Zeroth law: thermal equilibrium and temperature
If bodies A and B are in thermal equilibrium, and B and C are in thermal equilibrium, then A and C are also in thermal equilibrium. This law lets a thermometer (B) compare the temperatures of different bodies. Because it is more basic than the other laws, it was later named the "zeroth" law.
First law: conservation of energy
The change in a system's internal energy equals the heat added to the system minus the work done by the system (ΔU = Q − W). It was established in the mid-19th century through Joule's experiments on the mechanical equivalent of heat and the work of Mayer and Helmholtz. A perpetual motion machine of the first kind, which does work without an energy supply, is impossible.
Second law: entropy and the direction of processes
The second law sets the direction of natural processes. It has two classic statements.
- Clausius statement: heat cannot flow spontaneously from a colder body to a hotter one without some other change.
- Kelvin-Planck statement: no cyclic engine can take heat from a single reservoir and convert all of it into work (a perpetual motion machine of the second kind is impossible).
Clausius (1865) introduced the concept of entropy and formulated the law as: the entropy of an isolated system does not decrease. The maximum efficiency of the ideal heat engine analyzed by Carnot (1824) is η = 1 − Tcold/Thot (absolute temperatures), and real engines are less efficient. Social extensions of the entropy concept continue in the entropy law.
Third law: absolute zero
Growing out of Nernst's heat theorem (1906), the third law states that as temperature approaches absolute zero (0 K), the entropy of a system approaches a constant minimum. The entropy of a perfect crystal can be taken as zero at 0 K, and it follows that absolute zero cannot be reached in a finite number of steps.
Applications
The laws are used to calculate the performance of heat engines and refrigerators, judge whether chemical reactions are spontaneous (Gibbs free energy), and describe energy metabolism in living things. Thermodynamics deals with equilibrium states and energy balances, however, not with how fast heat moves; rates are the subject of heat transfer.
See also: Anthropocene, Boiling Point and Pressure
References
- Carnot, S. (1824). Réflexions sur la puissance motrice du feu. Paris: Bachelier.
- Clausius, R. (1865). Über verschiedene für die Anwendung bequeme Formen der Hauptgleichungen der mechanischen Wärmetheorie. Annalen der Physik und Chemie, 125(7), 353-400.
- Fermi, E. (1937). Thermodynamics. New York: Prentice-Hall.
- Callen, H. B. (1985). Thermodynamics and an introduction to thermostatistics (2nd ed.). New York: Wiley.