Assume a piston cylinder arrangement. External pressure always equals internal pressure ( reversible conditions ). Now consider these two scenarios: Scenario one: q heat is suddenly added to the arrangement causing a temperature rise and consequently pressure rise, but the volume is yet constant. That implies: q = n Cv dT Now lets decrease the pressure adiabatically to initial pressure p1 (before temp. rise). There will be adiabatic work done in expanding to lower pressure at the cost of internal energy: p dV = n Cv dT Scenario two: q heat is added at a constant pressure p1 (isobaric process) causing expansion and change in internal energy. Are the two scenarios equivalent in terms of change in internal energy, work done and final expansion (of volume)?