- Current can be pumped through a closed system by changing parameters (or fields) in time. Linear response theory (the Kubo formula) allows one 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 d Q=-G d X, where d X is the displacement of the scatterer. Thus the issue is to calculate the generalized conductance G.