We investigate mechanisms of energy extraction from a rotating black hole in terms of negative energy at infinity. In addition to the Penrose process through particle fission, the Blandford-Znajek mechanism by magnetic tension, and the magnetohydrodynamic Penrose process, we examine energy extraction from a black hole caused by magnetic reconnection in the ergosphere. The reconnection redistributes the angular momentum efficiently to yield the negative energy at infinity. We derive a condition for the process to operate in a simple situation, where the plasma is incompressible and the magnetic energy is converted completely to the plasma kinetic energy locally. Astrophysical situations of magnetic reconnection around the black holes are also discussed.
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