With the Komar mass formula we calculate the electromagnetic energy for a
charged Kerr black hole in a uniform magnetic field. We find that the total
electromagnetic energy takes the minimum when the Kerr black hole possesses a
non-zero net charge Q=2ξB0​JH​ where B0​ is the strength of the
magnetic field, JH​ is the angular momentum of the black hole, ξ is a
dimensionless parameter determined by the spin of the black hole.Comment: 9 pages, 1 figur