  For non-spinning, charged (Reissner-Nordstr\"om) black holes, the particles
with an opposite sign of charge with respect to that of the black hole will be
pulled into the black hole by the extra electromagnetic force. Such a hole will
be quickly neutralized so that there should not exist significantly charged,
non-spinning black holes in the universe. The case of spinning, charged
(Kerr-Newmann) black holes is more complicated. For a given initial position
and initial velocity of the particle, an oppositely charged particle does not
always more easily fall into the black hole than a neutral particle. The
possible existence of a magnetosphere further complicate the picture. One
therefore cannot straightforwardly conclude that a charged spinning black hole
will be neutralized. In this paper, we make the first step to investigate the
neutralization of Kerr-Newmann black holes without introducing a magnetosphere.
We track the particle trajectories under the influence of the curved spacetime
and the electromagnetic field carried by the spinning, charged black hole. A
statistical method is used to investigate the neutralization problem. We find a
universal dependence of the falling probability into the black hole on the
charge of the test particle, with the oppositely charged particles having a
higher probability of falling. We therefore conclude that charged, spinning
black holes without a magnetosphere should be quickly neutralized, consistent
with people's intuition. The neutralization problem of Kerr-Newmann black holes
with a co-rotating force-free magnetosphere is subject to further studies.
