Entropy change of black holes in disk-accretion

Entropy change of central black holes of accretion disks (including thin an d thick disks) is investigated by considering both accretion and the Blandf ord-Znajek (BZ) process in detail. It is shown that the contribution of the BZ process to the rate of change of black hole entropy is always less than that arising from accretion process in the case of thin disks, while the f ormer may be greater than the latter for a fast-spinning black hole surroun ded by a thick disk. In addition the non-monotonic characteristics of the r ate of change of black hole entropy in the BZ process are discussed.