Pressure–volume work and process paths
Boundary work depends on the process, while internal energy is a state function.
Key knowledge
For a mechanically quasistatic simple compressible closed system, work done by the system is W=∫p dV: expansion contributes positive work and compression negative work. A quasistatic isothermal ideal gas with fixed amount gives W=nRTln(V_f/V_i). Isochoric means constant volume; isobaric constant pressure; isothermal constant temperature; adiabatic means no heat transfer.
Values and equations
W_by=∫_(V_i)^(V_f)p dV; W_isothermal=nRTln(V_f/V_i)
- W_by,W_isothermal
- Boundary work done by the system and its isothermal ideal-gas value J
- p
- Quasistatic equilibrium pressure Pa
- V_i,V_f,V
- Initial/final and intermediate volume m³
- n
- Fixed amount of ideal gas mol
- R
- Molar gas constant J mol⁻¹ K⁻¹
- T
- Fixed thermodynamic temperature in the isothermal formula K
Conditions and scope
- In the displayed boundary-work integral, equilibrium system pressure equals the resisting boundary pressure in the quasistatic limit; all other work modes are handled separately.
- For a strongly irreversible boundary motion use the actual boundary-force/external-pressure model, not an undefined system p(V) curve. Adiabatic need not mean constant temperature or reversible.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Samuel J. Ling, Jeff Sanny and William Moebs — OpenStax, Rice University, University Physics (2016)2 sources
- University Physics Volume 2, §3.2: work heat and internal energySamuel J. Ling, Jeff Sanny and William Moebs — OpenStax, Rice University, University Physics (2016)Textbook chapter
- University Physics Volume 2, §3.4: thermodynamic processesSamuel J. Ling, Jeff Sanny and William Moebs — OpenStax, Rice University, University Physics (2016)Textbook chapter
Evidence and review
Read §3.2 boundary-work3.1–3.3 and quasistatic isothermal integral, plus §3.4 named process definitions. Irreversible external-pressure distinction bounds the equilibrium-p integral; it is not an extra work law claimed from a pressure–volume diagram.
2026-10-10 · /root/audit_life_earth_chemistry — independent Chemistry/Life/Earth editor. Independent AI editorial review of this entry's statements, equations, applicability conditions and source coverage. Source checks apply to these claims within their stated limits.
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