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Work [J] is a specific form of energy in the First Law of thermodynamics, and a specific form of exergy in the Second Law of thermodynamics, performed by a closed or open system on its surroundings (the environment). This is the definition of external work, which is zero in isolated systems. The term exergy includes external and internal work. Mechanical work is force [N] times path length [m]. The internal-energy change of a closed system, dU, is due to external exchange (e) of work and heat, and external total work (et, including pressure-volume work) is the internal-energy change minus heat,

detW = dU - deQ

Abbreviation: deW [J]

Reference: Gnaiger (1993) Pure Appl Chem

Communicated by Gnaiger E 2018-10-20; last update 2022-07-11

Pressure-volume work

Pressure-volume work, dVW, at constant pressure, is the gas pressure, p [Pa = JΒ·m-3], times change of volume, dV [m3],
dVW = -pΒ·dV [J]
The available work, deW, is distinguished from external total work as
deW = detW - dVW
The enthalpy change, dH, at constant pressure, is defined as internal-energy change minus pressure-volume work,
dH = dU - dVW


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  • Joule [J]; 1 J = 1 NΒ·m = 1 VΒ·C; 1 cal = 4.184 J
Fundamental relationships
Β» Energy
Β» Exergy
Β» Extensive quantity
Β» Force
Β» Pressure
Β» Intensive quantity
Forms of energy
Β» Internal-energy dU
Β» Enthalpy dH
Β» Heat deQ
Β» Bound energy dB
Forms of exergy
Β» Helmholtz energy dA
Β» Gibbs energy dG
Β» Work deW
Β» Dissipated energy diD


  1. Gnaiger E (1993) Nonequilibrium thermodynamics of energy transformations. Pure Appl Chem 65:1983-2002. - Β»Bioblast linkΒ«

MitoPedia concepts: Ergodynamics 

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