  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 $Q = 2\xi B_0 J_H$ where $B_0$ is the strength of the
magnetic field, $J_H$ is the angular momentum of the black hole, $\xi$ is a
dimensionless parameter determined by the spin of the black hole.
