The First Principle of Thermodynamics and the Non-Separability of the   Quantities "Work" and "Heat": The adiabatic piston controversy
  The impossibility of separating into work and heat the energy transmitted between two subsystems through a movable piston is analyzed in this article. The process here described, although "quasi-static", is not reversible. It is shown that the First Principle, dU=dW+dQ, introduced by Clausius, does not generally allow a physical identification of dW and dQ, although dU=-pdV+TdS is verified along the equilibrium points of each subsystem. 
Rodrigo de Abreu (Instituto Superior Tecnico)