Definitions Glossary

Thermodynamics & Kinetic Theory — Key Terms

Term	Definition
Thermodynamic system	Any defined quantity of matter under study, separated from the surroundings by a boundary.
Surroundings	Everything outside the thermodynamic system.
Thermal equilibrium	Two bodies are in thermal equilibrium if no heat flows between them when placed in contact.
Zeroth Law	If A is in thermal equilibrium with B, and B with C, then A is in thermal equilibrium with C. Defines temperature.
First Law	Q = $\Delta U$ + W. Energy conservation applied to thermodynamic systems.
Internal energy (U)	Total microscopic energy of all molecules — kinetic and potential. For ideal gas: U = (f/2)nRT.
Work (W)	W = ∫P dV. Area under PV curve. Positive when gas expands.
Heat (Q)	Energy transferred between system and surroundings due to a temperature difference.
Isothermal	Process at constant temperature. $\Delta U$ = 0 for ideal gas.
Adiabatic	Process with no heat exchange (Q = 0).
Isochoric	Process at constant volume. W = 0.
Isobaric	Process at constant pressure. W = P $\Delta V$ .
Carnot engine	Theoretical engine of maximum efficiency operating between $T_{1}$ and $T_{2}$ via two isothermal and two adiabatic steps.
Carnot efficiency	η = 1 − $T_{2}$ / $T_{1}$ . The upper limit for any heat engine.
COP	Coefficient of Performance. Figure of merit for refrigerators. COP = $Q_{2}$ /W.
Second Law	No cyclic engine converts all heat to work (Kelvin-Planck); heat does not spontaneously flow cold → hot (Clausius).
Entropy	Measure of disorder (microscopic randomness) of a system. Increases in any spontaneous irreversible process.
Degrees of freedom (f)	Number of independent ways a molecule can store energy. Monoatomic: 3, Diatomic: 5, Polyatomic: 6.
Equipartition theorem	Energy per degree of freedom = ½k_BT per molecule = ½RT per mole at thermal equilibrium.
v_rms	Root mean square speed: √(3RT/M). Used in pressure equation P = ½ρv_r $ms^{2}$ .
v_avg	Mean (average) molecular speed: √(8RT/πM).
v_mp	Most probable speed: √(2RT/M). Speed at the peak of the Maxwell distribution.
Mayer's relation	Cₚ − Cᵥ = R. Difference in specific heats equals the gas constant for any ideal gas.
Boltzmann constant	k_B = $1.38 \times 10^{-23}$ J/K. Relates microscopic energy to temperature.