  Current can be pumped through a closed system by changing parameters (or
fields) in time. Linear response theory (the Kubo formula) allows to analyze
both the charge transport and the associated dissipation effect. We make a
distinction between adiabatic and non-adiabatic regimes, and explain the subtle
limit of an infinite system. As an example we discuss the following question:
What is the amount of charge which is pushed by a moving scatterer? In the low
frequency (DC) limit we can write dQ=-GdX, where dX is the displacement of the
scatterer. Thus the issue is to calculate the generalized conductance $G$.
