Laws of thermodynamics

Four fundamental principles constrain energy, heat, work, temperature, and entropy. They describe limits on physical processes, from energy conservation to the impossibility of converting heat entirely into work in a cycle.

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Zeroth law of thermodynamics: If two systems are each in thermal equilibrium with a third, they are in thermal equilibrium with each other. It establishes temperature and thermal equilibrium, the basis for comparing system states.
Thermodynamic system: A specified quantity of matter or region in space chosen for thermodynamic analysis. The laws apply to systems and their exchanges with surroundings.
Carnot cycle: An ideal reversible cycle whose efficiency sets the maximum possible efficiency for an engine operating between two temperatures. It derives a universal engine-efficiency limit from the second law.
Sadi Carnot: A French physicist whose 1824 analysis of ideal heat engines established a foundational limit on their efficiency. His engine analysis helped reveal the second law’s restrictions before entropy was defined.
First law of thermodynamics: A system’s energy change equals energy transferred as heat minus work done by the system. It formalizes energy conservation for thermodynamic processes.
Temperature: A physical quantity that determines the direction of spontaneous thermal energy transfer between systems. The zeroth law makes temperature a consistent property of thermal equilibrium.
Heat engine: A device that converts heat drawn from a hot source partly into work while rejecting remaining heat to a cooler sink. The laws determine its energy balance and maximum efficiency.
James Prescott Joule: An English physicist who established the mechanical equivalence of heat through experimental measurements. His work supported the first law by showing heat and mechanical work are interconvertible.
Second law of thermodynamics: The total entropy of an isolated system cannot decrease over time. It gives thermodynamic processes a direction and limits heat-to-work conversion.
Heat: Energy transferred between systems because of a temperature difference. Heat is one of the energy transfers accounted for by the first and second laws.