of interest being unknown. This includes friction and hammering, and all similar forces that result in decoherency of energy—that is, conversion ofcoherentor directed energy flow into an indirected or moreisotropicdistribution of energy.
Energy
edit"The conversion of mechanical energy into heat is called energy dissipation." –François Roddier[3]The term is also applied to the loss of energy due to generation of unwanted heat in electric and electronic circuits.
Computational physics
editIncomputational physics,numerical dissipation (also known as "Numerical diffusion") refers to certain side-effects that may occur as a result of a numerical solution to a differential equation. When the pureadvectionequation, which is free of dissipation, is solved by a numerical approximation method, the energy of the initial wave may be reduced in a way analogous to a diffusional process. Such a method is said to contain 'dissipation'. In some cases, "artificial dissipation" is intentionally added to improve thenumerical stabilitycharacteristics of the solution.[4]
Mathematics
editA formal, mathematical definition of dissipation, as commonly used in the mathematical study ofmeasure-preserving dynamical systems,is given in the articlewandering set.
Examples
editIn hydraulic engineering
editDissipation is the process of converting mechanical energy of downward-flowing water into thermal and acoustical energy. Various devices are designed in stream beds to reduce the kinetic energy of flowing waters to reduce theirerosive potentialon banks andriver bottoms.Very often, these devices look like smallwaterfallsorcascades,where water flows vertically or overriprapto lose some of itskinetic energy.
Irreversible processes
editImportant examples of irreversible processes are:
- Heat flow through a thermal resistance
- Fluid flow through a flow resistance
- Diffusion (mi xing )
- Chemical reactions[5][6]
- Electrical current flow through an electrical resistance (Joule heating).
Waves or oscillations
editWavesoroscillations,loseenergyovertime,typically fromfrictionorturbulence.In many cases, the "lost" energy raises thetemperatureof the system. For example, awavethat losesamplitudeis said to dissipate. The precise nature of the effects depends on the nature of the wave: anatmospheric wave,for instance, may dissipate close to the surface due tofrictionwith the land mass, and at higher levels due toradiative cooling.
History
editThe concept of dissipation was introduced in the field of thermodynamics byWilliam Thomson(Lord Kelvin) in 1852.[7]Lord Kelvin deduced that a subset of the above-mentioned irreversible dissipative processes will occur unless a process is governed by a "perfect thermodynamic engine". The processes that Lord Kelvin identified were friction, diffusion, conduction of heat and the absorption of light.
See also
editReferences
edit- ^Escudier, Marcel; Atkins, Tony (2019).A Dictionary of Mechanical Engineering(2 ed.). Oxford University Press.doi:10.1093/acref/9780198832102.001.0001.ISBN978-0-19-883210-2.
- ^Planck, M.(1926). "Über die Begründung des zweiten Hauptsatzes der Thermodynamik",Sitzungsber. Preuss. Akad. Wiss., Phys. Math. Kl.,453—463.
- ^Roddier F.,Thermodynamique de l'évolution (The Thermodynamics of Evolution),parole éditions, 2012
- ^Thomas, J.W. Numerical Partial Differential Equation: Finite Difference Methods. Springer-Verlag. New York. (1995)
- ^Glansdorff, P.,Prigogine, I.(1971).Thermodynamic Theory of Structure, Stability, and Fluctuations,Wiley-Interscience, London, 1971,ISBN0-471-30280-5,p. 61.
- ^Eu, B.C. (1998).Nonequilibrium Thermodynamics: Ensemble Method,Kluwer Academic Publications, Dordrecht,ISBN0-7923-4980-6,p. 49,
- ^W. ThomsonOn the universal tendency in nature to the dissipation of mechanical energyPhilosophical Magazine, Ser. 4, p. 304 (1852).