  Within a multi-phase transport (AMPT) model with the string melting mechanism
and imported initial electric quadrupole moment, the difference between the
elliptic flow of positive and negative pions is calculated. The slope parameter
$r$ of the linear dependence of $\Delta v_{2}=v_{2}(\pi^{-})-v_{2}(\pi^{+})$ on
$A_{ch}=(N^{+}-N^{-})/(N^{+}+N^{-})$ is yielded owing to the parton cascade,
which converts the initial electric quadrupole distribution into the final
charge-dependent elliptic flow. The slope parameter $r$ is found to be
increased by the hadronization given by the coalescence, and decreased by the
resonance decays. The slope parameter $r$ is very sensitive to both the initial
electric quadrupole percentage and centrality bin and consequently a helpful
constraint on the quadrupole moment of the chiral magnetic wave is obtained for
Au+Au collisions at the top RHIC energy.
