Temperature
Temperature is what two systems in thermal contact end up sharing once energy stops flowing between them.
Two systems in thermal contact exchange energy until they reach the same temperature — thermal equilibrium. The zeroth law of thermodynamics: if A is in equilibrium with B, and B with C, then A is in equilibrium with C — the principle that makes a thermometer's reading meaningful.
Celsius, Fahrenheit and Kelvin measure the same physical quantity with different zero points and step sizes.
Conversion between Celsius, Kelvin and FahrenheitConventionT1.2.1Most materials expand slightly as temperature rises, in proportion to the temperature change and the material's original size.
Change in length or volume with temperatureResultT1.3.1An idealized gas of point particles with no intermolecular forces obeys a single equation relating pressure, volume, amount, and temperature.
Pressure, volume and temperature of an ideal gasLawT1.4.1Heat and the First Law
Heat and work are two different ways of moving energy across a system's boundary; the first law tracks both.
Different materials require different amounts of heat to raise their temperature by the same amount.
Heat required for a given temperature changeDefinitionT2.1.1During a phase change, added or removed heat changes the proportion of substance in each phase rather than the temperature.
Heat required for a phase change at constant temperatureDefinitionT2.2.1A gas does work on its surroundings by changing volume against an external pressure.
Work done by a gas changing volumeDefinitionT2.3.1Energy conservation applied to a system that can exchange both heat and work with its surroundings.
Energy conservation for heat and work combinedPrincipleT2.4.1Kinetic Theory
Pressure, temperature and heat capacity are all statistical consequences of enormous numbers of colliding molecules.
Gas pressure arises from the cumulative momentum transferred by molecules colliding with a container's walls.
Gas pressure from molecular collisionsResultT3.1.1At thermal equilibrium, energy distributes equally among every independent way a molecule can store it.
Average thermal energy per degree of freedomPrincipleT3.2.1Heat capacity follows directly from how many degrees of freedom a gas molecule has to store energy in.
Molar heat capacity from degrees of freedomResultT3.3.1Not every molecule in a gas moves at the same speed: the Maxwell-Boltzmann distribution describes the spread, skewed toward higher speeds with a long tail. The most probable, mean, and root-mean-square speeds are three distinct values, in that increasing order, and all shift higher as temperature rises.
Entropy and the Second Law
Heat never flows spontaneously from cold to hot; every engine, refrigerator and irreversible process obeys that one restriction.
A heat engine converts part of the heat flowing from a hot reservoir into useful work, exhausting the rest to a cold reservoir.
Fraction of heat input converted to workDefinitionT4.1.1A refrigerator or heat pump uses work input to move heat against its natural direction, from cold to hot.
Heat moved per unit of work, for a refrigerator or heat pumpDefinitionT4.2.1No engine operating between two fixed temperatures can exceed the efficiency of an idealized, fully reversible cycle.
Maximum possible efficiency between two reservoirsResultT4.3.1Entropy quantifies the direction in which spontaneous processes run: toward greater disorder, never the reverse.
Entropy change from heat exchanged at a given temperatureDefinitionT4.4.1