KERR BLACK-HOLES AS A CARNOT ENGINE

It is shown through a series of thought experiments that rotating black holes immersed in a strong magnetic field can act, in principle, as a Carnot engine. In contrast with the original Carnot engine, however, work is extracted in the form of electric power, so that a Kerr hole actually functions as an electric powerhouse which is driven by the energy of Hawking's thermal radiation. The whole cycle consists of (i) isothermal spin-down, (ii) adiabatic spin-up, (iii) isothermal spin-up and (iv) adiabatic spin-down processes of the hole. The maximum efficiency of this engine is attained if step (iii) can be performed in the extreme Kerr state.