Thermodynamics
Thermodynamics is the branch of physics that studies heat, work, temperature and energy, especially how energy moves and changes form. Its four laws describe thermal equilibrium, conservation of energy, the direction and limits of energy transformations, and the behavior of entropy near absolute zero. Developed in large part through the study of heat engines, thermodynamics now helps explain and design processes in physics, chemistry, engineering and materials science.
Scope and central concepts
Classical thermodynamics describes the large-scale state and behavior of physical systems using measurable properties such as temperature, pressure, volume and internal energy. A thermodynamic system may exchange energy with its surroundings as heat or work. Heat is energy transferred because of a temperature difference; it is not a substance stored inside an object. Temperature characterizes a system's thermal state, while internal energy accounts for energy associated with the microscopic motion and interactions of its constituents. Thermodynamics focuses on macroscopic relationships rather than tracking each atom individually. Statistical mechanics complements it by relating those bulk properties to the behavior of particles.
The four laws
The zeroth law states that if two systems are each in thermal equilibrium with a third, they are in equilibrium with one another. This makes temperature comparison and thermometers meaningful. The first law is conservation of energy: a system's internal energy changes through energy transferred as heat and work, but energy is not created or destroyed. The second law constrains which processes occur spontaneously. Heat flows naturally from hotter to colder regions, not the reverse without external work; no cyclic heat engine can convert all the heat it absorbs into work. In entropy terms, the total entropy of an isolated system does not decrease: it is unchanged in an ideal reversible process and increases in an irreversible one. The third law says that the entropy of a perfect crystal approaches zero as its temperature approaches absolute zero, providing a reference for absolute entropy. These principles describe both energy accounting and the limits on how effectively energy can be put to work.
Development and applications
Thermodynamics took shape during the nineteenth century as engineers and scientists investigated heat engines and the limits on converting heat into useful work. Sadi Carnot's analysis of engine cycles in 1824 helped establish the field; Rudolf Clausius and William Thomson (Lord Kelvin) later formulated the first and second laws. Walther Nernst developed the third law in the early twentieth century, and Ralph Fowler named the zeroth law in the 1930s. The framework is now used to analyze engines and refrigerators, phase changes, chemical reactions and the properties of materials. Modern research and engineering also rely on measured and critically evaluated thermodynamic and transport-property data, including datasets maintained by the U.S. National Institute of Standards and Technology.
Key facts
- Subject
- Relationships among heat, work, temperature and energy
- First law
- Energy is conserved; changes in internal energy reflect transfers of heat and work
- Second law
- The total entropy of an isolated system does not decrease
- Third law
- The entropy of a perfect crystal approaches zero as its temperature approaches absolute zero
- Historical origin
- The field developed in the nineteenth century, in part from analysis of heat engines
Timeline
1824
Sadi Carnot analyzes heat-engine cycles, laying groundwork for the science of thermodynamics.
Around 1860
Rudolf Clausius and William Thomson (Lord Kelvin) formally develop the first and second laws.
1906–1912
Walther Nernst develops the third law of thermodynamics.
1930s
Ralph Fowler gives the name “zeroth law” to the principle underlying temperature comparison.
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References
- 1.Thermodynamics Laws, Definition, & Equations
Encyclopaedia Britannica · encyclopedia · reliability: high · retrieved 17 October 2026
Supports: Overview of thermodynamics, macroscopic state variables, the laws, historical development and relation to statistical mechanics.
- 2.Laws of thermodynamics Definition, Physics, & Facts
Encyclopaedia Britannica · encyclopedia · reliability: high · retrieved 17 October 2026
Supports: Explanations of the zeroth, first, second and third laws, including their historical formulation.
- 3.Ch. 1 Introduction - University Physics Volume 2
OpenStax, Rice University · other · reliability: high · retrieved 17 October 2026
Supports: Introductory physics text on heat, temperature, work and the scope of thermodynamics.
- 4.3.3 First Law of Thermodynamics - University Physics Volume 2
OpenStax, Rice University · other · reliability: high · retrieved 17 October 2026
Supports: Textbook explanation of energy conservation and the relation among heat, work and internal energy.
- 5.Ch. 4 Introduction - University Physics Volume 2
OpenStax, Rice University · other · reliability: high · retrieved 17 October 2026
Supports: Explains why energy conservation alone does not determine process direction, introducing the second law and limits on heat-to-work conversion.
- 6.4.6 Entropy - University Physics Volume 2
OpenStax, Rice University · other · reliability: high · retrieved 17 October 2026
Supports: Textbook treatment of entropy as a state function and its relation to the second law and reversible or irreversible processes.
- 7.Thermodynamics
National Institute of Standards and Technology · other · reliability: high · retrieved 17 October 2026
Supports: Describes NIST research applying thermodynamics to materials and industrial and scientific processes.
- 8.ThermoData Engine
National Institute of Standards and Technology · other · reliability: high · retrieved 17 October 2026
Supports: Describes NIST's critically evaluated thermodynamic and transport-property data resource.
References are suggested by the research engine and may be incomplete. Unverified URLs are omitted rather than guessed.
Article history
- Version 117 October 2026 · EditorCreated by Zapia