Isochoric temperature change, reversible

The second path consists of [1] an adiabatic reversible expansion to Vi, and a temperature T less than Tj, and [2] an isochoric temperature increase from T to Tj,. The entropy change for the gas is again given by the sum of the changes for the two steps ... 

Recall from Section 12.1 that a true reversible process is an idealization it is a process in which the system proceeds with infinitesimal speed through a series of equilibrium states. The external pressure therefore, can never differ by more than an infinitesimal amount from the pressure, P, of the gas itself. The heat, work, energy, and enthalpy changes for ideal gases at constant volume (called isochoric processes) and at constant pressure (isobaric processes) have already been considered. This section examines isothermal (constant temperature) and adiabatic (q = 0) processes. 

---