An isothermal process is a thermodynamic process in which the temperature of the system stays constant; ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir, and the system changes slowly enough to allow it to adjust to the temperature of the reservoir. The opposite extreme in which a system exchanges no heat with its surroundings is known as an adiabatic process.
Assuming that the quantity n0 of moles of gas of the system also remains constant, then the internal energy E of the system also remains constant, viz.
- ,
An isothermal process is shown as a hyperbolic line (T0 = constant) on a P-V (Pressure-Volume) diagram which asymptotically approaches both the V (abcissa) axis and the P (ordinate) axis. The line is called an isotherm and its equation is
- .
See also: adiabatic process, isochoric process, isobaric process, cyclic process.