Overview
The First Law of Thermodynamics establishes the principle of energy conservation within thermodynamic processes. It asserts that energy can neither be created nor destroyed, only transformed or transferred. Central to energy accounting in physical systems, it relates changes in internal energy to heat supplied and work done by or on the system.
"Energy can be transformed from one form to another, but the total energy remains constant." -- Rudolf Clausius
Definition
Energy conservation principle applied to thermodynamic systems; internal energy change equals net heat and work exchange.
Significance
Foundation for all energy-related calculations, dictates constraints for heat engines, refrigerators, and natural processes.
Scope
Applies universally to closed, open, and isolated systems; valid for reversible and irreversible processes.
Historical Background
Preliminary Concepts
Caloric theory dominated early heat studies; heat considered a fluid. Experiments by Rumford challenged caloric permanence.
Key Contributors
Joule: mechanical equivalent of heat. Mayer: conservation of energy. Helmholtz: formal energy conservation principles.
Formulation Milestones
Mid-19th century, thermodynamic energy formalized. Clausius and Kelvin refined laws to modern expressions.
Statement and Formulation
Classical Statement
"The increase in internal energy of a system is equal to the heat supplied to the system minus the work done by the system."
Mathematical Form
ΔU = Q - W, where ΔU: change in internal energy, Q: heat added, W: work done by system.
Alternative Formulations
Sign conventions vary; some use W as work done on system. Integral and differential forms used in analysis.
Thermodynamic Systems
Open Systems
Mass and energy cross boundaries. First law includes flow work and enthalpy changes.
Closed Systems
Fixed mass, energy transfer via heat or work only. Simplest application of the First Law.
Isolated Systems
No mass or energy exchange. Internal energy constant; ΔU = 0.
Internal Energy
Definition
Total microscopic energy: kinetic + potential of molecules, atoms, electrons within the system.
State Function
Depends solely on state variables, independent of process path.
Measurement and Estimation
Cannot be measured absolutely; changes inferred from heat and work interactions.
| Internal Energy Components |
|---|
| Translational, rotational, vibrational kinetic energies, chemical bond energies, nuclear energies |
Heat and Work
Heat (Q)
Energy transfer due to temperature difference; path function, not a state function.
Work (W)
Energy transfer via force acting through displacement; includes boundary work, shaft work.
Sign Conventions
Heat added to system: positive. Work done by system: positive (engineering convention).
| Quantity | Positive When |
|---|---|
| Heat (Q) | Energy enters system |
| Work (W) | System does work on surroundings |
Mathematical Expressions
Differential Form
dU = δQ - δW; infinitesimal changes, δ denotes inexact differentials.
Boundary Work Expression
Work done by expanding gas: W = ∫P dV, integral over volume change at pressure P.
Energy Balance Equation
For closed system: ΔU = Q - W; for open systems, includes flow terms.
First Law (Closed System):ΔU = Q - WDifferential form:dU = δQ - δWBoundary work:W = ∫ P dV Applications
Heat Engines
First Law governs conversion of heat to work; efficiency limitations arise from energy conservation.
Refrigerators and Heat Pumps
Energy input required to transfer heat against gradient; first law defines work requirements.
Calorimetry
Measurement of heat changes in chemical and physical processes; relates heat transfer to internal energy change.
Chemical Thermodynamics
Enthalpy and reaction energy calculations depend on first law principles.
Limitations and Extensions
No Directionality
First Law does not address process spontaneity or entropy; second law complements it.
Non-Mechanical Work Forms
Electrical, magnetic, surface tension work extensions required in some systems.
Quantum and Relativistic Cases
Energy conservation holds; internal energy definitions extend to microscopic scales.
Experimental Verification
Joule's Experiments
Mechanical stirring raised water temperature; established mechanical equivalent of heat.
Calorimetric Measurements
Heat flow quantified; confirmed energy balance in chemical and physical changes.
Modern Techniques
Calorimeters, bomb calorimeters, differential scanning calorimeters for precise energy measurements.
Summary
The First Law of Thermodynamics is a universal conservation principle: energy within a system changes only via heat and work transfer. It defines internal energy as a state function and provides a quantitative framework for numerous physical, chemical, and engineering systems. Limitations include lack of directionality and entropy considerations, addressed by subsequent laws.
ΔU = Q - WEnergy conserved in all thermodynamic processes.Applies to closed, open, isolated systems.Basis for heat engines, refrigerators, calorimetry. References
- J. P. Holman, Thermodynamics, McGraw-Hill, 2010, pp. 15-45.
- R. C. Reid, J. M. Prausnitz, B. E. Poling, The Properties of Gases and Liquids, McGraw-Hill, 1987, pp. 1-30.
- H. B. Callen, Thermodynamics and an Introduction to Thermostatistics, Wiley, 1985, pp. 30-65.
- C. Kittel, H. Kroemer, Thermal Physics, W. H. Freeman, 1980, pp. 10-50.
- F. W. Sears, G. L. Salinger, Thermodynamics, Kinetic Theory, and Statistical Thermodynamics, Addison-Wesley, 1975, pp. 20-60.