In chemical engineering, the principles of thermodynamics are commonly applied to " open systems ", i.e. those in which heat, work, and mass flow across the system boundary. In general, flow of heat Q ˙ ˙ , flow of shaft work W ˙ ˙ S and pressure-volume work P V ˙ ˙ across the system boundaries cause changes in the entropy of the system. Heat transfer entails entropy transfer Q ˙ ˙ / T , where T is the absolute thermodynamic temperature of the system at the point of the heat flow. If there are mass flows across the system boundaries, they also influence the total entropy of the system. This account, in terms of heat and work, is valid only for cases in which the work and heat transfers are by paths physically distinct from the paths of entry and exit of matter from the system.