In mesoscopic physics the principle of virtual work provides a notion of current. It associates the current to a virtual variation variation of energy with respect to magnetic field. Once the system is open the ownership of interaction energy is in question. The energy of system and corresponding notion of current become ambiguous. We study this ambiguity inside the Markovian framework and propose a principle of virtual work that is free of ambiguity.
This is a join work with J. Avron and G. M. Graf. | 
