  The directed flow of identified hadrons is studied within the
parton-hadron-string-dynamics (PHSD) approach for the asymmetric system Cu+Au
in non-central collisions at $\sqrt{s_{NN}}$ = 200 GeV. It is emphasized that
due to the difference in the number of protons of the colliding nuclei an
electric field emerges which is directed from the heavy to the light nucleus.
This strong electric field is only present for about 0.25 fm/c at
$\sqrt{s_{NN}}$ = 200 GeV and leads to a splitting of the directed flow $v_1$
for particles with the same mass but opposite electric charges in case of an
early presence of charged quarks and antiquarks. The microscopic calculations
of the directed flow for $\pi^\pm, K^\pm, p$ and $\bar{p}$ are carried out in
the PHSD by taking into account the electromagnetic field induced by the
spectators as well as its influence on the hadronic and partonic quasiparticle
trajectories. It is shown that the splitting of the directed flow as a function
of pseudorapidity $\eta$ and in particular as a function of the transverse
momentum $p_t$ provides a direct access to the electromagnetic response of the
very early (nonequilibrium) phase of relativistic heavy-ion collisions and
allows to shed light on the presence (and number) of electric charges in this
phase.
