Isothermal process
An isothermal process is a thermodynamic change in which a system’s temperature remains constant. Heat may enter or leave the system as work is done.
Ideal gas law: The equation of state relating an ideal gas’s pressure, volume, temperature, and amount of substance. At fixed temperature and amount, it makes pressure inversely proportional to volume.
Thermodynamic system: A specified quantity of matter or region in space chosen for thermodynamic analysis. The system boundary determines which heat and work transfers count.
Adiabatic process: A thermodynamic process in which no heat crosses the system boundary. Unlike an isothermal process, an adiabatic change generally alters temperature.
Boyle's law: The gas law stating that pressure and volume are inversely proportional at fixed temperature and amount of gas. It describes the ideal-gas pressure–volume relation along an isothermal path.
Entropy: A thermodynamic state function that quantifies energy dispersal and determines the direction of spontaneous change. Reversible heat transfer at constant temperature changes entropy by heat divided by temperature.
First law of thermodynamics: The principle that energy is conserved, relating a system’s internal-energy change to heat and work. It determines how heat transfer balances work during an isothermal change.
Thermodynamic state: The condition of a system described by variables such as pressure, volume, temperature, and composition. An isothermal path connects states that share the same temperature.
Isobaric process: A thermodynamic process that occurs at constant pressure. Its fixed variable is pressure rather than temperature.
Carnot cycle: An ideal reversible heat-engine cycle consisting of two isothermal and two adiabatic processes. Its heat absorption and rejection occur on isothermal legs.
Helmholtz free energy: A thermodynamic potential equal to internal energy minus temperature times entropy. At fixed temperature and volume, its change constrains the maximum useful work.