  It has been thought for decades that rotating black holes (BHs) power the
energetic gamma-ray bursts (GRBs) and active galactic nuclei (AGNs), but the
mechanism that extracts the BH energy has remained elusive. We here show that
the solution to this problem arises when the BH is immersed in an external
magnetic field and ionized low-density matter. For a magnetic field parallel to
the BH spin, the induced electric field accelerates electrons outward and
protons inward in a conical region, centered on the BH rotation axis, and of
semi-aperture angle $\theta \approx 60^\circ$ from the BH rotation axis. For an
antiparallel magnetic field, protons and electrons exchange their roles. The
particles that are accelerated outward radiate off energy and angular momentum
to infinity. The BH powers the process by reducing its energy and angular
momentum by capturing polar protons and equatorial electrons with net negative
energy and angular momentum. The electric potential allows for negative energy
states outside the BH ergosphere, so the latter does not play any role in this
electrodynamical BH energy extraction process.
