First law of thermodynamics
Keep the heat and work sign convention explicit in an energy balance.
Key knowledge
For a closed thermodynamic system, with changes of bulk kinetic and gravitational potential energy neglected, the internal-energy change is ΔU = Q − W: Q is heat entering the system and W is work done by it.
Values and equations
ΔU = Q − W
- ΔU
- Change in internal energy J
- Q
- Heat transferred into the system J
- W
- Work done by the system J
Conditions and scope
- No matter crosses the boundary; include all relevant forms of work.
- The stated sign convention applies; other conventions reverse the work term.
- Use equilibrium end states to assign U in this introductory balance.
Sources · 3
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
- University Physics Volume 2, 3.3 First Law of ThermodynamicsSamuel J. Ling, William Moebs and Jeff Sanny — OpenStax / Rice UniversityTextbook chapter
- First Law — Internal EnergyNASA Glenn Research CenterTeaching resource
- Thermodynamics · Physics (2026–27 reprint)National Council of Educational Research and Training (NCERT)Textbook chapter · PDF
Evidence and review
OpenStax 3.3, equation 3.7 and sign table; NASA state-to-state Q-in/W-out balance.
2026-10-10 · Closed-system internal-energy balance under stated simplifications only; flowing matter and changing bulk motion need additional energy terms.
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
With work done by a closed system positive, heat input minus work output changes internal energy when bulk-energy changes are excluded.
Changing the work sign convention changes the equation’s sign, not the energy balance.
National Council of Educational Research and Training (NCERT)
- Thermodynamics · Physics (2026–27 reprint) IndiaEnglish
11.4–11.5, equation11.1
- Thermodynamics · Physics (2026–27 reprint)
Entry details
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- physics-first-law-thermodynamics
- Revision
- core.1
- Structured reference record
- Essential core · first collection · University core expansion