Entropy of isothermal process in terms of pressure


An isothermal process is a change of a system, in which the temperature remains constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir (heat bath), and the change in the system will occur slowly enough to allow the system to continue to adjust to the temperature of the reservoir through heat exchange. In contrast, an adiabatic process is where a system exchanges no heat with its surroundings (Q = 0). In other words, in an isothermal process, the value ΔT = 0 and therefore the change in internal energy ΔU = 0 (only for an ideal gas) but Q ≠ 0, while in an adiabatic process, ΔT ≠ 0 but Q = 0.
For the expansion (or compression) of an ideal gas from an initial volume V0 and pressure P0 to a final volume V and pressure P at any constant temperature, the change in entropy is given by what is shown here.

Related formulas


ΔSChange in Entropy (J/K)
nnumber of moles (mole)
Rmolar gas constant
Pfinal pressure (pa)
P0intial pressure (pa)