Second law: isolated-system entropy
Apply entropy increase to the whole isolated system.
Key knowledge
For an isolated macroscopic thermodynamic system, total entropy does not decrease: ΔS_total ≥ 0. A subsystem can lose entropy when its surroundings gain enough to satisfy the total balance.
Values and equations
ΔS_total ≥ 0
- ΔS_total
- Entropy change of the isolated total system J K⁻¹
Conditions and scope
- Apply the inequality to the whole isolated system, including relevant surroundings.
- This is macroscopic thermodynamic entropy; microscopic fluctuations and other entropy definitions need separate treatment.
Sources · 3
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
- 8.044 Statistical Physics I (2013), Lecture 8: entropy and the second lawMassachusetts Institute of Technology — MIT OpenCourseWareCourse notes · PDF
- Thermodynamics, Module 5, Lecture 23: Principle of Entropy IncreaseIndian Institute of Technology KanpurCourse notes
- Classical Thermodynamics · EntropyP. Mahanta · IIT GuwahatiCourse notes
Evidence and review
MIT Lecture 8 pages 6–8: total isolated entropy and bath/subsystem balance; IIT Kanpur Lecture 23: isolated-system inequality.
2026-10-10 · Macroscopic isolated-system inequality only; closed-to-matter is insufficient if entropy can cross the boundary with heat.
Foundational review
2026-10-10 · Fresh primary-source inspection of the complete compact record, including every statement, equation, quantity and material condition, followed by independent AI editorial review. This check does not certify all linked lessons or every passage of every cited publication.
Sources from other places
Agreement within this scope
Macroscopic isolated-system entropy is nondecreasing; reversible change gives equality.
An individual subsystem can lose entropy while the combined isolated system satisfies the inequality.
Indian Institute of Technology Guwahati · NPTEL
- Classical Thermodynamics · Entropy IndiaEnglish
Module1 Lecture4 page2, equations1.35–1.38
- Classical Thermodynamics · Entropy
Entry details
Permanent link to this entry ↗- Record identity
- physics-second-law-thermodynamics
- Revision
- core.1
- Structured reference record
- Essential core · first collection · University core expansion