  Charging of dust grains in low-pressure plasmas is reviewed critically. A
theory based on the Fokker-Planck equation and orbital motion limited
approximation is proposed. The theory predicts that dust grains can acquire a
positive charge in low-pressure electropositive plasmas having a sufficiently
high plasma potential, in agreement with experimental observations. It is also
shown that variations in the plasma potential (electron temperature) can lead
to spatial regions in which grains have opposite charges.
