A recent result, relating the (irreversible) work performed on a system
during a non-quasistatic process, to the Helmholtz free energy difference
between two equilibrium states of the system, is discussed. A proof of this
result is given for the special case when the evolution of the system in
question is modelled by a Langevin equation in configuration space.Comment: Conference talk in Zakopane, Poland; 11 pages + 3 figure
